Health and environmental evidence, challenges, and sustainable strategies of Planetary Health Diet: A mixed-methods review

Abstract

The Planetary Health Diet (PHD) was proposed by the Lancet Commission in 2019. It is a new way of eating that aims to improve both human health and the environment simultaneously. However, recent practice demonstrates persistent cognitive gaps in terms of systematic quantification of health benefits, robust evidence of environmental impacts, and region-specific implementation pathways. This study systematically integrates epidemiological evidence to clarify the associations between PHD and reduced risks of diseases, including cardiovascular disease, diabetes, and certain cancers. Additionally, the study talks about how it affects the demand for agricultural land, the stress on freshwater resources, the release of greenhouse gases, and the imbalance in nitrogen and phosphorus cycles. The study elucidates various multidimensional obstacles to global PHD adoption, including limitations in natural resources, regional economic inequalities, scaling challenges in food technology innovation, and resistance to cultural and intergenerational transmission. To tackle these issues, the study proposes a four-dimensional synergistic approach: encouraging innovation in food production systems, creating fair food distribution systems, building policy intervention toolkits, and designing behavioral incentives. These evidence-based solutions offer both theoretical foundations and actionable paradigms for sustainable food system transformation.

Graphical abstract

Highlights

• •Evidence synthesis confirms PHD reduces chronic disease risks (CVD, cancer, etc.).
• •PHD relieve environmental stress: farmland, freshwater, GHG, energy, nitrogen and phosphorus cycles.
• •Identify global barriers: resource limits, econ gaps, tech bottlenecks, cultural resist.
• •Propose integrated strategy: production innovation, fair access, policy tools, incentives.

1. Introduction

Diet is closely linked to human health and environmental health. However, factors like processed foods, rapid urbanisation and lifestyle changes [1] have caused people to consume more high-calorie foods, refined grains, sugar or salt. Unhealthy diet increases disease prevalence and even leads to premature death around the world [2]. The 2021 Global Burden of Disease Assessment estimates that approximately 8 million deaths around the world can be attributed to dietary risk factors [3]. In addition, food production driven by unhealthy diets is also challenging the earth’s carrying capacity, such as climate change, environmental pollution, and biodiversity loss [4]. The food system contributes to over one-third of greenhouse gas (GHG) emissions, while animal-based foods emitting double as much as plant-based foods [5]. The upsurge in GHG emissions has intensified ultraviolet radiation, elevated temperatures, altered precipitation patterns, and heightened the occurrence of extreme weather event [6]. Moreover, the escalating intensification of agricultural production, coupled with the excessive utilization of pesticides and fertilizers, can potentially result in the deterioration of soils and the degradation of ecosystems [7]. Diet-related environmental challenges pose significant threats to both economic development and public health. Consequently, adopting healthier dietary practices emerges as a critical solution for improving human well-being and ensuring the sustainable development of our global ecosystem.

The Lancet Commission issued a novel eating guideline in 2019 called the Planetary Health Diet (PHD) [8], and reiterated the scope and importance of PHD for health and the environment in the October 2025 update report (PHD 2.0) [9]. The PHD is a plant-based diet that limits red meat, added sugars, and saturated fats to regulate and alleviate the detrimental impacts of diet on human health and ecosystem [9]. The updated report also emphasizes that the PHD is not rigidly prescriptive; its implementation is flexible and adaptable to regional resource availability, cultural contexts, and dietary traditions [9]. Projections indicate that this dietary pattern could sustainably nourish over 10 billion people while preventing 10.8 million to 11.6 million annual premature deaths by 2050. In addition, PHD reduces soil degradation from monoculture-based farming through moderate grazing of non-arable resources and reduces dependence on water-intensive plant foods through animal protein supplementation, such as fish and crustaceans [10,11].

The potential influence of PHD on public health and Earth’s ecosystem is extensive. However, most available research concentrates on a single dimension to assess its environmental or health outcomes, while lacking comprehensive systematic evidence regarding PHD’s influence on global health trajectories. This study employs a mixed-methods approach, wherein the evidence into the health and environmental effects of PHD was synthesized according to systematic review protocols, while the insights on associated challenges and strategies were drawn from a wider range of sources, including peer-reviewed academic studies, institutional reports, and policy documents. The complete methodological details, including methodology, the flowchart, registration, and PRISMA checklist, are available in the Supplementary Files 1 and 2. This work explores the dual role of the PHD on human health and environmental sustainability throughout the life cycle, and identifies potential challenges and strategies to promote it. The project intends to build a scientific basis for healthcare policy and offer actionable guidance for population-wide dietary decisions, effectively promoting the development of PHD.

2. The PHD and health outcomes: Evidence from population studies

The emerging PHD, as a multisystem disease modulator, underscores its potential effects on human health. However, the association among them remains unclear. This work systematically evaluated and quantified the effects of PHD on cardiovascular diseases (CVDs), diabetes, cancer, metabolic syndrome, respiratory disease, neurological disease, and all-cause mortality (Fig. 1, Fig. 2). The assessment of the evidence quality was conducted based on the scales in Tables S3 and S4. The assessment results are presented in Tables S5 and S6. The included studies were of moderate to high quality (Tables S5 and S6). Table S1 presents the specific characteristics of each study.

Fig. 1.

The analysis of the effect size between Planetary Health Diet (PHD) and health outcomes. HR, hazard ratio; CI, confidence interval. All HR are for the highest vs. the lowest category of PHD adherence.

Fig. 2.

Regional analysis of Planetary Health Diet and health outcomes. CVDs, cardiovascular diseases. All HR are for the highest vs. the lowest category of PHD adherence.

2.1. CVDs

CVDs, the leading cause of mortality and disability worldwide [12], are closely related to dietary patterns. The relationship between PHD and CVD outcomes has been systematically examined in a number of large cohort studies conducted recently. A prospective study based on the UK Biobank (n = 114,165) showed a significant reduction in the risk of major cardiovascular events in the high PHD-scoring group compared to the low PHD-scoring group: the overall CVDs hazard ratio (HR) was 0.79 [95% CI (confidence interval): 0.74–0.84], with a 31% reduction in the risk of heart failure (HR = 0.69, 95% CI: 0.59–0.82) [13]. This finding was also validated in three major US cohorts (the Nurses’ Health Study I, the Nurses’ Health Study II, and the Health Professionals Follow-up Study), where pooled analyses showed that high PHD adherence reduced CVD risk by 17% (HR = 0.83, 95% CI: 0.78–0.89) [14]. Furthermore, findings from China’s Health and Nutrition Survey indicated that participants with the highest PHD scores exhibited a 51% reduction in CVD risk compared to those with the lowest scores (HR = 0.49, 95% CI: 0.37–0.66) [15]. Analyses of specific cardiovascular subtypes showed that adherence to the PHD reduced the risk of coronary heart disease (CHD) by 12%–19% [14,16], hypertension by 13%–21% [17,18], and ischemic heart disease by 20%–28% [13,19,20]. The Danish cohort (n = 55,016) found that PHD was linked to a 70% lower incidence of subarachnoid haemorrhage (HR = 0.30, 95% CI: 0.12–0.73) [21]. The Swedish prospective cohort demonstrated that the PHD index was significantly negatively correlated with eight biomarkers of heart failure over 25 years of follow-up [22]. In addition, the implementation of PHD significantly improved several cardiovascular metabolic indices, including a reduction in systolic blood pressure by 0.84 mmHg (95% CI: −1.66 to −0.01) [23], a 12% reduction in the probability of high blood cholesterol [odds ratio (OR) = 0.88, 95% CI: 0.78–0.99] [18], a reduction in total cholesterol by 0.80 mg/dL (95% CI: −1.13 to −0.46) [24], low-density lipoprotein cholesterol (LDL-C) decreased by 4.10 mg/dL (95% CI: −5.9 to −2.23) [23], high-density lipoprotein (HDL) increased by 0.21 mg/dL (95% CI: 0.03–0.39) [25], and levels of C-reactive protein (CRP), an inflammatory marker decreased by 14.95% [26]. Notably, a prospective cohort study of Chinese elderly individuals (n = 3742) indicated that PHD promoted cardiometabolic health through the gut microbiota-blood protein interaction [27]. However, in contrast to the aforementioned findings, data from a Spanish cohort study (n = 18,656) at baseline (HR = 0.77, 95% CI: 0.51–1.18) and after 10 years of follow-up (HR = 0.82, 95% CI: 0.55–1.23) indicated no statistically significant correlation between PHD adherence and CVDs [28]. The observed results may stem from minimal differences in dietary component intake across different PHD scores. For instance, monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA) intakes were comparable, and fat intake showed no variation between scores. Given the strong association between these dietary components and CVDs, the lack of intake differences may explain the absence of a link between PHD and CVDs.

An overall effect assessment (Fig. 1) incorporating 15 studies demonstrated that compared with the low-adherence group, the high-adherence group exhibited a 13% reduction in the risk of major CVD events (combined HR = 0.83, 95% CI: 0.79–0.87) [[13], [14], [15], [16], [17], [18], [19], [20], [21], [22],[28], [29], [30], [31]]. Specific subgroup analyses revealed that high PHD adherence reduced the risk of cerebrovascular events by 21% (HR = 0.83, 95% CI: 0.74–0.94) [14,16,19,21,29], the risk of heart disease by 21% (HR = 0.81, 95% CI: 0.74–0.88) [13,14,16,19,20,22,29], and the risk of vascular disease by 17% (HR = 0.83, 95% CI: 0.76– 0.91) [17,18].

