Greenspace and intestinal diseases: a systematic review and meta-analysis

Abstract

Greenspace has been reported potential therapeutically beneficial for intestinal diseases, yet the findings remain controversial Current researches demonstrate diversity in the assessment of greenspace and involve broad population groups, with the publication bias remaining a concern. This study offers convincing and reliable evidence supporting the beneficial effects of greenspace exposure on intestinal health, through a systematic review and meta-analysis examining the association between greenspace and intestinal disorders. The Greenspace exposure was primarily assessed using the Normalized Difference Vegetation Index (NDVI) and residential proximity to greenspaces. A comprehensive assessment was conducted concurrently by integrating various methods, such as land-use and satellite image data. This research encompassed a variety of intestinal conditions, such as inflammatory bowel disease (IBD), Crohn’s disease (CD), ulcerative colitis (UC), colorectal cancer (CRC), and diarrhea. A search was performed cross four English and two Chinese databases, yielding nine studies for systematic review and eight for meta-analysis from 11,497 identified records. The results illustrated a significant protective effect of greenspaces against CD and IBD as well as a potentially protective effect against diarrhea. Nonetheless, no significant association was detected for UC or CRC. The study established that greenspaces could influence intestinal health through potential mechanisms such as improved air quality, enhanced microbial diversity, and reduced stress. These findings provide a solid basis for developing healthier urban environments and advancing strategies for prevention and treatment of intestinal diseases.

Trial registration Registration number: CRD42024625968.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12889-025-23629-9.

Introduction

Greenspace refers to areas designated as natural zones or urban vegetation,encompassing parks, gardens, courtyards, urban forests, urban farms, and natural open spaces [43]. The positive impact of greenspace on human well-being has been recognized since the 1800 s [44]. Based on this consensus, urban construction policies that advocate for health and well-being have gradually developed. In 1986, the World Health Organization (WHO) launched the “Healthy Cities” program, which established urban development principles aimed at promoting health within cities [59]. The World Bank first introduced the concept of nature-based solutions (NBS) in 2000 [27], emphasizing the role of natural systems in addressing environmental and societal challenges, advocating for solutions that leverage natural systems to confront pressing global challenges, thereby benefiting both the environment and human society [42].

Numerous empirical studies demonstrated that greenspace positively impact human health by promoting cardiovascular health [10, 38], regulating pulmonary function [20, 52], enhancing cognitive function [4, 6], and reducing stress [37]. Despite these well-documented benefits, intestinal diseases have emerged as a considerable global health challenge. Loftus et al. highlighted the growing global prevalence and impact of inflammatory bowel disease (IBD), especially in regions such as Asia, Southern Europe, and numerous developing nations [26]. Colorectal cancer (CRC), the most common gastrointestinal malignancy [3], ranks as the second leading cause of cancer-related mortality worldwide, with approximately 1.9 million new cases and 900,000 deaths reported globally in 2020 [51]. According to the 2021 Global Burden of Disease (GBD) report, although the global burden of diarrheal diseases has improved, diarrhea remains the 14th leading cause of global mortality and the 11th in terms of disease burden [17]. Establishing a potential association between greenspace exposure and intestinal health could provide a novel and cost-effective preventive strategy against intestinal diseases and potentially reduce their prevalence. This would improve public health outcomes and mitigate substantial human and economic costs associated with the treatment of intestinal diseases.

The Potential association between greenspace and intestinal diseases may be mediated through biopsychosocial pathways, as proposed in previous theoretical studies, which include restoring and building capacities [30], as well as reducing harm. First, regarding the mitigation of harm, numerous studies indicated that inhalation of atmospheric particulate matter can alter gut microbiota composition, disrupt oxidative and inflammatory balance and damage intestinal epithelial permeability [38, 45]. Urban greenspaces have been shown to diminish environmental contaminants, including air particulates, thereby reducing their adverse effects on intestinal health. Second, with respect to restoring capacities, empirical evidence supports the role of greenspaces in alleviating anxiety and depressive behaviors [18, 46]. The gut-brain axis (GBA) theory posits that, the gut and brain are connected through a neural-immune-humoral signaling pathway termed the gut microbiota-brain axis, which involves components such as the vagus nerve and the immune system [41]. Inflammasomes related to neuroinflammation, anxiety, and depression manifestations in mice can influence gut microbiota community [40]. Furthermore, the “old friends hypothesis” and “biodiversity hypothesis” [39] suggest that exposure to diverse microbiota in natural environments can enhance immune system modulation [23]. The gastrointestinal tract has the biggest clusters of largest lymphoid tissue aggregates in the body and is highly susceptible to microbial immune dysregulation [29], leading to intestinal diseases. Consequently, the inhibitory effect of greenspaces on microbial immune dysregulation can be perceived as part of the building capacity mechanism.

