Understanding nutrition research for better patient care

Introduction

The field of nutrition research is rapidly evolving, and interest in understanding food and nutrition spans laypeople, professionals and researchers alike. However, the growth and diverse methodological approaches taken by researchers can make it challenging to critically appraise research and apply the evidence to clinical practice.

Nutrition is a multifaceted subject, and well-designed research using appropriate methods is an essential tool for developing a sound evidence base on how food impacts our health, well-being and overall quality of life.^{1 2} Nutrition research encompasses many areas and has relevance in physical sciences, medicine, public health, sociology, psychology and increasingly, planetary health. It aims to uncover the scientific processes that explain nutrient actions and interactions in living bodies, inform dietary choices, recommendations and policy decisions to prevent disease. Food choice is increasingly linked to concepts of sustainability, food security and the broader social determinants of health. At its core, nutrition research seeks to answer fundamental questions about our food choices, their nutritional value and their relationship with health and disease. To address these questions, researchers employ various methods, each with its strengths and limitations.

Research method hierarchy

The research method hierarchy can lead people to judge the quality of the findings based on their preferred research design without considering the relevance of the design to the questions being explored. In research, the research method hierarchy refers to the ranking of study designs based on the strength and reliability of the evidence they provide. At the top of this hierarchy are systematic reviews and meta-analyses, which synthesise data from multiple studies to offer comprehensive conclusions. Randomised controlled trials (RCTs) follow and are considered the gold standard for establishing cause-and-effect relationships due to their rigorous design and control over variables. Cohort studies and case–control studies come next, providing valuable insights into associations between dietary factors and health outcomes, though they are less definitive in proving causality. Cross-sectional studies and case studies offer descriptive data and can highlight potential areas for further research but are limited in their ability to establish temporal relationships. At the base of the hierarchy are expert opinions and anecdotal evidence, which, while informative, can lack the methodological rigour to be considered strong evidence. Increasingly, nutrition research includes the direct evidence and opinions of participants to measure impact and sustainability.

However, even within this hierarchy, the value of research may be influenced by the methods of data analysis and applicability of the findings to clinical or individual cases. The findings of well-designed RCTs, while providing clear evidence of cause-and-effect, may be challenging to apply in diverse clinical/health settings due to factors controlled during the study, such as adherence to regimens proposed. In contrast, case studies such as that on type 2 diabetes management in a single GP practice³ provide useful practical insights that can resonate with general practitioners and add to the body of evidence to guide general practitioners with type 2 diabetes management in their own setting and stimulate more research based in ‘real-world’ settings.

Approaches to nutrition research

Narrative inquiry

Narrative inquiry is both a method and a phenomenon in qualitative research that focuses on collaboration between the researcher and the participants, and on the stories people tell as they engage, in this example, with food. Both historical and current narratives can help to understand cultural drivers to dietary patterns and food choices, beliefs and values associated with food preferences, and how agrarian and technological developments have shaped food production over time. Current health concerns associated with food processing can be better understood when contextualised to food production changes over time, the need to store food in times of plenty, and the development of food supply chains. Likewise, the interest in farming ‘local’ and the use of indigenous foods⁴ and the importance of knowing what foods can withstand climatic change is of increased relevance to human health. This knowledge, often passed from generation to generation, can identify areas for future research and inform current thinking around food sovereignty, food security and policy development.

Case studies

Case studies in nutrition research provide detailed and in-depth analyses of individual or group dietary patterns, health outcomes and lifestyle factors. These studies offer valuable insights into the real-world application of nutritional interventions and their effects on specific populations. By focusing on unique cases, researchers can explore the complexities of nutrition-related issues, identify potential causal relationships and generate hypotheses for further investigation. Case studies often highlight the interplay between diet, genetics, environment and behaviour, offering a holistic view of how these factors influence health. They are particularly useful for examining rare conditions, novel dietary approaches, or the impact of personalised nutrition plans. Elmi et al., 2023⁵ published a case–control study of serum vitamin D concentrations in hospitalised patients and hospitalised controls suffering from respiratory tract infections of differing aetiology and found that both COVID-19 and acute respiratory tract infections are equally associated with low vitamin D levels. This identified that there is a need for further research to better understand the accuracy of this observation and the implications of this finding for improved patient management and outcomes. Through detailed documentation and analysis, case studies contribute to a richer understanding of nutrition science and its practical implications for improving health and well-being.

Observational studies

Observational studies^{6 7} track individuals or populations over time, collecting data on dietary habits, health outcomes and lifestyle factors. These studies are invaluable for understanding the impact of diet and lifestyle on health outcomes and provide insights into associations between nutrition and disease risk. Pot et al⁸ used this approach in their publication on the Nutrition and Lifestyle intervention in type 2 diabetes: pilot study in the Netherlands showing improved glucose control and reduction in glucose. However, they cannot establish causality, and as in the example cited, concluded that further studies are required to confirm results. For example, an observational study may find a link between high fruit consumption and reduced heart disease risk, but it cannot prove that eating more fruit directly prevents heart disease. Long-term studies such as the Nurses’ Health Study, Seven Countries Study and ALSPAC have significantly shaped health policies and dietary guidelines worldwide.