The cardio-protective effects of PHD may be related to the synergistic action of multiple components. Firstly, phytochemicals, such as flavonoids, can elevate antioxidant enzyme activity by activating the Nrf2/Keap1 pathway [32]. Dietary soluble fibre forms a gel in the gut that binds to bile acids and lowers blood LDL-C [33]. In addition, the gut microbe-metabolite axis plays an important role, with short-chain fatty acids (SCFAs) generated by the metabolic process of gut bacteria reducing serum proinflammatory factor IL-6 levels by inhibiting the TLR4/NF-κB pathway [34].

Despite the significant overall protective effect of PHD, clear population heterogeneity remains. The correlation between PHD and CVD risk was greater in women under 60 years of age, and this age-dependence may result from diminished oestrogen regulation of lipid metabolism and oxidative stress in the vascular endothelium [14]. Notably, Polygenic Risk Score (PRS) analyses demonstrated that the benefits of PHD adherence for CVDs were not influenced by genetic susceptibility in the UK follow-up study (n = 111,669) [13]. Individuals at high genetic risk still achieved a 20% risk reduction with strict PHD (HR = 0.80, 95% CI: 0.70–0.91), indicating that adhering to PHD, regardless of genetic susceptibility, has a key role in preventing CVDs [13].

2.2. Diabetes

The 21st century has seen a dramatic increase in the number of people with diabetes, making it one of the world’s most pressing health concerns [24]. Dietary modification may be effective in preventing and controlling diabetes [24]. Preliminary evidence of the positive impact of PHD on diabetes has been obtained from numerous epidemiological studies. The EPIC-Oxford cohort study (n = 46,069) revealed a significant inverse correlation between strict adherence to the PHD and the incidence of diabetes, with participants who adhered most strictly experiencing a 59% lower risk (HR = 0.41, 95% CI: 0.33–0.50) [19]. This outcome has been validated across multiple nations (Denmark [35], Sweden [36], the U.S. [37], China [15]), demonstrating a decrease in diabetes risk ranging between 3% and 65% with high compliance to the PHD. In addition, a UK cohort study based on the UK Biobank found that Body Mass Index (BMI) played a substantial mediating role, explaining 44% of the total association between adherence to PHD and reduced risk of diabetes [38]. Meanwhile, PHD reduced several metabolic markers of diabetes, lowering total cholesterol by 0.80 mg/dL (95% CI: −1.31 to −0.46) and LDL-C by 0.10 mg/dL (95% CI: −0.19 to −0.08) [24]. However, inconsistent with the results of the above studies, the Mexican cohort study indicated no correlation between PHD and diabetes (HR = 0.90, 95% CI: 0.75–1.10) [39]. The population included in this study was female, and for female participants who may be more prone to dietary questionnaire misreporting [40], the true correlation between PHD and diabetes risk may be greater than that observed in this study. An overall effect assessment of seven studies with heterogeneous outcomes (Fig. 1), which indicated a 21% lower risk of diabetes in the high-scoring group compared with the low PHD-scoring group (HR = 0.69, 95% CI: 0.56–0.85) [15,19,[35], [36], [37], [38], [39]].

PHD may exert mechanistic regulation of diabetes through a variety of food components. High dietary fibre (25–30 g/d) may improve hepatic insulin sensitivity by promoting the proliferation of butyric acid-producing bacteria, enhancing intestinal barrier function, and decreasing circulating lipopolysaccharide (LPS) [41]. It may also enhance haemoglobin A1c levels by increasing the secretion of glucagon-like peptide-1 (GLP-1) induced by SCFA [41]. Limiting the use of red meats that contain high levels of heme iron and nitrites may reduce inflammation, insulin resistance, and pancreatic cell damage [42]. Soy may restore the function of islet cells and increase insulin sensitivity by increasing plasma adiponectin levels and stimulating adenosine monophosphate-activated protein kinase (AMPK) phosphorylation [43].

2.3. Metabolic syndrome

Metabolic syndrome is the largest non-communicable disease burden globally [44]. Obesity is a key issue among these metabolic conditions, and preventing its increase is crucial for the health of the population as a whole [45]. The beneficial role of PHD in controlling obesity and metabolic syndrome has been demonstrated in a growing number of studies. A cohort study in Iran (n = 6465) indicated that high PHD index (PHDI) adherence was linked to a lower likelihood of both metabolic syndrome and abdominal obesity (P < 0.05) [46]. The Brazilian Longitudinal Study of Adult Health (n = 14,515) indicated a 24% reduction in the risk of overweight (OR = 0.76, 95% CI: 0.67–0.85) and obesity (OR = 0.76, 95% CI: 0.65–0.88) in people with high adherence to PHD compared to those with low adherence [47]. The prevalence of overweight and obesity was 7.16% higher among people not adhering to PHD than among those following PHD in the Polish region [48]. Multi-ethnic cohort studies (African American, European American, Japanese American, Native Hawaiian, and Latino) have further validated that populations with high PHD adherence had lower obesity rates (HR = 0.76, 95% CI: 0.73–0.79) [37]. In addition, the Generation XXI birth cohort highlighted that the beneficial effect of PHD on metabolic syndrome was irrespective of age differences, with each 10-point increase in PHD score reducing the risk of metabolic syndrome by 17.4% in 7-year-olds [Prevalence Ratio (PR) = 0.83, 95% CI: 0.70–0.97] and 16.3% in 13-year-olds (PR = 0.84, 95% CI: 0.73–0.97) [49]. An overall effect assessment (Fig. 1) of the above studies indicated a 24% lower risk of obesity in the high-scoring group compared to the low PHD-scoring group (HR = 0.76, 95% CI: 0.73–0.79) [37,47].

Meanwhile, PHD reduced numerous obesity indicators [50], such as BMI by 0.50 kg/m² (95% CI: −0.73 to −0.27) [51] and waist circumference by 1.70–1.90 cm (95% CI: −2.28 to −1.12 [47] and −2.50 to −1.20 [52], respectively). The protective effect of PHD on waist circumference was more pronounced with age, by 9.6% at age 7 (PR = 0.90, 95% CI: 0.83–0.98), 10.1% at age 10 (PR = 0.90, 95% CI: 0.83–0.97), and 14.2% at age 13 (PR = 0.86, 95% CI: 0.78–0.94) [49]. Moreover, a negative relationship was identified between PHD and both fat-free mass index (FFMI) and body fat percentage (P < 0.05) [51,53].

PHD intervenes in the development of metabolic syndrome through multiple mechanisms. Firstly, PHD may reduce the risk of metabolic syndrome by lowering inflammation [32]. Fruit and vegetables are rich in antioxidants, which help to reduce oxidative stress by neutralising free radicals [32]. Dietary fibre may have anti-inflammatory effects in adipose tissue and on immune cell function by increasing the production of lactic acid by the intestinal flora, either dependently or independently on G protein-coupled receptor 81 (GPR81) [34]. In terms of energy metabolism, plant proteins (such as soybean and pea proteins) can prevent the overactivation of mammalian target of rapamycin protein complex 1 (mTORC1) by decreasing the concentration of plasma branched-chain amino acids (BCAAs). This may suppress key lipogenic enzymes, such as fatty acid synthase and acetyl-CoA carboxylase 1, and promote the phosphorylation of adipose triacylglyceride lipase (ATGL) [54]. Whey protein in dairy products may reduce hunger, lower blood glucose levels, and increase anorectic hormone secretion [55]. Dietary fibre binds to bile acids, thereby activating the G protein-coupled bile acid receptor (TGR5) and the farnesoid-X-receptor [56]. This enhances satiety and regulates key metabolic processes, such as hepatic glycogen synthesis, insulin secretion, and energy expenditure in tissues like the liver, brown fat, and muscle [56]. All of the above collectively indicate the potential of PHD as a promising novel strategy for preventing and controlling metabolic disorders like obesity.

2.4. Cancer

The World Health Organization (WHO) states that diet and nutrition may account for 20% to 25% of the global cancer burden [57], emphasizing the importance of a balanced diet for maintaining a healthy weight and preventing cancer. The European Prospective Investigation into Cancer and Nutrition (EPIC) cohort (n = 443,991) showed that a balanced diet of PHD will prevent 10% to 39% of cancers over a 20-year risk period [58]. In a UK Biobank prospective cohort with over 11.5 years of follow-up, close adherence to the PHD was associated with a 9% lower incidence of overall cancer (HR = 0.91, 95% CI: 0.87–0.95) [31]. Analyses of specific cancer subtypes showed that adherence to PHD in the UK population reduced the risk of lung cancer by 36% (HR = 0.64, 95% CI: 0.51–0.80) and squamous cell lung cancer by 54% (HR = 0.46, 95% CI: 0.23–0.94) [59]. Of these associations, 1.44% to 10.00% of the risk was mediated by nine biomarkers, including inflammatory cell parameters (e.g., leukocyte count), erythrocyte distribution width, SII, alkaline phosphatase, CRP, and cystatin C, with leukocyte count acting as a predominant mediator (10.00%) [59]. This suggests that PHD may reduce the risk of lung cancer by modulating inflammatory processes. Additionally, results from an Iranian case-control study indicated that high adherence to the PHDI reduced the risk of colorectal cancer by 61% (OR = 0.39, 95% CI: 0.18–0.86) [60]. In contrast to these results, the prospective NutriNet-Santé cohort found no evidence linking the PHD to overall cancer risk (HR = 0.96, 95% CI: 0.84–1.10) [30]. This may be because the study’s participants were mostly young, educated women leading healthy lifestyles who volunteered for the research. Compared to the general population, this healthier profile may have diluted the observed association between the PHD and cancer risk. An overall effect assessment (Fig. 1) showed that a high PHD score was associated with a 21% reduction in cancer risk compared to a low score (HR = 0.78, 95% CI: 0.65–0.95) [30,31,59,60].