Nevertheless, the absence of robust evidence linking greenspaces to intestinal diseases greatly hinders the understanding of the mechanisms through which greenspaces might influence health outcomes. An animal study revealed that mice exposed to high-biodiversity soil dust exhibited greater intestinal microbiota diversity than those exposed to low biodiversity and dust-free soil [24]. A cohort study conducted by [29] also found that childhood exposure to farm environments could reduce the risk of IBD. However, other studies have shown that the diversity of the gut microbiota in infants exposed to natural environments may be reduced [34]. These contradictory findings may stem from methodological constrains including limited sample sizes and insufficient regional and population diversity. Therefore, it is imperative to perform a meta-analysis and systematic review to explore the connection between greenspaces and intestinal diseases. This study aimed to thoroughly evaluate both observational and interventional evidence regarding the relationship between greenspaces and intestinal health outcomes, aiming to uncover potential mechanisms by which greenspaces may influence intestinal diseases. Such an understanding could provide novel approaches for the prevention and non-pharmacological adjunctive treatment of intestinal diseases, while also offering valuable insights for urban health planning. Moreover, it can guide policymakers in promoting sustainable development between humannity and nature, thereby enhancing public health and well-being.

Methods

This meta-analysis and systematic review were conducted in strict accordance with the Cochrane Handbook for Systematic Reviews of Interventions, ensuring methodological rigor. Additionally, the study adhered to the guidelines outlined in the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) [36]statement, which provides a structured framework for enhancing the transparency and reproducibility of systematic reviews and meta-analyses. "Endnote (X9) citation management software was employed to identify duplicates within the selected literature records and manage and screen them. This systematic review has been officially registered in the International Prospective Register of Systematic Reviews (PROSPERO), ensuring transparency and methodological (registration number: CRD42024625968).

Data sources

A comprehensive search was conducted across four English-language databases (PubMed, Web of Science, Embase, and Cochrane Library) and two Chinese databases (CNKI and Wanfang). The search period included the establishment of the database until November 2024. The publications were limited to those written in Chinese and English. Additionally, citation lists were manually retrieved from review articles on greenspaces and intestinal diseases to minimize potential omissions.

Search strategy

The search strategy was formulated by researchers after extensively reviewing relevant studies on greenspaces and human intestinal diseases, as well as existing systematic reviews [5, 22, 31, 55]. The strategy consisted of two main components: 1) professional terms related to greenspaces and 2) terms related to intestinal diseases. The selection of intestinal disease types was based on a section on lower digestive tract in the report on the burden of digestive system diseases in the United States [12] and was appropriately narrowed according to findings from the literature review. The search included terms such as “greenspace,” “greenness,” “green environment,” “natural area,” “outdoor,” “wild land,” along with terms for intestinal diseases such as “irritable bowel syndrome”, “colorectal neoplasm,” “intestinal polyps,” “ruptured appendicitis,” “Crohn’s disease,” and “inflammatory bowel disease” (Table S1). Intestinal diseases were classified according to the International Classification of Diseases, 10th Revision (ICD-10).

Study selection

Based on the Population, Exposure, Comparator, Outcome, and Study (PECOS) standards, specific criteria used are as follows: the population refers to the general public, irrespective of gender or age, while exposure refers to higher levels of greenspace contact, with greenspace exposure considered as an independent variable in the analysis and evaluated at an individual level; comparator refers to lower or no exposure to greenspace; outcomes refer to any health outcomes related to intestinal diseases; and study refers to human epidemiological research, encompassing both observational and intervention studies. A comprehensive outline of the inclusion and exclusion criteria is presented in Table 1.

Table 1.

Inclusion and exclusion criteria using PECOS framework

Item	Inclusion criteria	Exclusion Criteria
Population	The general population without distinction of age, gender, and underlying health conditions	Not conducted in humans
Exposure/Intervention	Greater exposure to greenspaces, including but not limited to green areas surrounding residences, urban greenspace, parks, forests, and natural environments Greenspace exposure is regarded as an independent variable in the analysis Conducting an assessment of the exposure to greenspace at the individual level	No assessment of greenspace exposure or specific studies on greenspace exposure for individuals has been conducted
Comparator	Low or no exposure to greenspaces	/
Outcomes	Evaluating the association between the incidence of the included types of intestinal diseases and greenspace exposure	No quantitative or qualitative description
Study	Observational and studies intervention studies	Case report review, in vitro and in vivo experimental research

Titles and abstracts of all studies were read and those that were clearly irrelevant to the research topic, such as animal experiments and toxicological studies, were excluded. Subsequently, the full text was independently reviewed and assessed based on the inclusion and exclusion criteria. Discrepancies were resolved through discussion among reviewers, with a third reviewer arbitrating if necessary.

Data extraction

Data was extracted following information from eligible studies: authors, publication year, experimental methods, total sample size (number of patients), age (and follow-up), greenspace exposure assessment, disease type, covariate adjustment, effect evaluation, and full-text screening results. Data extraction methods used in previous reviews were adopted [53] and Microsoft Excel was used to organize and compile the data into tables (Table 2). In instances where inconsistencies arose during the data extraction process, resolution was achieved through collaborative consultation with an independent third-party researcher to ensure methodological rigor and maintain the integrity of the research outcomes. This approach aligns with established best practices in empirical research, where inter-rater reliability is enhanced through triangulation of evaluative perspectives, thereby minimizing potential biases and enhancing the validity of the extracted data.

Table 2.