Randomised controlled trials

RCTs are considered the gold standard for assessing cause-and-effect relationships.⁹ Participants are randomly assigned to interventions (eg, diets, supplements) or control groups. By comparing outcomes between these groups, researchers can determine whether a specific dietary intervention leads to desired health outcomes. Grammatikopoulou et al¹⁰ challenged the robustness of RCTs to produce dietary guidelines based on the Mediterranean diet through a systematic exploration of RCTs in this field. While RCTs can provide robust evidence, they can be expensive, time-consuming and challenging to design and conduct.

Meta-analyses and systematic reviews

Meta-analyses combine results from various RCTs or observational studies¹¹ offering a comprehensive view of the evidence. Systematic reviews follow a rigorous process to assess the quality of existing research. Both approaches help identify trends, inconsistencies and gaps in the literature. Pineda et al¹² completed a systematic review and meta-analysis of the food environment and obesity and concluded that food outlets selling predominantly ultra-processed food were associated with higher levels of obesity in the locality, and that the regulation of food outlets by zoning laws may not be enough to address the issue of obesity. Such findings can be explored further, and the availability of large datasets, such as the UK Biobank, has created opportunities to revisit known concerns, such as obesity and cardiometabolic health, dietary intake and cancer risk, and the impact of gender and age on perimenopausal health outcomes. Mason et al,¹³ in an observational study of 335 046 UK Biobank participants, concluded that individuals with an increased genetic risk for obesity may be more sensitive to a fast-food environment and called for better designed residential environments to promote healthy weight. Meta-analyses and synthesis of findings within and between datasets can deepen our understanding of these complex phenomena. In the examples cited above, the link between the design of the built environment, access to ultra-processed foods and obesity is emerging, opening opportunities for future research and work that includes others such as town planners and architects.

New statistical methodologies

The 21st century is heralding new research approaches in nutrition science such as causal inference analysis¹⁴ and Mendelian randomisation (MR) analysis.¹⁵ Pathways developed drawing on tools from causal inference analysis support greater understanding of the relationship between variables. In the work of Lima do Vale et al,¹⁶ the links between diet, as well as conventional and emerging physiological risk factors, and cardiovascular disease (CVD) risk, incidence and mortality were translated to causal pathways, highlighting the gaps in research. MR adds to the understanding of causal inference by using genetic variants as instrumental variables for modifiable risk factors (eg, diet) that impact population health. Genetic variants are fixed at conception, and knowing these can support understanding of the influence of confounding factors on modifiable risk factors when describing causal inference. This type of analysis is important for analysing observational studies¹⁵ and evaluating the causal impact of nutritional exposures on long-term health outcomes but assumes equivalency of exposure and genetic proxy on the outcome. Kohlmeier and Baah¹⁷ consider the importance of MR analysis in understanding observational studies and the reported association between levels of vitamin D on COVID-19 disease risk and outcomes, when 25-OHD concentration in serum is a less-than-ideal proxy for vitamin D status of cells involved in the immune response.

The importance of quality and scientific rigour in nutrition research

The quality and scientific rigour of nutrition research ensures that evidence is valid and robust,¹⁸ countering the widespread nutrition opinions on social media that may not be factual. Enhanced quality and rigour can be achieved by well-designed, rigorously conducted and appropriately analysed nutrition research (table 1).

Table 1. Principles to enhance research quality and rigour.

Enhancing research quality and rigour	Rationale
Accuracy and reliability	Rigorous methods prevent flawed or incomplete evidence from guiding dietary advice
Uncovering hidden relationships	Research methods can reveal unexpected connections, such as a seemingly unrelated nutrient affecting a specific health marker, informing personalised dietary strategies.
Policy and public health impact	Policy-makers rely on robust research to create guidelines and regulations. Nutrition research informs public health campaigns, school meal programmes and food labelling policies
Challenging assumptions	Research methods encourage critical thinking, prompting us to question prevailing beliefs and explore new avenues. For instance, since the work ofl Keys19 methodical research, enhanced data analysis techniques and developing methodologies have shifted perspectives on dietary fats and understanding of the complex relationship between dietary intake and health outcomes
Transparency	Researchers should disclose their methods, data sources and potential conflicts of interest, building trust and allowing others to replicate studies
Interdisciplinary approach	Collaboration between pure scientists, physicians, health professionals, nutritionists, epidemiologists, geneticists, social and behavioural scientists enrich our understanding
Long-term studies	Longitudinal studies provide insights into lifelong dietary patterns and their health impacts, making investment in such studies essential
Technology and big data	Leveraging technology and analysing large datasets can yield novel insights, leading to the development of wearable devices, mobile apps and genetic profiling, ultimately supporting healthier dietary patterns.

Conclusions

Rigorous and valid research methods in nutrition shape our understanding of food, health and well-being. Likewise, adopting a rigorous approach to reading research empowers individuals, healthcare professionals and policymakers to critically appraise the literature, make informed decisions and consider the implications for their practice domains. BMJNPH Education special collection contains a series of articles to enhance understanding and to provide guidance on how to read nutrition research to support evidence-based practice.