The composition of the PHD may be crucial for preventing cancer. The intake of high dietary fibre (whole grains, legumes, and so on), fruits, and vegetables may reduce cancer risk by increasing antioxidant capacity [32], promoting the repair of DNA [61], regulating oestrogen metabolism [62], lowering insulin levels, and elevating growth regulators (mainly the insulin-like growth factor 1) [41]. Phytonutrients (such as tocopherols) may counteract tumour transformation of cells through multiple mechanisms, including reducing the activity of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, penetrating the cell nucleus, binding to DNA, and preventing nucleic acid chain breaks and mutations [63]. Reducing the consumption of red and processed meats limits exposure to various carcinogens, both from external sources and those generated within the body, such as N-nitroso compounds. It may reduce the formation of cytotoxic and genotoxic aldehydes, which in turn reduces the adverse prognosis of cancers such as colorectal cancer [42]. A diet using oils rich in linoleic acid may lead to a significant reduction in pro-inflammatory cytokines and inhibit STAT-3 and NF-κB signalling pathways [64].

Notably, each one-point increment in the PHD score was specifically associated with a 9% reduction in the risk of lung cancer mortality in the high PRS group. This was not evident in the low-PRS cohort. This suggests that the effect of PHD may appear to be more pronounced in populations with higher genetic susceptibility. A decreasing trend in lung cancer burden (incidence and mortality) was associated with improved PHD scores and lower genetic risk (P < 0.05) [59]. However, this effect was not significant within the low PRS group [59]. A reduction of 46.6% in lung cancer incidence and 47.2% in mortality was observed among individuals with the most favourable genetic and dietary profile (lowest genetic risk and highest PHD adherence) compared to those with the least favourable profile. The PHD score showed no significant interaction with genetic risk in terms of its effect on lung cancer incidence and mortality (P > 0.05) [59]. This suggests that PHD confers a protective effect against lung cancer, regardless of genetic predisposition.

2.5. Neurological disorders

Neurological disorders arise from a combination of genetic and environmental factors. Many studies have focused heavily on nutrition due to its role as a modifiable environmental factor. An 8-year follow-up study of 11,737 middle-aged and older adults in the ELSA-Brasil cohort found a link between sticking more closely to the PHD and reduced cognitive decline (P = 0.009) [65]. This was further confirmed in the B-proof study [66]. In the cross-sectional analysis, an increase of 3.7 points in the PHD score was associated with an increase of 0.04 units in the overall cognitive synthesis index (95% CI: 0.00–0.08) [66]. In the longitudinal analysis, an increase of 3.7 points in the PHD score was associated with an increase of 0.05 units in the overall cognitive composite index (95% CI: 0.02–0.08) [66]. A study on adults of Chinese in Singapore (n = 16,736) clearly highlighted that the risk of poor cognitive function in the overall population was reduced by 11% for each 1-SD increase in PHD score (OR = 0.89, 95% CI: 0.85–0.93) [67]. Regarding specific domains of cognitive function, PHD slowed down memory decline (P_trend = 0.04) [65], improved executive function (β per SD = 0.07, 95% CI: 0.01–0.12) [66], improved working memory (β per SD = 0.11, 95% CI: 0.04–0.18) [66], improved information processing speed performance (β per SD = 0.06, 95% CI: 0.00–0.12) [66], and improved attentional accuracy (P < 0.001) [68]. However, strict adherence to PHD in the UK population had no effect on situational memory (P = 0.440), reaction time (P = 0.886), and focused attention (P = 0.789) [68]. It is possible that the overall efficacy of PHD on cognitive health may be attenuated by limiting the intake of red meat and poultry rich in folate, iron, zinc, etc., and reducing the nutrient density of the diet in some populations. In addition, the PERSIAN Organizational Cohort Study in Mashhad (POCM) (n = 4579) indicated that subjects with the highest PHD score were 35% less likely to experience depression than those with the lowest PHD score (OR = 0.65, 95% CI: 0.48–0.88) [69]. Consistent with the findings of the UK study, those with stricter adherence to PHD were less depressed (β = −0.153, 95% CI: −0.020 to −0.003) [68]. Another notable finding was that adhering to PHD substantially reduced the risk of depression by 19.4% (HR = 0.81, 95% CI: 0.73–0.89) and anxiety by 18.2% (HR = 0.82, 95% CI: 0.75–0.89) [70]. It was also associated with a lower likelihood of the two conditions occurring together [70]. An overall effect assessment (Fig. 1) showed that participants with high PHD scores were 24% less likely to experience depression than those with low PHD scores (HR = 0.76, 95% CI: 0.63–0.92) [69,70].

PHD may support brain health to some extent throughout the life course [71]. Whole grains may support cognitive function in children and adolescents through the gamma-aminobutyric acid (GABA) component, promotion of brain-derived neurotrophic factor (BDNF) production, and development of the hippocampus and amygdala [72]. Anti-inflammatory and antioxidant properties derived from dietary components such as carotenoids and polyphenols, which are commonly found in fruits and vegetables, may improve metabolic profiles, mitigate oxidative damage, and counteract the formation of AβO and tau aggregates, particularly in elderly individuals [73]. Furthermore, they may inhibit the transcription factor NF-κB to reduce the expression of inflammatory cytokines and exert antidepressant potential [34]. Propionate, a short-chain fatty acid derived from the fermentation of dietary fibre, may act through a CD14-dependent process to counteract pathways involved in non-specific microbial infections [74]. It also suppresses LRP-1 expression while activating NRF2 (NFE2L2) signalling, thereby preserving the integrity of the blood-brain barrier against oxidative stress [74]. In addition, it may improve intestinal permeability and hypothalamo-pituitary-adrenal (HPA) axis reactivity associated with depression, generating anti-depressant effects [75]. Dairy intake may increase serum BDNF levels and brain glutathione peroxidase concentrations, contributing to good cognitive function during aging [76]. Omega-3 polyunsaturated fatty acids (PUFAs), primarily found in fish and nuts, may improve the properties of neuronal membranes, have antioxidant properties, and reduce cellular impairment, thereby maintaining cognitive function [77].

Interestingly, the beneficial effect of PHD on memory was more pronounced in high-income compared to low-income populations (P < 0.001) [65]. This may be related to the greater tendency of high-income populations to choose healthier and greener foods, thereby exaggerating the effect of PHD on memory. A strong effect modification by APOE ε4 status was observed (P = 0.042) [67]. A higher PHD score was associated with a 11% reduction in the odds of cognitive dysfunction for each SD increase among non-carriers (OR = 0.89, 95% CI: 0.83–0.96) [67]. However, no such benefit was observed in carriers of the allele (OR = 1.04, 95% CI: 0.89–1.23) [67]. It suggests that genetic effects may significantly attenuate the behavioral intervention effect of PHD.

2.6. Respiratory diseases

There is a paucity of literature on PHD and respiratory diseases. To date, only one study has found an association between adherence to the PHD and a 47% lower risk of dying from respiratory diseases (HR = 0.53, 95% CI: 0.48–0.59) [78]. Most research has primarily focused on asthma subtypes. Asthma, a chronic respiratory disease prevalent in children and adults, is a serious global health problem. Studies have shown that PHD containing high levels of green foods is linked to a lower likelihood of developing asthma. The National Health and Nutrition Examination Survey (NHANES) (n = 32,388) showed that asthma risk was 14% lower among participants with the highest PHD scores compared to those with the lowest (OR = 0.86, 95% CI: 0.75−0.98) [79]. BMI accounted for 33.85% of this association (P < 0.001), suggesting that the protective effect of the diet may be partly achieved through weight management, thereby lowering asthma risk [79]. In addition, in non-overweight/non-obese populations, the odds of airway inflammation were reduced by 3% as PHD scores increased (OR = 0.97, 95% CI: 0.93–0.99) [80]. Patients with overweight/obesity may tend to report healthier diets than they actually do [81]. As a result, the protective effect of PHD in overweight/obese patients may not be observed.

The development of many respiratory conditions is closely linked to dual pathogenic mechanisms involving oxidative stress and inflammation. The antioxidant and unsaturated fatty acids in PHD may mediate the inhibition of NLRP3 expression, the promotion of phagocytosis and lymphocyte function, the modulation of cytokine and histamine concentrations, and the enhancement of expiratory volume and vital capacity [79]. Intake of whole grains may reduce serum CRP and tumour necrosis factor α receptor-2 levels, inhibit nuclear factor κB activity, and activate G-protein-coupled receptors, thereby inhibiting immune cell recruitment [34].