Characteristics of the included studies

Author/Year	Country	Study type	Total sample (people with disease)	Age	Greenspace exposure assessment		Disease type	Main outcome	Effect estimation
					Metrics	Radius
Agrawal/2024 [2]	Denmark	Cohort	1,438,487 (3,768)	Baseline:/Follow-up: 14 (Median age)	The proportion of land cover (forest, natural greenspace, or urban greenspace)	/	IBD	Early-life exposure to agricultural land use elevates the risk of CD, whereas biodiversity and greenspace exposure serve as protective factors against CD. In contrast, early-life environmental factors do not exhibit a significant influence on the risk of ulcerative colitis	HR: 0.84 (95%CI: 0.75, 0.95)
Odebeatu/2024 [35]	Britain	Cohort	44,393 (2,152)	Baseline: 55.6 ± 8.17 Follow-up:/	Percentage of total greenspace coverage, private residential gardens, and other types of greenspaces surrounding participants’ residential addresses	100 m, 300 m, 500 m, and 1000 m	CRC	Greater exposure to private residential gardens was associated with a lower risk of CRC, especially breast and uterine cancer	HR: 0.95 (95%CI: 0.92, 0.98)
Cao/2023 [7]	Britain	Cohort	401,189 (4917)	Baseline: 56.3 ± 8.1	Percentage of outdoor greenspace around each participant’s place of residence	300 m, 1000 m	CRC	Exposure to residential greenspace was inversely linked to prostate cancer risk	HR: 0.99 (95% CI: 0.94, 1.04)
Xinying Wu/2022 [50]	China	Cohort	917,450 (600)	Baseline: 39.40 ± 22.17 Follow-up:/	NDVI	1000 m	CRC	Higher NDVI values were associated with a reduced risk of overall cancer and several specific cancers	GLMM results: OR: 0.85 (95% CI: 0.84, 0.86) NCE-TS result: AR: −0.19% (95% CI: −0.21%, −0.16%)
Zhang/2022 [57]	Britain	Cohort	216,868 (1,271)	Baseline: 57.4 ± 8.0 Follow-up:/	Outdoor greenspace surrounding each participant’s residence	300 m, 1000 m	IBD	Increased exposure to residential greenspace, blue spaces, and natural environments was associated with a reduced risk of IBD, with more pronounced effects observed in socioeconomically deprived regions and among males for certain types of exposure	HR: 0.99 (95% CI: 0.97, 1.00)
Elten/2021 [11]	Canada	Cohort	2,715,318 maternal and infant pairs (3,444)	Baseline: 29.5 ± 5.8 (mother) Follow-up: 16.9 (child)	NDVI	250 m	IBD	More childhood greenspace exposure was linked to a lower risk of pediatric IBD, with no link for prenatal exposure	HR: 0.82 (95% CI: 0.77, 0.87)
Sakhvidi/2021 [56]	France	Cohort	19,408 (367)	Baseline: 43.74 ± 3.50 Follow-up: 71.05	NDVI	100 m, 300 m, 500 m, and 1000 m	CRC	Greater exposure to greenspace was linked to a higher risk of all-site and prostate cancer, while potentially offering protective benefits against breast, lung, and CRCs. These associations differed depending on the cancer type and specific greenspace category	HR: 0.99 (95% CI: 0.82, 1.19)
Datzmann/2018 [9]	Germany	Cohort	1918,449 (11,976)	Baseline: 49.3 ± 25.33 Follow-up: 75.04	NDVI	250 m	CRC	Higher PM10 increased risks of some cancers, a 10% NDVI increase protected against certain cancers, and CRC was unaffected	RR: 0.95 (95% CI: 0.87, 1.04)
Johnson/2013 [21]	Malawi	Cross-sectional	23,000 households	Range: 0–5 (children); 15–49 (mother)	Percentage of forest cover (VCF products, NDVI)	5,000 m	Diarrhea	Children residing in regions experiencing net forest loss tended to have poorer dietary quality, whereas those in areas with greater forest cover or gains showed improved diets and lower rates of diarrhea. However, this relationship is correlational and doesn’t imply causation	OR: 0.08

Abbreviation: IBD Inflammatory Bowel Disease, CD Crohn’s Disease, ORC Obesity-Related Cancer, CRC Colorectal cancer, VCF Vegetation Continuous Fields, HR Hazard Ratio, RR Risk Ratio, OR Odds Ratio, AR Attributable Risk, NDVI Normalized Difference Vegetation Index, PM₁₀ Particulate Matter 10

Quality evaluation

The methodological rigor and potential biases of the nine investigations included in this systematic review were rigorously appraised through the application of the Newcastle–Ottawa Scale (NOS). This well-established evaluation framework, which has gained substantial recognition within the scientific community, provides a comprehensive and standardized approach for quality assessment in non-randomized observational research [47]. The NOS primarily evaluates three key aspects: selection, comparability, and exposure/outcome, and consists of eight items. The evaluation system utilizes a semi-quantitative approach based on a star-based scoring mechanism, with specific weightings assigned to different assessment criteria. Notably, the comparability dimension is allocated a maximum of two stars due to its critical role in study design, while other quality indicators are each assigned one star. This structured scoring approach allows for systematic quality assessment while recognizing the relative importance of different methodological aspects in observational research. The resulting scores provide a standardized metric for evaluating and comparing study quality across the reviewed literature. The maximum possible score was nine, with higher scores indicating better study quality. Scores of 7–9, 4–6, and 0–3 signify high, medium, and low quality, respectively. The evaluations were initially performed by the first author and subsequently verified by another author to check for discrepancies. Any inconsistencies identified during the evaluation process were addressed through thorough discussion among the reviewers. If disagreements persisted, a third author was consulted for re-evaluation, ensuring that all differences were systematically analyzed and resolved. A final consensus was reached through deliberation, enhancing the reliability and objectivity of the study assessment.