2.7. Longevity and total mortality

The health risks posed by unhealthy diets are greater than those posed by unsafe sex, alcohol, tobacco, and drug abuse combined [8]. A good and sensible diet is an important aspect of health and longevity. One study suggested that 54% to 63% of deaths were avoided over 20 years if PHD is fully followed [58]. The greatest reduction in mortality was observed for CHD, with 2 million and 1.9 million averted deaths among men and women, respectively [82]. In addition, approximately one-sixth of premature deaths from cancer and stroke were prevented by PHD, and one million women and seven hundred thousand men can avoid dying from respiratory diseases by following PHD [82]. According to three large U.S. cohorts (n = 206,404), raising the global PHDI to 120 would prevent an estimated 15 million fatalities each year, accounting for over a quarter (27%) of all deaths [83]. Among these, preventable deaths ranged from 2.5 million due to CVDs to 0.7 million attributable to neurodegenerative diseases [83]. In a U.S. prospective cohort study that followed participants for 34 years, higher adherence to the PHD was associated with a 23% lower risk of all-cause mortality (HR = 0.77, 95% CI: 0.75–0.80) and a 47% reduction in respiratory disease mortality (HR = 0.53, 95% CI: 0.48–0.59) [78]. Further cross-continental studies (in the UK, China, Sweden, and Singapore) confirmed that strict adherence to the PHD was associated with an 8%–25% lower risk of death from any cause [15,31,[84], [85], [86]]. Additionally, an analysis combining the Cohort of Swedish Men and the Swedish Mammography Cohort (n = 68,000) revealed a significant protective effect of the PHD across genders [87]. A clear dose-response relationship was observed between PHD score and all-cause mortality. Each 3-point increase in the PHD score (approximately 2 SD) corresponded to a 7% lower all-cause mortality in men (HR = 0.93, 95% CI: 0.90–0.97) and a 9% reduction in women (HR = 0.91, 95% CI: 0.87–0.95) [87]. This indicates that the protective effect of PHD on overall mortality may be significant in both sexes.

An overall effect assessment (Fig. 1) of the nine included studies showed that participants with higher PHD scores had an overall mortality risk that was 14% lower than those with lower scores (combined HR = 0.82, 95% CI: 0.78–0.86) [15,19,31,59,78,[84], [85], [86], [87]]. Specifically, higher PHD scores were associated with a reduction in mortality across multiple disease categories, including a 14% decrease in CVD mortality (HR = 0.86, 95% CI: 0.79–0.93) [78,[85], [86], [87]]. This was alongside a 15% reduction in cancer mortality (HR = 0.86, 95% CI: 0.79–0.95) [59,78,85,86], a 33% reduction in respiratory disease mortality (HR = 0.66, 95% CI: 0.43–0.99) [78,86], a 28% reduction in neurological diseases mortality (HR = 0.72, 95% CI: 0.67–0.78) [78] and a 38% reduction in infectious diseases mortality (HR = 0.62, 95% CI: 0.51–0.76) [78].

Potential drivers of this protective association may arise from PHD-specific food groups. Beneficial components of PHD may reduce systemic markers of inflammation (15%−21% reduction in CRP [88]), improve metabolic markers (a reduction in glycated haemoglobin HbA1c of 2.00 mmol/mol, 95% CI: −3.30 to −0.71 [89]), alter and enrich the diversity of the gut microbiome [90], boost immunity, and thereby exert a protective effect against death from disease. Among them, PUFA can lower the risk of death by 24% (HR = 0.76, 95% CI: 0.74–0.78) [78]. In addition, restriction of animal foods may reduce the intake of L-carnitine and choline, and harmful metabolites produced by the gut microbiome (such as trimethylamine N-oxide, tryptophan metabolites, and sulphides) may be reduced in circulating concentrations [91].

It is noteworthy that despite the significant overall protective effect of PHD, for infectious diseases, a mortality risk reduction effect was demonstrated only in women (HR = 0.62, 95% CI: 0.51–0.76) [78]. This gender dependence may stem from the fact that women are at an immune disadvantage after menopause due to oestrogen levels. PHD precisely compensates for the vulnerability of this group through the synergistic effect of phytoestrogens and key nutrients. Therefore, PHD may be more protective for women than for men against infectious diseases. In addition, the correlation between PHD scores and mortality was stronger in smokers (P for interaction = 0.039) and urban dwellers (P for interaction = 0.014) compared to never-smokers and rural dwellers [84]. This suggests that the benefits of PHD may be more pronounced in adequately nourished populations than in undernourished populations. It is possible that the beneficial effects of PHD have not yet been realized because rural populations are at a relatively early stage of the nutrition transition.

3. The PHD from an environmental perspective

Given the rapid growth of the global population and dietary demands, PHD shows potential for environmental improvement. However, the impacts of PHD on the environment are unclear. This work systematically evaluated the influences of PHD on land occupation, water use, GHG emissions, cumulative energy demand (CED), and nitrogen and phosphorus cycles (Fig. 3, Fig. 4). The evidence quality was assessed based on the Newcastle-Ottawa scale (Table S3) and the Agency for Healthcare Research and Quality (AHRQ) (Table S4). The assessment results are presented in Tables S5 and S6. The quality of the included studies was moderate or above (Tables S5 and S6). Table S2 provides an overview of the fundamental attributes of all the studies included in the analysis.

Fig. 3.

The global-scale relationship between Planetary Health Diet and environmental sustainability. Given the large number of countries involved in GHG emissions, the specific number of countries is not shown in the figure. Please refer to Fig. 4. As the remaining environmental indicators involve fewer countries, the specific number of countries is indicated in the figure. Each green downward arrow indicates a decrease, and each red downward arrow indicates an increase. GHG, Greenhouse gas; CED, cumulative energy demand.

Fig. 4.

Regional analysis of Planetary Health Diet and environmental indicators. As GHG involves a greater number of countries, it is represented by colour in the diagram. As land use, water use, nitrogen and phosphorus cycling, and CED involve fewer countries, they are described in text within the diagram. Each blue downward arrow indicates a decrease, and each red downward arrow indicates an increase.

3.1. Land occupation

The global demand for food has doubled due to a growing population and changing dietary habits, putting progressively greater pressure on global resources and the environment. Livestock occupy most of the available land, with cattle accounting for 30% and poultry for 14% [92], contributing to increasing land scarcity. With the gradual improvement of the PHD framework, its effect on land occupation has been gradually identified [93]. This study identified eight publications examining the association between land occupation and PHD. Among the 13 countries studied, only China reported a positive correlation between PHD and land occupation. The results of the German DONALD cohort study (n = 298) indicated that PHD adherence was negatively associated with land use (β = −0.40, 95% CI: −0.52 to −0.29, P < 0.001) [53], which was further validated in the Israeli population [92]. Cross-continental studies (Europe, North America) have highlighted that high adherence to PHD reduced land use by 3.9% to 62% [16,58,78,94]. However, the high compliance of PHD increased land use by 10.3% in the Singapore Chinese population [3.2 (2.9–3.7) m² vs 2.9 (2.6–3.3) m², P < 0.001] [84,86]. As animal-based food is likely to be a key factor, the positive correlation between adherence to PHD and land use may reflect a discrepancy between the recommended and actual levels of dairy consumption relative to other dietary components. Therefore, the environmental benefits of the PHD on land use may not be fully realized in the short term.

The PHD recommends reducing the consumption of red and processed meat. This would reduce the demand for land used for livestock production and enhance soil carbon sequestration through more diverse cropping systems [95]. This shift may not only reduce land tensions but also reduce biodiversity loss. Importantly, the adoption of the PHD could reduce the need for deforestation, which is currently ongoing, driving climate change, and is unsustainable. A significant finding was that increased land use was associated with a higher overall risk of mortality (HR = 1.18, 95% CI: 1.15–1.21), with the most significant association observed for CHD mortality risk (HR = 1.38, 95% CI: 1.27–1.49) [58]. This may be associated with a decline in exposure to PM_2.5 due to the reduction in nitrogen emissions from intensive agriculture [96]. This reveals that PHD may be a model for the dual benefits of conserving environmental resources and thereby prolonging healthy lifespan.

3.2. Water use

Growing demand and widespread pollution are putting severe pressure on global water scarcity, jeopardizing sustainable development worldwide [97]. Research findings show that fruits, vegetables, and dairy products account for 40%, 12%, and 11% of water used in food production, respectively [92]. The impact of PHD, which consists of these ingredients as major components, on water resources is bidirectional. This study collected six articles concerning PHD and water usage. Among these, three countries reported that PHD increases water consumption, while two countries reported that PHD reduces water consumption. A three-year Israeli cohort study (n = 525) confirmed a positive correlation between adherence to PHD and water consumption (P < 0.001) [92]. This is consistent with the findings from China [84] and Singapore [86] as adherence to the PHD increased, water footprint consumption also gradually rose (P_trend < 0.001). In addition, the EPIC-NL cohort (n = 35,496) clearly demonstrated that high adherence to PHD increased water use by 32.1% (95% CI: 28.5%−35.7%) [16]. However, in the U.S., areas with the highest PHD scores had a 13% reduction in irrigation water demand compared to areas with the lowest PHD scores [78], consistent with findings from the Iranian study [46].

Beef consumption is a significant factor in water scarcity within the American diet [98], and PHD may reduce blue water consumption primarily through reducing water use in the livestock production chain. Regional differences of water use may also be due to the expansion of fruit, vegetable, and nut cultivation [99], with higher irrigation demand and thus increased blue water stress. Furthermore, the aforementioned investigation, which used the Food Frequency Questionnaire (FFQ) to assess dietary patterns, may have limited reliability of self-reported data on vegetable and fish consumption [100]. This suggests that there may be measurement errors in vegetable and fish intake, and therefore the impact on environmental impact indicators may have been underestimated. In conclusion, optimization of irrigation techniques, intensified wastewater reuse, and selection of seasonal fruits and vegetables suitable for local cultivation are essential for the implementation of PHD to achieve wide coverage in the districts and to reduce the blue water footprint.

3.3. GHG emissions

Every stage of the food production, processing, transportation, and consumption chain generates greenhouse gas (GHG). These emissions trap atmospheric heat, driving climate change. The largest proportion of GHG related to food comes from agriculture and land use [101], with dairy products contributing 26% [92], meat contributing 14% [92], and vegetables contributing 14% [92]. In addition, methane from cattle digestion and manure disposal contributes a large share [101]. Therefore, reducing global animal food consumption will be key to achieving environmental sustainability goals. This study collected 16 articles concerning PHD and GHG. The majority of countries indicated that PHD reduces GHG emissions, while a minority reported that PHD increases GHG emissions. Interestingly, some contradictory results were also observed. Enriching PHD with plant foods reduced GHG emissions (β = −0.22, 95% CI: −0.30 to −0.14) [53], which is in line with studies in Israel, Iran, and France [46,92,102,103]. NHANES indicated that higher PHD adherence was associated with lower GHG emissions (P_trend < 0.001) [104]. A comparative study of dietary patterns in Italy revealed that PHD produced a smaller carbon footprint than the Mediterranean diet (P < 0.05) [105]. Cross-continental studies (Europe, North America, Asia) showed that high adherence to PHD reduced GHG emissions by 2.4%–56% [16,58,78,86,94]. In the catering of American college students, using PHD as a purchasing standard reduced GHG emissions by 46.1% while significantly improving nutritional quality [106]. In addition, if all countries and regions adopted PHD, there would be a net reduction in GHG emissions of 17%–23% [107,108]. Among them, in high-income countries, PHD reduced per capita GHG emissions by 10% [107]. However, the implementation of PHD in low- and middle-income economies was associated with a rise in per-capita GHG emissions of over 10% [107], mirroring the findings of the investigation into the Chinese cohort [84]. PHD scores were positively associated with GHG emissions in China (P < 0.001) [84].

Although most of the aforementioned studies indicated that PHD significantly reduced GHG emissions, notable regional heterogeneity existed (Fig. 4). High-income countries typically demonstrated emission reductions, whereas some low- and middle-income countries experienced increased emissions. This inconsistency may stem from multiple factors, including socioeconomic contexts, agricultural structures, and methodological variations [109]. In high-income countries, where animal-based foods constitute a substantial proportion of current diets, transitioning to PHD could substantially reduce ruminant meat consumption, thereby lowering GHG emissions [110]. Conversely, in low- and middle-income countries, baseline diets may already be predominantly plant-based. However, PHD necessitates increased intake of fruits, nuts, and vegetables, whose production may rely on high-carbon energy sources or cold-chain logistics, potentially elevating GHG footprints [111]. It is noteworthy that some studies within the same country report inconsistent findings (e.g., China). This may stem from discrepancies in GHG accounting scopes. Some studies focused solely on the agricultural phase [92] and excluded emissions from food transport, processing, and waste disposal, which potentially yielded markedly different results. Furthermore, the quality of dietary data relied upon directly influences study outcomes. The validity of FFQ in assessing vegetable and fish consumption may be poor [100], potentially introducing bias into carbon footprint estimates. The environmental benefits of PHD are typically derived by comparison with current average diets, differing “control diets” set by studies yield varying emission-reduction magnitudes. Moreover, substantial socio-economic and agro-geographical disparities may exist within the same region. Entirely different agricultural models may be employed across distinct areas of a single country. In regions extensively adopting precision agriculture [112] and drip irrigation techniques, the water and carbon footprints required to produce PHD-recommended fruits and vegetables are lower. Conversely, in areas reliant on traditional flood irrigation and coal-dominated energy structures, the environmental costs of producing the same foods are significantly higher, potentially leading to increased emissions from PHD. The agricultural suitability of the PHD-recommended food combinations varies across regions. In areas well-suited for growing nuts and fruits, the environmental impact is positive. However, in regions requiring substantial resource inputs to cultivate these crops, the environmental impact may become negative. In summary, achieving global GHG emission reductions necessitates differentiated strategies. High-income countries should focus on reducing red meat consumption, while low-income countries need to invest in low-carbon agricultural infrastructure and localised food systems to minimise the emission costs of the PHD.

Interestingly, higher all-cause mortality corresponded with elevated GHG emissions (HR = 1.13, 95% CI: 1.10–1.16), with the strongest association observed for mortality due to CVDs (HR = 1.19, 95% CI: 1.10–1.28) [58]. It is suggested that PHD may indirectly reduce the burden of disease by reducing carbon emissions from the healthcare system.

3.4. CED

There is a growing scientific consensus that diets emphasizing plant-derived foods have a positive environmental impact [113]. Currently, the world is facing a serious energy crisis [114]. One study showed that a higher PHD index reduced CED by 31% compared to a lower PHD index [94]. This may be mainly attributed to PHD cuts in energy-intensive animal husbandry and resource-consuming food processing. In addition, PHD may reduce energy consumption along the healthcare supply chain by decreasing the incidence of diet-related diseases and reducing the demand for healthcare resources. Nevertheless, realizing the full energy-saving potential of PHD depends on locally tailored implementation strategies and supportive policy incentives.

3.5. Nitrogen and phosphorus cycle

Our food choices quietly influence the future of our planet. As a green diet, the far-reaching effects of PHD on the nitrogen and phosphorus cycle still need to be comprehensively examined. A 15.1-year follow-up cohort (n = 35,496) in the Netherlands found that high adherence to PHD reduced freshwater eutrophication by 0.5% (95% CI: −2.6 to −1.6), seawater eutrophication by 3.3% (95% CI: −5.8 to −0.8), and terrestrial acidification by 7.7% (95% CI: −10.8 to −4.6)¹⁶. Eutrophication refers to water pollution caused by excessive levels of plant nutrients such as nitrogen and phosphorus [115]. This phenomenon is closely linked to the use of agricultural chemicals, especially fertilisers. In the U.S., the adoption of PHD reduced the demand for fertilizer by 21% [78], which may mainly stem from a reduction in the scale of cultivation of feed crops such as soybeans and corn. In addition, increased consumption of organic plant foods in PHD may further stabilize the natural nitrogen and phosphorus balance, while protecting the environment from chemical products.

4. The challenges to the PHD

The PHD aims to balance human health with the sustainability of Earth’s ecosystems. However, its global promotion faces numerous and multifaceted challenges. These challenges encompass not only individual behavioral changes but also systemic barriers across multiple levels, including environmental, economic, technological, and cultural factors (Fig. 5).

Fig. 5.

The challenges to the Planetary Health Diet.

4.1. Environmental constraints

4.1.1. Resource-intensive agriculture

Livestock currently occupies 77% of global agricultural land while contributing only 18% of global calories intake [116]. Regarding GHG emissions, eutrophication, acidification, and land utilization intensity, the effect of the least-impact livestock products outweighs the median impact of alternative plant proteins [116]. A PHD that encourages the use of plant-based meals while reducing unhealthy food intakes like red meat. However, its implementation remains challenging in some regions. In Kenya, for instance, approximately one-third of the land is classified as arid and semi-arid, and most herdsmen rely on livestock for livelihoods and nutrition. Similarly, in some African countries, malnutrition and micronutrient deficiencies result in health problems, and higher intake of animal-sourced foods could benefit nutritionally [117].

In addition, soil degradation caused by the misuse of chemical fertilizers is of equal concern as water pollution. Over the last twenty years, the worldwide yield of major crops has climbed by 56%, while pesticide use has risen by 70% [44]. In 2022, the Americas were responsible for half of the pesticides used globally. The total amount of inorganic fertilizer used in agriculture amounted to 185 million tons, of which 58% was nitrogen, an increase of 37% since 2000 [44]. The overuse of chemical fertilizers accelerates soil acidification and decreases soil fertility and carbon storage [118]. Furthermore, the misuse of chemical fertilizers also pollutes water bodies. To mitigate soil salinization, soil leaching is often employed, which involves excessive irrigation. However, this practice leads to pesticide contamination of water supplies. In regions of intensive agricultural production in Mexico, 24% of irrigation water was utilized for crops, while 60% was discharged into the Gulf of Mexico via runoff from the drainage system, causing significant water pollution [119].

4.1.2. Climate change

Climate change is among the most important global challenges, exerting huge effects on agriculture and substantial threats to the availability of PHD. Between 1961 and 2020, climate change impeded the growth of total factor productivity (TFP) in agriculture, resulting in a cumulative reduction of global agricultural TFP by 21% [120]. This effect is more pronounced in low-income and food-insecure regions, such as Africa, where climate change has caused a 34% decline in agricultural TFP [120]. The influence of average temperature and precipitation on agricultural TFP has been increasing. By 2050, climate change could reduce global staple crop yields by 10%–25% [121].

4.1.3. Loss of biodiversity

Biodiversity is the cornerstone of human well-being and is crucial for maintaining ecological balance [122]. The loss of biodiversity can affect billions of people worldwide by impacting ecosystem services [122]. Large-scale monocropping is a prevalent practice in modern agriculture; however, this model has led to significant harm to biodiversity over time, including reduced genetic diversity, increased crop vulnerability to pests and diseases, and increased reliance on chemical pesticides. A meta-analysis comprising 67 effect sizes indicates that genetic diversity serves as a robust defense against infection [123]. Furthermore, monocropping disrupts ecological balance by diminishing both the variety and abundance of wildlife in agricultural areas. Large-scale monocropping of crops, such as soybeans, has caused severe ecosystem degradation. According to the Food and Agriculture Organization of the United Nations (FAO), 75% of global food production relies on merely 12 plant and 5 animal species. This highly concentrated pattern of crop cultivation exacerbates biodiversity loss.