Meta-analysis

In this meta-analysis, the incidence of intestinal diseases was classified as the main outcome of the effect size synthesis. Effect sizes related to intestinal diseases, such as relative risk (RR), odds ratio (OR), hazard ratio (HR), and their corresponding 95% confidence intervals (CIs), were extracted and combined from each study. The authors of the included studies were required to provide sufficient and accurate data for these effect sizes to ensure smooth progression of subsequent calculations. The meta-analysis was conducted using Stata 15 software (StataCorp LLC; College Station, TX, USA). To ensure consistency in selecting outcome indicators, we predefined a hierarchical decision protocol: 1. Highest greenspace exposure: When studies reported multiple exposure quantiles (e.g., quartiles, tertiles), we prioritized the highest quantile (e.g., Q4 for quartiles) as defined by the original study. If multiple definitions coexisted (e.g., NDVI-based vs. coverage-based metrics), NDVI-derived measures were selected due to their standardized scalability. 2.Smallest buffer zone: Among overlapping buffer zones (e.g., 100 m, 300 m, 500 m), the smallest reported buffer (e.g., 100 m) was chosen to represent immediate residential exposure. This protocol was applied uniformly across studies to minimize selection bias.

Before the analysis, the collected data were rigorously cleaned and preprocessed to ensure completeness and accuracy and meta-analysis proceeded after this process. A heterogeneity test was conducted to determine an appropriate model for effect size synthesis. According to the 2024 Cochrane Handbook for Systematic Reviews of Interventions, statistical heterogeneity was identified when the p-value was below the set threshold (usually 0.10). Subgroup analysis was employed to identify heterogeneity sources, which may include differences in geographical analysis units, measurement methods across studies, variations in the included populations, and factors not accounted for in the meta-analysis.

The assessment of heterogeneity was conducted through the I² statistical measure, with specific thresholds established for interpretation: 0–40% indicating minimal heterogeneity, 30–60% suggesting moderate levels, 50–90% representing substantial variation, and 70–100% denoting significant heterogeneity [19]. For studies demonstrating P-values exceeding 0.1 accompanied by I² values falling within the 0–40% range, effect sizes were combined using a fixed-effects approach. In all other cases, a random-effects framework was implemented. To evaluate the stability of the meta-analytic results, sensitivity assessments were conducted employing the sequential exclusion method, systematically removing individual studies. The potential for publication bias was investigated through visual inspection of funnel plot symmetry and statistical evaluation using Egger’s regression test, performed in Stata 15 (StataCorp LLC, College Station, TX, USA). Both graphical representations and statistical outcomes were carefully analyzed to detect any indications of potential bias in the published literature.

Results

A total of 11,497 articles were retrieved from six databases, with two additional articles identified through snowball sampling. Following duplicate screening using EndNote, 9,723 articles were excluded. The titles were further evaluated, leading to the exclusion of review papers, systematic reviews, animal studies, and articles evidently irrelevant to the research topic. A total of 2,316 articles were retained for abstract review. Subsequent to the initial screening of abstracts, publications that failed to satisfy the predetermined inclusion criteria were systematically excluded, yielding a refined pool of 2,303 potentially relevant articles. Subsequently, 15 full-text articles were reviewed, and four were removed due to the lack of greenspace measuring data. Ultimately, nine studies were identified as suitable for inclusion in the systematic review. Among these, eight articles containing appropriate quantitative data were subsequently subjected to meta-analytic procedures, as illustrated in Fig. 1.

Fig. 1.

Flow chart of studies

Study characteristics

Within the nine studies selected for systematic evaluation, a substantial majority (seven publications) were published in the five years, demonstrating the growing scientific interest in investigating connections between greenspaces and intestinal diseases within environmental health research. The geographical distribution of these investigations spanned seven nations, with two studies from the United Kingdom, two from China, and one each from Denmark, Canada, Malawi, Germany, and France respectively. These studies spanned Europe, Asia, America, and Africa, including both developed and developing countries. Among the nine studies, seven were cohort studies, and two were cross-sectional studies. The sample sizes varied between 44,393 and 2,715,318. Two studies focused on children, while the other seven were conducted on adults without age restrictions. The primary greenspace assessment indicators used in the review were the Normalized Difference Vegetation Index (NDVI) and greenspace coverage rate. Among the nine studies, four utilized NDVI as a primary indicator, four employed the greenspace coverage rate, and one assessed greenspace by calculating the forest coverage rate and its changes. All nine studies used the incidence of intestinal diseases as an assessment indicator of health outcomes.