4.1.4. Waste and loss of food

Globally, almost one-third of all food is lost or thrown away, which harms the environment and food security [124]. Food loss and waste not only use up vital resources such as water, land, and energy but also cause more damage to the environment when they are thrown away in landfills and break down [124]. According to research, food loss and waste account for 8%−10% of worldwide GHG emissions, almost half of all emissions from the food system [124]. Food waste also constitutes the biggest part of landfills, accounting for 22% of urban solid waste, worsening the environment [125]. Food loss and waste not only have negative effects on the environment but also threaten food security. For instance, fresh fruits and vegetables, which contain lots of sugar and moisture, are more likely to go bad than other foods [126]. Additionally, the large amount of food that homes and restaurants throw away could have helped with hunger but instead it increases food shortages.

4.2. Imbalances in regional development

The global promotion of healthy diets depends not only on changing food production and consumption patterns, but also on improving food cost and acceptance across regions [127]. Due to disparities in economic capacity, resource distribution, and developmental stages, implementing healthy diets worldwide remains a complex challenge.

4.2.1. Inequalities in economic capacity and access to food

Unequal access to food worldwide poses a significant difficulty to achieving healthy diets generally. Nutritious foods, including whole grains, vegetables, and plant-based proteins, are often inaccessible to low-income populations due to high production and distribution costs in comparison to current diets dominated by starchy staples [128]. Globally, around 3 billion individuals cannot afford a nutritious diet for economic reasons [128]. This inequality makes regions distinct from each other. People in developed countries and regions usually have enough money and strong food supply chains, which means they can get abundant healthy foods, including organic fruits and vegetables, plant-based foods, and minimally processed items [129]. In contrast, many developing countries and low-income regions have limited economic capacity, forcing them to rely on cheap, nutritionally unbalanced foods, such as processed and ultra-processed products. This not only contributes to rising rates of obesity and chronic diseases but also aggravates malnutrition and hinders the global adoption of healthy dietary practices [130]. Meanwhile, the EAT-Lancet 2.0 report also places significant emphasis on equity and justice for the first time, positioning healthy diets as a human right and a shared responsibility [9]. It advocates for the establishment of universal access to healthy diets, clean environments, and fair work for all [9].

4.2.2. Agricultural production patterns and economic stability

Significant variations in agricultural production patterns across regions can directly influence the production and accessibility of healthy food [131]. Developed regions often use modern agricultural and sustainable production technologies, such as organic farming and precision agriculture [131]. These approaches not only enhance production efficiency but also reduce environmental impacts [131]. In contrast, economically underdeveloped regions often depend on conventional farming practices, which are highly susceptible to climatic and environmental changes [132]. Frequent extreme weather occurrences, including droughts and floods, can reduce crop yields and cause economic instability [120]. These challenges not only limit individual food choices and accessibility but also hinder the global promotion of healthy eating practices.

4.2.3. Imbalance in policy and economic incentives

Policy and economic incentives are very important for encouraging healthy eating habits. Through policy reform and economic incentives, healthy food can be made more available and less expensive. Measures such as subsidies for organic farming, tax regulations on unhealthy foods, and advertising nutritious foods can reduce the cost of healthy options and make them more competitive in the market [133]. There are large differences in policy support and market incentives between regions. In economically underdeveloped areas, limited resources and inadequate policy support make it hard to create and use effective incentives for producing and consuming healthy foods.

In conclusion, the promotion of the PHD must consider economic capacity, agricultural production patterns, policy, and economic incentives to establish sustainable food systems and promote healthy dietary habits.

4.3. Restrictions on technology and innovation

The PHD can be based entirely on existing foods, and many traditional healthy diets, such as the Mediterranean Diet, should be preserved and protected where possible. Unfortunately, most countries have moved away from these nutritious dietary traditions. While many people find it challenging to transition to a more plant-based diet, innovative foods that replace meat and dairy could play an important part in developing healthy and sustainable food systems. Despite significant advances in food technology, including production, processing, and distribution, major barriers remain. High costs limit accessibility, ecological and ethical concerns remain, as do issues of imbalance and sustainability that continue to impede widespread adoption.

4.3.1. High costs and limited accessibility

Many advanced food production technologies, such as vertical farming, cell-cultured meat, and plant-based food innovations, hold significant potential for reducing environmental impacts. However, their high costs hinder widespread adoption. For instance, vertical farming demands substantial infrastructure investment and technical maintenance, resulting in prices significantly higher than traditional agricultural products, making them less accessible to average consumers [134]. Similarly, the production of cell-cultured meat relies on complex biotechnology and high research and development costs, rendering it uncompetitive with conventional meat in the short term and limiting its market penetration [135]. In contrast, plant-based foods consume fewer natural resources and exert less environmental pressure compared to animal products. Yet, enhancing their taste and nutritional value requires advanced technologies, which also drives up costs. Despite these challenges, technological advancements and growing consumer demand for sustainable foods suggest that plant-based foods retain substantial market potential [135].

4.3.2. Ecological and ethical controversies

The implementation of technological innovations in the food sector is frequently accompanied by ecological and ethical controversies, including gene-editing, synthetic biology and nanotechnology, and artificial intelligence and automation technologies. Gene-editing technologies, including CRISPR, have the potential to increase disease resistance and yield in crops. However, their potential impact on ecosystems has not been fully scientifically validated. The introduction of gene-edited crops may disrupt local biodiversity and ecological balance [136]. Similarly, the application of synthetic biology and nanotechnology in food processing can create novel food products and enhance nutritional value, but it may also introduce new environmental pollutants [137]. The lasting impacts of these pollutants are poorly understood and can pose potential threats to water bodies, soils, and biodiversity. Furthermore, the application of artificial intelligence and automation technology in the food supply chain, which can greatly improve efficiency, has also raised concerns about job loss, food safety controls, and data privacy [138,139]. These issues highlight the need for a strong regulatory framework to address ecological and ethical challenges.

4.3.3. Imbalance and sustainability challenges

The global application of technological innovation is highly uneven, with developed countries and regions enjoying greater access to advanced technologies and financial support, while developing countries and economically disadvantaged areas have difficulty applying these technologies due to inadequate infrastructure and insufficient funds [132]. Furthermore, some technological innovations are designed to mitigate environmental impact, but may introduce sustainability challenges. For instance, the production of plant-based foods relies heavily on specific raw materials, such as soybeans and peas, whose cultivation often requires extensive land and water resources, potentially leading to over-exploitation. Additionally, new food processing technologies may produce environmental pollutants and pose a potential threat to the ecosystem.

4.4. Cultural and traditional resistance

Globally, people are increasingly recognizing the influence of PHD on human health and environmental sustainability [140]. However, promoting PHD faces significant cultural and traditional barriers, including deep-rooted dietary preferences, religious and ethical restrictions, and the two-way influence of intergenerational transmission [141]. Consequently, altering eating habits is not only a technical challenge but also a profound sociocultural transformation.

4.4.1. Deep-rooted dietary preferences

Dietary cultures across different regions are deeply rooted in traditional foods, such as white rice in Asia and red meat and dairy products in Europe and North America [142]. These dietary habits, shaped by long-term cultural evolution, require not only time and patience to change but also sustained educational efforts and cultural adaptation [143].

4.4.2. Religious and ethical constraints

Religious beliefs significantly influence dietary choices. For instance, Hinduism prohibits beef consumption, while Islam forbids pork. Such religious dietary restrictions limit the food choices available to certain populations. Additionally, promoting vegetarianism or veganism may cause ethical controversies, particularly concerning animal rights and ecological balance, as individuals from diverse cultural backgrounds often hold differing ethical and cultural perspectives [144].

4.4.3. The two-way influence of intergenerational transmission

Family cooking traditions serve as a means of transmitting healthy and sustainable dietary practices across generations, yet they may also constrain openness to novel dietary patterns due to entrenched eating habits. Research demonstrated that adolescents’ fruit and vegetable intake is positively related to the availability of nutritious foods at home [145]. Furthermore, positive family dietary habits have lasting benefits, improving dietary quality and health outcomes across generations [146]. Conversely, the persistence of traditional home cooking practices may foster dietary inflexibility, reducing younger generations’ openness to alternative eating patterns. A study involving second-generation Americans and international university students found that food is deeply tied to cultural identity and well-being [147]. Dietary habits evoke food-related memories, reinforcing connections to familial and cultural heritage while sustaining traditions, customs, and norms. This dual nature of intergenerational transmission suggests that promoting PHD requires integrating beneficial aspects of traditional food cultures with contemporary dietary practices.

5. Sustainable development strategies for PHD

The sustainable development of a PHD is a global challenge that requires multifaceted efforts and innovations. This section examines strategies to achieve sustainable development of a PHD, focusing on innovative food system transformations, equitable food access, food system interventions, and the transformations of dietary behaviors (Fig. 6).

Fig. 6.

Sustainable development strategies for Planetary Health Diet.