Risk of bias assessment

All nine articles were independently evaluated for quality using the NOS to assess the risk of bias. The median NOS score in these studies was seven (range 6–8). Therefore, all studies included in this review could be classified as medium to high quality, and no study was excluded due to a poor quality score. Specifically, two studies received eight points, four studies were rated seven points, and three studies attained six points. Among the evaluation criteria, indicators most prone to losing points were: “4. Demonstration that the outcome of interest was not present at the start of the study,” “5. Comparability of cohorts based on the design or analysis,” and “8. Adequacy of follow-up of cohort” (Tables S2 and S3).

Narrative analysis

Of the nine investigations incorporated in this systematic analysis examining potential associations between greenspaces and intestinal diseases, three examined the association between greenspaces and IBD [2, 11, 57], five investigated the relationship between greenspaces and CRC [7, 9, 35, 50, 56], and one explored the link between greenspaces and diarrhea [21].

Of the studies reporting the relationship between greenspace use and IBD risk, two studies [11, 57] showed a significant negative correlation. One study, conducted on 216,868 adults in the UK, calculated the percentage of greenspace in the 300-m and 1,000-m buffer zones around the participants’ residences. It was determined that residential greenspaces may serve as a protective factor in reducing the incidence of IBD, with this association being stronger among participants in poorer areas [57]. Another study conducted in Canada measured the exposure of 2,715,318 mother-infant pairs to greenspaces throughout pregnancy and childhood using NDVI. The investigation revealed an inverse relationship between the spatial extent of greenspaces during childhood and subsequent pediatric IBD development risk [11]. Another independent study [2], while demonstrating no significant overall association between greenspace exposure and IBD incidence in its primary analysis, identified a potential protective effect when comparing extreme exposure groups, specifically between the highest and lowest quartiles of greenspace accessibility.

Five studies investigated the relationship between greenspaces and CRC. Three investigations demonstrated significant associations between various greenspace metrics and cancer risk reduction. Specifically, these studies identified protective relationships between: (1) the expansion of residential garden areas within a 100-m radius of participants’ homes [35], (2) the enhanced vegetation density measured through NDVI at the community level [50], and (3) increased exposure to natural environments within multiple buffer zones (100 m, 300 m, 500 m, and 1000 m) surrounding residential locations [56]. These environmental factors were associated with decreased risks of both obesity-related malignancies and colorectal cancer, suggesting potential protective mechanisms against CRC development. In contrast, the other two studies reported that increases in the NDVI of residential greenspaces within the 300-m and 1000-m buffer zones [7], as well as in the areas surrounding residences [9], were not associated with CRC.

A study conducted on 23,000 households in Malawi measured and calculated the percentages of forest cover. From 2000 to 2010, the incidence of diarrhea among children living in areas with a net increase in forest coverage decreased by 34%, demonstrating the protective effect of increased forest coverage on children’s diarrhea [21].

Meta-analysis

For the meta-analysis of the three studies on IBD, heterogeneity was initially assessed using Q and I² statistics. The results showed I² = 57.5%, indicating moderate heterogeneity. Given these findings, the random-effects model (DerSimonian-Laird method) was chosen to analyze the outcome measures. The sensitivity analysis revealed that removing any single study did not affect the research results, indicating that the results were consistent and reliable and supported the protective effect of greenspace on IBD (Fig. 2 and S1). However, due to the limited number of included articles, further subgroup analyses to determine heterogeneity was infeasible. Since IBD includes ulcerative colitis (UC) and Crohn's disease (CD), we performed a meta-analysis of these two diseases. The above-mentioned heterogeneity tests, model selection, outcome index combination, and sensitivity analyses were then performed separately for each disease. The results showed no heterogeneity in the CD subgroup, and the sensitivity analysis validated the robustness of the results (Figs. S2 and S3). In contrast, the UC subgroup exhibited high heterogeneity (I² = 86% > 50%, P = 0.001 < 0.1), suggesting that the protective effect of greenspaces on IBD is more consistent in CD, whereas the effect on UC incidence requires further empirical evidence (Figs. 3 and 4).

Fig. 2.

Meta-analysis of the effects of greenspace exposure on IBD

Fig. 3.

Meta-analysis of the effects of greenspace exposure on CD

Fig. 4.

Meta-analysis of the effects of greenspace exposure on UC

The initial meta-analytic synthesis incorporating five CRC investigations demonstrated substantial heterogeneity, with I² = 96.7% > 50%, P < 0.001. To investigate the sources of this variability, a sequential sensitivity analysis was performed through iterative exclusion of individual studies (Fig. S4). This methodological approach identified the research conducted by Wu and colleagues as exerting a disproportionately significant impact on the overall effect estimates, suggesting its substantial contribution to the observed heterogeneity. On re-examining this article, we found that the heterogeneity most derives from:(1) geographic clustering of their sample in Pingyi County, a small-scale region where localized demographic factors may limit the generalizability of NDVI-CRC associations (2) reliance on the China Monthly 1KM Vegetation Index Dataset, whose 1-km resolution likely inadequately captured greenspace variations in small administrative units like Pingyi County, introducing exposure misclassification. After excluding this study, a retest was performed, which reveling a significantly reduction in heterogeneity, with I² = 0% < 50% and P = 0.472 > 0.1, indicating the absence of heterogeneity. Following this, a fixed-effects analytical approach utilizing the Mantel–Haenszel statistical method was implemented to calculate pooled effect estimates with corresponding 95% confidence intervals. The quantitative synthesis demonstrated no statistically meaningful protective association between greenspace and CRC, with a relative risk of 1.01 (95% CI: 0.98–1.04).

Subsequently, an analysis was performed using the fixed-effects model (Mantel–Haenszel method), and the combined effect size with a 95% confidence interval was calculated. The results revealed that the protective effect of greenspaces on CRC was not statistically significant (RR = 1.01, 95% CI: 0.98–1.04). These findings are illustrated in Fig. 5.

Fig. 5.

Meta-analysis of the effects of greenspace exposure on colorectal cancer

All four meta-analyses presented funnel plots and Egger’s test was conducted. However, since each analysis included fewer than 10 studies and the P-values from Egger’s test exceeded 0.05, the limited sample size constrained the test’s power, making it impossible to completely rule out the risk of publication bias.

Discussion

Existing reviews on the relationship between greenspace and intestinal diseases have primarily focused on single disease outcomes, with limited scope in terms of the number of studies included. This article presents the most comprehensive systematic review of the relationship between greenspaces and intestinal diseases. Nine studies were selected from 11,497 documents, including seven cohort studies and two cross-sectional studies. The geographical distribution of the included research primarily encompassed European and Asian regions, with a specific focus on gastrointestinal disorders including IBD, CRC, and diarrheal conditions. Four distinct meta-analytic investigations were conducted to evaluate potential associations between greenspace exposure and various intestinal pathologies, namely IBD, CD, UC, and CRC. The analysis revealed statistically significant correlations for IBD (pooled effect size = 0.86, 95% confidence interval: 0.79–0.94) and CD (pooled effect size = 0.87, 95% confidence interval: 0.81–0.93). In contrast, no significant relationships were observed for UC (effect size = 0.85, 95% CI: 0.70–1.04) or CRC (effect size = 1.01, 95% CI: 0.98–1.04). Several methodological limitations were identified, including substantial variability in greenspace measurement approaches and demographic characteristics across studies. Additionally, the relatively small number of available studies constrained the ability to conduct comprehensive publication bias assessments, potentially undermining the robustness of the findings.

The association between exposure to greenspace and intestinal diseases

The current study confirmed that greenspace exposure is associated with IBD and CD, aligning closely with previous research findings. An epidemiological review focusing on pediatric IBD [22] comprehensively examined various environmental determinants associated with the onset of childhood inflammatory bowel disease, incorporating an analysis of greenspace exposure. The synthesis suggested that proximity to natural environments might potentially function as a preventive element against pediatric IBD development. In contrast, research conducted by Azreen and colleagues [31] demonstrated no statistically correlation between greenspace accessibility and colorectal cancer risk. Furthermore, a systematic evaluation performed by [55] highlighted methodological limitations, including insufficient data and substantial variability across studies, which precluded the establishment of conclusive evidence regarding the association between greenspaces and colorectal cancer incidence. Conversely, the quantity of studies included in our review surpassed that of other reviews, encompassing all studies that have considered and conducted systematic reviews of several major intestinal diseases, including IBD, CRC, and diarrhea. This study emphasized the subsequent key findings:

1. Greenspace exposure has varying effects on various intestinal diseases as well as the incidence rates of different types of IBD, including UC and CD.

2. Analysis revealed a significant inverse relationship between greenspace exposure and CD risk, whereas no significant correlation was detected between natural environment exposure and UC and CRC occurrence.

In addition to studies establishing a direct correlation between greenspace exposure and intestinal diseases, studies on the intestinal microbiota as a critical biomarker for intestinal disorders, along with studies on greenspace, offer substantial indirect evidence to support the hypotheses presented in this article. Notably, comprehensive investigations have systematically analyzed the interplay among environmental factors, genetic predispositions, and the human intestinal microbiome. Research demonstrates that multiple environmental elements within residential settings, particularly the level of engagement with natural surroundings, can significantly impact both the structural configuration and functional capacity of intestinal microbial communities [14]. An investigation examining a British twin population [5] evaluated the association between residential characteristics and gut microbiome profiles among 2,443 participants. The findings suggested that proximity to natural green areas potentially influences microbial diversity and taxonomic distribution within the gastrointestinal tract. Notably, fluctuations in the prevalence of specific bacterial taxa were correlated with the spatial extent of accessible green areas. Specifically, residents living in areas with larger greenspace areas tended to have a greater diversity of gut bacteria, which is typically considered a biomarker associated with improved intestinal health and ecosystem stability.

Considering the potential link between intestinal diseases and the microbiota, current methods for measuring greenspace are predominantly limited to macroscopic assessments and potential exposure considerations. However, these approaches lack precise insight into the underlying mechanisms. To further clarify the association between greenspaces and intestinal diseases, it is essential to focus on the actual exposure conditions of individuals, such as the duration and frequency of their activities in greenspaces, as these factors may differentially affect the gut microbiota. Moreover, different vegetation types, such as deciduous and evergreen forests, may exert distinct seasonal effect on microbial communities. Nonetheless, the research in this field is limited. Forests, being important carriers of soil, harbor a broad array of distinctive soil microorganisms. Zhou et al. [58] conducted experiments on mice by exposing them to sterile soil, analogous to inhaling small amounts of soil microorganisms from the air. They found that this exposure increased the diversity of the intestinal microorganisms, suggesting that soil can facilitate the proliferation and reproduction of certain gut microorganisms. These findings offer novel perspectives on the intricate link between greenspaces and gut microbial ecosystems, while simultaneously highlighting the critical importance of pursuing additional investigations in this emerging research domain. The results underscore the necessity for more comprehensive studies to elucidate the underlying mechanisms and potential health implications of this environmental-microbial interaction.

Potential mechanisms

Numerous potential mechanisms contribute to the influence of greenspaces on intestinal diseases, particularly in reducing the incidence and prevalence of IBD and its association with diarrheal diseases. These mechanisms can be elucidated from several perspectives, encompassing air pollution, biodiversity, water quality improvement, and stress alleviation.

Initially, regarding air pollution and its impacts on the gastrointestinal tract, the gastrointestinal system is vulnerable to both direct and indirect exposure to air pollution, resulting in the rapid transfer of particulate matter from the lungs to the intestines [13]. Previous studies have confirmed that particulate matter (PM) in air can affect intestinal tight junctions, increase intestinal permeability, and promote inflammation [38]. Urban greenspace, including forests, parks, and private gardens, have been shown to effectively mitigate environmental pollutants [32] and improve air quality [49]. Compared to the urban built environment, this greenspace characteristic can significantly diminish the potential threat of air particles to the gastrointestinal tract.

Secondly, the “Biodiversity Hypothesis” posits that regions with greater exposure to biodiversity can regulate the composition of human microbiota, thereby improving immune function [1]. Biodiversity-rich environments typically contain a multitude of microbial species. These micro-organisms can directly enter the human body through contact with or via soil and plant surfaces, ultimately influencing the human microbiome. Microbial imbalance is a well-established characteristic of IBD and the bidirectional mechanism of microbial imbalance in IBD pathogenesis has received considerable attention in recent years [16].

Third, a high incidence of diarrhea is most commonly observed in low- and middle-income countries [28], where inadequate hygiene conditions and water quality contribute significantly to the high prevalence of diarrheal diseases [48]. Greenspace can exert beneficial effects on water quality. In ecosystems such as forests and wetlands, the soil purifies water through processes including adsorption, desorption, ion exchange, and metabolic decomposition [8]. Improvement in water quality is an important factor in alleviating diarrheal diseases [25], which may explain the association between greenspaces and diarrhea. However, only single study has been conducted on greenspaces and diarrheal diseases, and further research is required to provide more evidence for this connection.

Finally, the impact of greenspaces on intestinal diseases may also be facilitated by mental health. Research indicates that exposure to natural settings can reduce cortisol levels in individuals [15]. The brain-gut axis, mediates the alleviation of stress and reduction of anxiety and depression, which may influence intestinal inflammatory factors, and subsequently affect intestinal diseases [33, 54] (Fig. 6).

Fig. 6.

Potential mechanisms underlying the impact of Green Spaces on Brain-Gut Axis

Owing to insufficient investigations, it is unable to assess the contribution of the above mechanisms to the association over a longer time span, across larger geographical areas, or with larger samples of patients with intestinal disease. Furthermore, the interactions between these mechanisms remain unclear, posing new challenges and avenues for future research. Further in-depth and comprehensive studies are required to address these issues.

Strengths and limitations

This investigation represents, to our current knowledge, the inaugural comprehensive synthesis and quantitative analysis examining associations between greenspace and intestinal diseases. The study encompassed a systematic evaluation of multiple digestive system conditions, including IBD, CRC, and diarrheal illnesses, while specifically performing meta-analytic procedures to quantify relationships between greenspace exposure and the incidence of both IBD and CRC. This study quantitatively reveals this correlation, offering valuable insights for clinical education and urban greenspace development.

For healthy urban planning, designers should prioritize the increasement of the proportion and distribution of greenspaces when planning new urban areas or revamping existing ones. This would enhance the intestinal health of the residents. Specific measures include optimizing the layout of greenspaces and prioritizing the planning of green areas in densely populated areas and regions with a high incidence of intestinal diseases to ensure easy access to the natural environment. Additionally, urban planners should emphasize the diversity and quality of greenspaces by introducing community gardens, rooftop and vertical greening, and water body distribution to promote ecological diversity. Furthermore, legislation should be enacted to safeguard existing greenspaces, prevent excessive development, and encourage new projects to incorporate greenspace design standards. Encouraging active community engagement in the design and maintenance of greenspace is essential to enhance public awareness and maximize the utilization of green areas for health promotion. From a clinical perspective, the evidence derived from this research enables healthcare providers to deliver specific recommendations regarding greenspace exposure. Particularly, patients residing in regions with elevated IBD prevalence or those with genetic susceptibility to gastrointestinal disorders should be advised to incorporate regular interaction with natural environments into their lifestyle as a potential preventive measure. Additionally, medical institutions can collaborate with urban planning departments to promote the “green prescription” project, facilitating access to greenspace a supplementary measure for the prevention and management of intestinal diseases.

Nevertheless, several methodological constraints and limitations should be acknowledged in the present investigation. First, because we only included Chinese and English literature and the accessibility of gray literature is limited, some relevant studies published in other languages or gray literature that have not been officially published but contain valuable information were inevitably excluded. Secondly, the assessment of greenspaces remains challenging to refine. The greenspace measurement indicators included in the literature primarily focus on NDVI and greenspace coverage rate, which assess greenspace exposure solely from a quantitative perspective, neglecting characteristics such as vegetation structure, species, and proportion. Moreover, variations in the geographical analysis units across studies, which may contribute to the heterogeneity shown in the results. For instance, distinct types of vegetation may have influence air purification and microbial diversity differently; however, the study has not explored this aspect in depth, limiting the precision with which the quality and characteristics of greenspaces can be linked to intestinal diseases. Third, due to the variations in the populations included in different studies, some factors were not incorporated into the meta-analysis, and the impact of greenspaces on intestinal health may also vary with age. Fourth, although all included studies were assessed to be of moderate to high quality (NOS scores 6–8), we did not conduct a sensitivity analysis based on study quality. Future reviews with a broader quality distribution may consider stratified sensitivity analyses to further test the robustness of pooled results. Moreover, this study did not systematically differentiate the heterogeneity between urban and rural green spaces. Due to significant disparities in pollution levels, vegetation types, and human behavioral patterns across urban and rural environments, future research should incorporate stratified analyses tailored to distinct regions to enable more precise assessment of the health benefits associated with green spaces. However, due to the limitations of the current study, we were unable to comprehensively analyze these variables.

Conclusion

This study comprehensively explored the relationship between greenspace and intestinal diseases and provided new evidence for this association. The research findings indicated that greenspaces exert a protective effect against IBD, particularly CD, while the results for UC and CRC were insignificant. Furthermore, in the context of limited research, greenspaces have shown potential protective effects against diarrhea. Greenspaces may influence intestinal health through various mechanisms, including improving air quality, enhancing microbial diversity, and alleviating stress mediated by the gut-brain axis. The addition of randomized controlled trials strengthens the argument for causal relationships.

To improve the assessment of greenspaces, it is essential to consider not only the quantity but also the types, structures, and seasonal variations of vegetation, as these factors may have distinct impacts on intestinal health. Moreover, emphasis on different subgroups of the population must be increased to accurately elucidate the relationship between greenspace and intestinal diseases, and to further investigate the potential mechanisms involved.

These findings provide valuable evidence for urban planning and the formulation of public health policies that comprehensively consider the health benefits of greenspaces. This approach can significantly enhance population health outcomes while providing empirical evidence to inform the development of health-oriented urban environments and intestinal diseases prevention strategies. In order to optimize the potential health benefits of greenspaces on digestive system well-being, urban design initiatives should incorporate customized greenspace configurations and typologies that align with demographic characteristics and regional specificities. Based on the findings of this study, public health policies can be developed to facilitate greater access to greenspaces, which would enhance residents’ quality of life and health.

Abbreviations

NDVI

Normalized Difference Vegetation Index

IBD

Inflammatory bowel disease

CD

Crohn’s disease

UC

Ulcerative colitis

CRC

Colorectal cancer

WHO

World Health Organization

NBS

Nature-based solutions

GBD

Global Burden of Disease

GBA

Gut-brain axis

PRISMA

Preferred Reporting Items for Systematic Reviews and Meta-Analyses

PROSPERO

Prospective Register of Systematic Reviews

NOS

Newcastle–Ottawa Scale

RR

Relative risk

OR

Odds ratio

HR

Hazard ratio

CIs

Confidence intervals

PM

Particulate matter

Authors’ contributions

A.Z.: Conceptualized the study, designed the methodology, curated data, and wrote the original draft. T.Z.: Validated data, designed visualization tools, and edited the manuscript. H.L. (Hongyi Li): Performed technical validation. T.X. (Tianyu Xia): Conducted statistical modeling, interpreted results, and prepared figures/tables. S.Z. (Shijie Zhang): Performed systematic literature searches and curated clinical/public health data. Y.Z. (Yuxin Zhu): Extracted data, assessed study quality for meta-analysis, and drafted supplementary materials. K.D. (Kang Ding): Supervised the project, acquired funding, revised the manuscript critically, and administered the study. All authors reviewed and approved the final manuscript.

Funding

The General Project of Jiangsu Province’s Science and Technology Development Program for Traditional Chinese Medicine in 2020 (Grant Number: YB2020029); Jiangsu Province Postgraduate Practical Innovation Program 2025 (Grant Number: SJCX25_1061).

Data availability

Data is provided within the manuscript or supplementary information files.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.