5.1. Innovative food system transformation

5.1.1. Sustainable agriculture

With a rising world population and increasing resource demand, the high emissions and low efficiency of some traditional agricultural models can no longer meet the needs of sustainable development. Sustainable production models such as ecological, precision, and regenerative agriculture have emerged [148]. Ecological agriculture can protect natural resources and ecosystems while minimizing agriculture’s detrimental influence on the environment. For example, cover crops and no-till agriculture can benefit agricultural systems by controlling weeds and managing soil microbial diversity [149]. Precision agriculture uses modern information technologies, including GPS and sensors, to monitor soil, crop, and climate conditions in real time, improving production efficiency and reducing environmental footprint. For example, smart irrigation and drone monitoring can optimize the use of water, fertilizers, and pesticides, and reduce waste and environmental pollution [112]. Regenerative agriculture increases agricultural output while enhancing environmental resilience by improving soil health, biodiversity, and the water cycle. Studies have shown that agricultural profits from reclaimed fields are 78% higher compared to conventional production systems [150].

5.1.2. Reducing food loss and waste

Food loss occurs during production, harvesting, storage, transportation, and processing, whereas food waste primarily arises in retail sectors, food services, and households [151]. Losses during every step of the supply chain have cascading effects on subsequent stages. At the production stage, selecting seeds with longer shelf lives or implementing planned breeding to reduce animal diseases can mitigate food loss and waste. During harvesting, optimizing timing and closely monitoring temperature and humidity minimizes mold growth and spoilage. Processing-stage interventions, such as drying and irradiation, extend preservation time, while cold chain transportation from harvest to retail reduces spoilage. Additionally, improving logistical infrastructure can further curb transportation-related losses [152]. Crucially, end-consumer interventions, such as promoting moderate purchasing and scientifically informed storage practices, are critical in reducing household food waste.

5.1.3. Food technology innovation

Food technology innovation is critical to altering food systems. A key challenge in achieving the Sustainable Development Goals across many regions is the transformation of unsustainable consumption patterns. This challenge is further compounded by systemic pressures, including global population growth, escalating resource constraints, and the adverse effects of climate change. Through technological innovation, we can improve food production efficiency, reduce environmental impact, and provide healthier dietary options for consumers. Current innovations in the food industry include precision fermentation technology, novel food processing and packaging methods, as well as vertical and urban agriculture.

Precision fermentation technology is a process that uses microbes, such as yeast, bacteria, or fungi, to efficiently produce specific food ingredients or substances via fermentation [153]. Biomass-fermented meat products offer protein and nutrient content comparable to traditional meat products and can be modified to mimic meat textures [154]. The efficiency of precision fermentation to produce specific animal proteins is far greater than traditional animal husbandry [155]. This results in a 10- to 100-fold reduction in land use, a 10-fold decrease in water use, and a 20-fold increase in production efficiency [155,156]. These estimates primarily reflect direct use of land and water resources. For instance, microbial cultivation avoids the substantial land resources required for animal grazing or feed crops, excluding upstream raw-material production stages.

Innovative food processing technologies, including high-pressure processing and pulsed electric fields, can extend shelf life and reduce food waste [157,158]. Additionally, sustainable packaging materials, including biodegradable plastics and plant-based packaging, offer environmentally friendly alternatives to traditional plastics, reducing packaging waste pollution [159].

Vertical and urban agriculture represent innovative agricultural practices that aim to cultivate fresh vegetables and fruits in indoor environments, utilizing limited spatial resources through multi-layered planting and hydroponics [160,161]. This model not only reduces transportation distances and carbon emissions but also provides urban residents with a localized, sustainable food supply [160,161].

While recognizing the potential of food technology innovation, it must be noted that its adoption and application face significant accessibility gaps and equity challenges around the world, especially in low- and middle-income nations. Many innovative technologies require substantial upfront investments, ongoing maintenance costs, and advanced infrastructure [132]. The conditions are often hard to attain in resource-constrained low- and middle-income nations. Furthermore, complex technologies and skills training gaps may exacerbate inequalities in technology access. Therefore, future efforts in these nations must prioritize developing and scaling low-cost and easy-to-maintain technologies. A greater focus on local crops, tailored technologies suited to regional conditions, and robust policy support are essential to achieve a truly global food transformation.

5.1.4. Digitalization and intelligent management

Digital and intelligent management play an important part in enhancing the PHD system, offering innovative approaches to optimize the industrial chain from farm to table. While traditional food systems can be sustainable, circular, and healthy in certain contexts, the integration of advanced technologies provides valuable tools to address specific challenges and improve efficiency.

In food production, the integration of sensors and Internet of Things technologies allows farmers to monitor soil moisture, nutrient content, and climate conditions in real time [162]. On the basis of data processing and analysis, irrigation and fertilization can be optimized, thereby enhancing crop yield and quality while minimizing water and fertilizer waste [163]. Furthermore, intelligent management systems facilitate the remote monitoring and operation of farm equipment, boosting production efficiency. For instance, automated planters and harvesters can operate according to predefined procedures, reducing manual errors and labor intensity [164].

Digital and intelligent management of the food supply chain includes blockchain technology and intelligent logistics distribution. Blockchain ensures full traceability of food products from farm to table, allowing consumers to access information on origin, processing, and transportation, reducing food losses and recalls [165]. Moreover, artificial intelligence algorithms optimize food logistics by analyzing inventory levels and traffic data, enabling efficient route planning and reducing transportation costs and losses [166].

Digitization and smart management promote healthy dietary choices among consumers. Through smartphone online platforms, consumers receive personalized dietary recommendations, encouraging healthier food choices [167]. Simultaneously, the intelligent retail systems analyze consumer purchasing behavior using big data, enabling adjustments in product offerings and marketing strategies to further drive the trend toward healthy eating [168].

5.2. Universal access to food

Universal food access policies are critical measures for ensuring that all individuals have access to adequate, nutritious, and affordable food. This policy framework covers various aspects, particularly the provision of essential food support to vulnerable groups, including poverty reduction and income support, food bank policies, and school feeding programs. In addition, attention should be paid to neglected and underutilized species in certain regions to solve the local food problems.

5.2.1. Poverty reduction and income support

A PHD extends beyond food production and selection; its core objective is to ensure that all people, especially the poorest in the world, can consistently access affordable and nutritionally adequate food. Poverty remains a fundamental barrier to healthy eating, constraining both purchasing power and dietary choices. Consequently, effective poverty alleviation and income-support policies constitute essential foundations for achieving equitable food access and PHD.

Cash transfer programs provide direct financial support to low-income households and serve as an immediate mechanism for poverty and hunger reduction [169]. Over the past two decades, Brazil’s cash transfer initiatives, while not significantly improving household nutritional outcomes, enhanced access to food and primary healthcare services [169]. Similarly, the programs in the U.S. demonstrably mitigated poverty’s adverse effects on child health [170].

Unemployment assistance similarly plays a critical role in safeguarding vulnerable populations, as evidenced during the COVID-19 pandemic. Spain, among the hardest-hit nations, faced severe economic contraction and widespread business closures, with unemployment surging to 3 million in 2020. Unemployment benefits emerged as a vital income source and a risk-mitigation tool for affected individuals [171].

Furthermore, enhancing the self-employment capacity of the poor is also a crucial aspect of poverty reduction efforts. Key interventions include providing skill training for the unemployed to create job opportunities as well as agricultural support programs, such as seed and fertilizer provision, cultivation training, and market access facilitation, to boost farm productivity and incomes.

5.2.2. Food bank policies and school feeding program

Food banks serve as a mechanism for redistributing surplus food to low-income and vulnerable populations. By collecting unsold or donated food, food banks distribute it to individuals and families in need. In the U.S., about 200 food banks are affiliated with the national group Feeding America, providing food to more than 40 million people annually via 49,000 food pantries [172]. As society has developed, some food banks have introduced visible nutritional ratings on their platforms in the U.S. to ensure access to nutritious and healthy food [173]. Green represents the healthiest foods, while red indicates the least healthy options. A review of the ordering records of the network’s 25 largest food banks indicated a rise in the number of green-rated products (39.3%–45.4%) and a drop in red-rated items (10.5%–5.1%) [173].

The PHD highlights the advantages of plant-based nutrition for both human health and environmental sustainability. However, adjustments are necessary to accommodate the elevated energy and micronutrient needs of children and adolescents, ensuring optimal nutritional adequacy [174]. Furthermore, the PHD promotes educational interventions in which schools and community-based programs raise awareness among families regarding the interplay between dietary choices, individual well-being, and planetary sustainability [174]. School feeding programs are a crucial measure to ensure that children receive a healthy and balanced diet, enhancing student nutrition and academic performance. As of 2018, 117 countries reported implementing such programs, with approximately 370 million youngsters receiving school lunches daily [175]. A cost-benefit analysis of 14 countries’ school feeding programs revealed an annual return on human capital of $180 billion, with $24 billion attributed to health and nutrition benefits and $156 billion to educational benefits [176]. Low-income nations have lower coverage rates for school feeding programs, where the need is most acute. Although the cost of school feeding programs in these regions is higher, the overall benefit ratio remains between 7 and 35 [176].

5.2.3. Neglected and underutilized species

Neglected and Underutilized Species (NUS) are plant species with substantial nutritional, economic, ecological, or cultural value that have been historically marginalized by mainstream agricultural systems, research institutions, and markets [177]. These species offer considerable potential for enhancing dietary diversity and improving health outcomes [177]. In sub-Saharan Africa, where diets predominantly rely on staple grains and root crops, many populations experience insufficient intake of protein, vitamins, calcium, and iron [178]. NUS can supply these missing nutrients, thereby alleviating hidden hunger [178]. For instance, minor cereals such as sorghum and finger millet, which are widely cultivated in sub-Saharan Africa, are rich in protein (8−13 g per 100 g), while Adansonia digitata is a valuable source of fiber, vitamins, and calcium [178].

Furthermore, NUS can enhance local biodiversity to address food security, soil erosion, and environmental degradation challenges [177]. NUS are often environmentally adaptive species, such as drought-tolerant Sorghum bicolor, which requires less water and soil nutrients than maize and rice [177]. They maintain yields under extreme climatic conditions, mitigating nutritional crises caused by reduced staple crop production. Additionally, NUS possesses significant genetic diversity, which facilitates the development of region-specific varieties through targeted breeding programs [179]. Such efforts can optimize nutrient content to better meet the dietary needs of local populations.

5.3. Food system interventions

To achieve PHD, the food system requires comprehensive interventions. These measures, including taxation, subsidies, and regulatory frameworks, are pivotal in promoting healthier dietary choices, reducing environmental impacts, and enhancing the sustainability of food systems.

5.3.1. Economic regulation

Differentiated taxation and targeted subsidies are important measures of the economic regulation system. Differentiated taxation serves as a primary policy instrument for food system regulation, seeking to achieve the dual goals of health improvement and environmental sustainability through variable tax rates. For instance, a 10% tax on sugary beverages reduces average consumption by 10% [180]; additionally, a $0.01-per-ounce tax on sugary drinks can also reduce adolescent BMI by 0.16 units [181]. However, such taxes may burden low-income households, necessitating complementary measures to ensure the affordability of nutritious options.

Precision subsidies represent a transformative lever for food systems. Government subsidies for healthy foods and sustainable agricultural practices can effectively increase the consumption of nutritious options and promote agricultural sustainability. For example, low-income individuals who participated in the Supplemental Nutrition Assistance Program (SNAP) and gained access to food through vouchers exhibited slower cognitive decline compared to non-participants [182]. Subsidies for sustainable agriculture encourage the use of eco-friendly methods, including organic farming, crop rotation, conservation tillage, and grassland management, reducing dependency on chemical inputs and enhancing lasting agricultural output [183]. The European Union’s Common Agricultural Policy (CAP) provides a range of subsidies and funding programs to support sustainable technologies in agriculture (such as organic farming, crop rotation, conservation tillage, and grassland management) [184].

5.3.2. Standardized regulatory system

The development and enforcement of stringent food labeling regulations and production standards are essential to empower consumers to make informed, healthy choices, ensure food safety and quality, and reduce environmental impacts. While existing food labeling regulations are prevalent, new regulations have been introduced to address the growing need to safeguard consumer health and promote sustainable development. For instance, in 2025, China’s National Health Commission issued the “General Rules for Nutrition Labeling of Pre-Packaged Food under National Food Safety Standards”, requiring manufacturers to provide more comprehensive nutritional information on labels [185]. In 2022, Health Canada amended the Food and Drug Regulations, requiring front-of-package (FOP) labeling for pre-packaged foods exceeding standard levels of sodium, sugar, and saturated fats [186]. These regulatory advancements enable consumers to make healthier dietary choices by offering more detailed and intuitive food information.

In addition to labeling, countries and international organizations have introduced rigorous food production and marketing standards to enhance food quality, safety, and environmental sustainability. For instance, the Japanese government launched a Food Traceability System in 2019 to increase transparency and safety in the food supply chain, thereby bolstering consumer confidence [187]. In 2024, China’s National Health Commission issued 47 new national food safety standards to improve food quality and safeguard consumer health [188]. These standards, supported by stringent regulations and advanced technologies, contribute to improved food quality, safety, and transparency, thereby advancing the sustainable development of the food industry.

5.3.3. Regulation and supervision of new food technologies

To advance new food production technologies, governments and enterprises should increase investment in research and development in innovative methods such as vertical farming, cell-cultured foods, and plant-based meat alternatives, while also focusing on reducing associated costs [189]. Encouraging enterprises to scale up production and strengthen upstream and downstream collaboration within the industrial chain can further enhance production efficiency and lower overall costs [190]. Governments can support these efforts by implementing policies such as tax incentives and financial subsidies for enterprises.

There must also be strong regulatory and ethical review systems in place for technological innovation in the food industry. For instance, gene-edited crops and novel food products should be thoroughly tested for safety and ethics to ensure they meet ecological and ethical standards [191]. Similarly, the use of AI and automation in the food supply chain needs rules to protect data privacy and food safety. International cooperation and exchanges to create uniform regulatory standards and ethical guidelines are also very important.

In summary, interventions such as taxes, subsidies, and regulations play critical roles in promoting a healthier and more sustainable global food system. These steps can help people choose healthier foods, protect the environment, and make food production more sustainable. However, their successful implementation depends on scientifically informed policy design, effective execution, and widespread public support. Governments, businesses, and consumers must collaborate to encourage the advancement of a sustainable food system, laying a solid foundation for future global sustainability.

5.4. Transformations of dietary behaviors

5.4.1. Economically driven changes in individual diets

A variety of factors influence individual eating choices, including not only food availability but also personal and familial preferences, health considerations, and convenience [131]. Even under unfavorable circumstances, people will exert their subjective initiative in food selection as much as possible according to their own culture and preferences. To facilitate dietary shifts, innovative market mechanisms and enhanced consumer awareness are crucial. Traditional economic tools, such as taxes and subsidies, often rely excessively on price signals and overlook irrational elements in consumer decision-making. Behavioral economic theory offers a more nuanced approach by employing “nudge” strategies to optimize policy effectiveness [192]. For instance, giving priority to plant-based foods or local produce in prominent positions on supermarket shelves can leverage default effects to increase purchase rates. Additionally, labelling food items with metrics such as “carbon footprint per meal” or “health benefits value” can reshape consumers’ cognitive frameworks regarding food pricing [193,194]. Digital technologies further enhance economy-driven strategies by providing precision tools. Integrating personal health data, such as metabolic markers monitored by wearable devices, with consumption records enables differentiated pricing for unhealthy foods like high-sugar products [195]. Moreover, based on individual metabolic profiles and budgetary constraints, an AI nutritional advisor can generate a customized shopping list [196].

5.4.2. Intervention-facilitated dietary modifications

Despite the deep-rooted nature of traditional dietary cultures, evidence demonstrates that diversified strategies can effectively drive transformation, including community-based interventions, living labs, and societal-wide guidance. For example, economic incentives combined with education have proven effective at the community level: an Australian intervention increased fruit and vegetable purchases by 12.7% through temporary store discounts, with effects persisting 24 weeks post-intervention [197]. Similarly, Canada’s Healthy Foods North program employed a 12-month integrated strategy—including healthy breakfasts, meal planning, cooking classes, and nutrition education—which significantly reduced consumption of high-fat meats, sugary beverages, and refined grains while promoting healthier cooking practices [198]. These results highlight the efficacy of community-based education and interventions in reshaping dietary behaviors.

Participatory models, such as Japan’s Citizen Health Living Laboratory, further demonstrate success by engaging individuals in self-monitoring, dietary tracking, and collaborative health planning [199]. This real-world, workshop-based approach helps overcome habitual resistance and sustains long-term healthy eating habits. Additionally, societal-level guidance, including shaping market environments, ongoing education, and cultural steering, can collectively enhance consumer awareness and improve food choices. For instance, long-term social guidance has led to a 96% increase in the consumption of plant-based alternative foods in the UK from 2008 to 2019, which also reflects their growing appeal [200]. At the same time, this transformation has also led to a reduction in risks related to CVDs and metabolic disorders [201].

Collectively, these strategies, spanning community interventions, living labs, and societal-wide guidance, provide actionable frameworks for overcoming cultural dietary barriers and fostering sustainable dietary shifts.

6. Conclusions

This study systematically evaluated the dual benefits of PHD in terms of human wellness and sustainability in the environment. PHD can significantly reduce the risk of chronic diseases such as CVDs, diabetes, obesity, and cancer, and play a certain role in environmental problems such as agricultural land pressure, GHG emissions, water consumption, and nitrogen and phosphorus cycling. However, the global implementation of PHD still faces constraints related to natural resource limitations, regional economic disparity, the increasingly severe food technological challenges, and cultural and intergenerational resistance. Based on the above, the proposed four-dimensional collaborative strategies—including the innovation of food production systems, equitable food distribution mechanisms, policy intervention toolkits, and behavioral economic incentives—provide practical pathways for sustainable food system transformation. Future research should focus on elucidating molecular mechanisms, conducting multi-center empirical analyses, and translating PHD from scientific consensus to global action by integrating scientific evidence, policy practice, and cultural perceptions. Ultimately, the goal of “health-environment” coordinated development will be achieved.

CRediT authorship contribution statement

Lan Jiang: Writing – review & editing, Writing – original draft, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization, Funding acquisition. Jia Wang: Writing – original draft, Validation, Software, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. Mengya Gao: Validation, Methodology, Data curation. Howard Bergman: Validation, Methodology, Investigation, Formal analysis, Conceptualization. Lei Cheng: Validation, Methodology, Investigation, Formal analysis, Conceptualization, Writing – review & editing. Yanan Liu: Validation, Methodology, Data curation. Hongmei Ji: Validation, Methodology, Investigation, Conceptualization, Writing – review & editing. Ru Yan: Validation, Methodology, Investigation, Conceptualization, Writing – review & editing. Vijaya Raghavan: Validation, Methodology, Investigation, Conceptualization, Writing – review & editing. Jin Wang: Writing – review & editing, Validation, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization.

Declaration of competing interest

The authors declare no conflicts of interest.

Appendix A. Supplementary data

The following are the Supplementary data to this article: