Sustainable diets and risk of overweight and obesity: A systematic review and meta‐analysis

Sustainable diets are gaining interest as a possible approach to tackle climate change and the global extent of obesity. Yet, the association between sustainable diets and adiposity remains unclear. We performed a systematic review and meta‐analysis, calculating summary relative risks and 95% confidence intervals (CI). We pooled maximally adjusted risk estimates, assessed heterogeneity and publication bias, calculated the E‐value, and evaluated the risk of bias across the included studies. A total of eight studies were eligible for analysis. Comparing the highest versus the lowest levels of adherence to sustainable diets, the pooled effect estimate was 0.69 (95% CI = 0.62–0.76) for overweight and 0.61 (95% CI = 0.47–0.78) for obesity. These results suggest that sustainable diets may decrease the risk of overweight/obesity and therefore could serve as enablers for improving both public and planetary health. An agreed‐upon clear definition of sustainable diets would enhance the comparability of future studies in this area.


| INTRODUCTION
Overweight and obesity are major public health concerns worldwide.
The global prevalence of overweight and obesity has increased substantially over the past few decades and is expected to continue to rise. 1 According to the World Health Organization (WHO), the global prevalence of obesity has almost tripled since 1975.In 2016, there were an estimated 1.9 billion adults with overweight and 650 million with obesity worldwide. 1In the European region, "overweight and obesity rates have reached epidemic proportions," with almost 60% of the population being either people with overweight or obesity. 2 Furthermore, low-and middle-income countries are experiencing significant increases in the prevalence of overweight and obesity. 3veral possible mechanisms lead to overweight and obesity, and dietary habits play an important role.An imbalance between the energy demand of the body and an increased energy intake resulting from greater availability and consumption of highly processed and energy-dense food is seen as the main cause. 4Combined with decreased physical activity levels due to the increasingly sedentary nature of many forms of work, changing modes of transportation, and increasing urbanization, 5 high energy intake can lead to severe adipose tissue accumulation and, thus, to overweight and obesity. 6rthermore, currently dominant dietary patterns are harmful to planetary health, contributing to the global syndemic-a confluence of the epidemics of obesity, undernutrition, and climate change. 7Food production is the single largest cause of global environmental change, 8 causing up to 30% of global anthropogenic greenhouse gas (GHG) emissions 9 while using around 40% of the planet's land 10 and 70% of its fresh water. 11Moreover, food systems are the main driver of global biodiversity loss. 12Consequently, innovation and transformation within the food system has the potential to significantly improve sustainability on a large scale.
To address the dual challenges of promoting health and sustainable development, sustainable diets have been suggested as a potential strategy that could play a key role from both a public and planetary health perspective. 8,13,14Sustainable diets as defined by the Food and Agriculture Organization of the United Nations are "those diets with low environmental impacts which contribute to food and nutrition security and to healthy life for present and future generations.Sustainable diets are protective and respectful of biodiversity and ecosystems, culturally acceptable, accessible, economically fair and affordable; nutritionally adequate, safe and healthy; while optimizing natural and human resources." 15For this article, we follow the definition by Willett et al. of sustainable diets being diets that promote health and well-being while reducing the environmental impact of food production and consumption. 8Besides mostly plant-based diets such as the Planetary Health Diet (PHD), organic food consumption meets several of the dimensions that characterize sustainable diets. 16stainable diets aim to ensure adequate nutrition of people worldwide while maintaining planetary boundaries and are thereby positively contributing to planetary health. 8,11Thus, sustainable diets are key for achieving the United Nations' Sustainable Development Goals, particularly Goal 2 (Zero Hunger), Goal 3 (Good Health and Well-being), Goal 12 (Responsible Consumption and Production), and Goal 13 (Climate Action). 17e relations between sustainable diets and health outcomes have gained increasing relevance in the last few years.For example, a systematic review by Karavasiloglou et al. 18 reported an inverse association between sustainable diets and cancer incidence and cancer-specific mortality. 15A systematic review and meta-analysis by Bhagavathula et al. 19 showed that organic food consumption is associated with an 11% reduced risk of obesity.1][22] However, a comprehensive synthesis summarizing the association between sustainable diets and the risk of overweight and obesity is lacking.
Therefore, we conducted a comprehensive systematic review and meta-analysis of sustainable diets in relation to the risks of body mass index (BMI)-defined overweight and obesity, carefully assessing potential bias and unmeasured confounding in the underlying studies.

| Search strategy and selection criteria
We conducted a systematic review and meta-analysis adhering to the PRISMA 2020 statement: an updated guideline for reporting systematic reviews (PRISMA). 23The PRISMA checklist is available in Data S1.The first author of one of the included studies 24 was contacted directly to obtain additional data, and therefore, we were able to compute the 95% CIs ourselves.To calculate high versus low levels of a sustainable diet, we inverted the RRs from one study, 22 proceeding analogously with the corresponding CIs.
Outcomes were determined as the natural logarithm of observed RRs (log (RRi)) to calculate the overall estimate.The corresponding sampling variances were specified as the squared standard error.
Standard errors were computed as the natural logarithm of the risk ratio subtracted from the upper bound CI (log (ci.ub)-log (RRi)) and the lower bound CI (log (ci.lb)-log (RRi)).We applied a random effects model to our data given high between-study variability.Restricted maximum likelihood estimation was used as an estimator of τ 2 .
Furthermore, we evaluated Q and I 2 statistics 25 to examine the effects of heterogeneity.
We assessed risk estimates primarily measuring sustainable diets as the exposure variables.In the primary meta-analysis, we included one risk estimate per study, except for one study that assessed men and women separately resulting in two RRs. 26In all analyses, we chose the most comprehensively adjusted risk estimate available.
To assess the risk of bias in the included studies measuring overweight/obesity as outcome, we used the Cochrane tool Risk of Bias in Non-randomized Studies of Exposures. 27The following adjustment factors were specified for the assessment of the confounding domain: sex, age, smoking, physical activity, and income/education.The assessment of risk of bias was performed by two researchers (A.M. S. and C. J.).Disagreements between the two authors were resolved by discussion between those authors.The robvis tool was used to visualize our risk of bias assessments. 28The overall certainty of the evidence was evaluated using GRADE. 29 assessed publication bias using a funnel plot and trim and fill analysis.Additionally, we performed Egger's regression test and Begg's rank correlation test.To measure each study's impact, influence diagnostics and leave-one-out analysis were evaluated. 30We calculated the E-value for estimating how strong an unmeasured confounder would need to be to explain away the observed association between exposure and outcome, apart from measured confounding variables. 31 performed several sensitivity analyses by study geographic region (Europe, North America, and South America), study design (cross-sectional and cohort), and sample size (<10,000 and ≥10,000 participants).To account for the possibility of overfitting the model because of the inclusion of multiple publications from a single cohort study (NutriNet-Santé), we conducted further analyses in which we included only one estimate at a time.Statistical analyses were carried out using the R program (version: 4.2.3).We used the packages metafor, robumeta, EValue, dplyr, and MetaUtility to extract risk estimates with 95% CIs.p-values <0.05 were considered statistically significant.

| Study selection and characteristics
Our systematic literature search of electronic databases and handsearching of reference lists and other publications resulted in 798 potential studies (Figure 1).After removal of duplicates, 795 studies remained for title and abstract screening, of which 18 were full text reviewed.Among those, four studies were excluded due to missing measurements of weight change as outcome, two studies were excluded due to missing ORs, and four studies were excluded because they failed to provide a measure of sustainability.
After exclusion, eight studies (four cohort and four cross-sectional studies) were eligible and were included in our systematic review and meta-analysis.The main characteristics of the eight studies included in the systematic review are presented in Table 1.Those studies yielded a total of 438,020 participants at baseline and ultimately 170,923 participants in the analytic sample.Six studies originated from Europe, 22,24,26,[32][33][34] one from North America, 20 and one from South America. 21Three studies 22,26,32 were based on the same cohort (NutriNet-Santé); however, we integrated all estimates in our main models because the time span of the data collection and the sustainability indices used to assess the exposure of the data collection were distinct in each study. 26,32Further, sensitivity analyses demonstrated no significant changes of the total risk estimate (Data S3).

| Dietary assessment methods
The studies included in our meta-analysis utilized different dietary assessment methods, among them questionnaires on food-related lifestyles, food frequency questionnaires, 24 h records (24HR), and information on organic products consumed in the past 12 months.To evaluate sustainability, studies used various indices or scores.The PHD Index 35 assesses adherence to a reference diet proposed by the EAT-Lancet Commission.The Sustainability Diet Index 36 is a score with a maximum of 20 points summed up using four subindices encompassing environmental, nutritional, economic, and sociocultural aspects.Kesse-Guyot et al. utilized an Organic Score ranging between 0 and 32 points across 18 food groups, and Andersen et al. 34 used an overall organic food score ranging from 6 to 24 points.Gosling et al. 33 used both a food propensity questionnaire and a 24HR to investigate frequency of organic food consumption across 12 food groups.
Pérez-Cueto et al. 37 made use of the food-related lifestyles concept measuring 23 lifestyle dimensions covering identification, preparation, and actual intake of food products, including high versus low organic food consumption.
We aimed to compare the highest versus the lowest levels of sustainable diet in relation to overweight and obesity.All studies used standard BMI categories according to the WHO as outcome variables.
The number of adjustment factors varied between 6 22 and 15 33,34 variables per study (Table 1).

| Sustainable diets and risk of overweight/ obesity
The funnel plot of the obesity risk estimates showed considerable asymmetry indicating potential publication bias, confirmed by Egger's regression test (z = À6.6636,p < 0.0001) (Data S5).Begg's rank correlation test showed a p of 0.0247 and a Kendall's tau of À0.6111.
Influence diagnostics noted a rather large externally standardized residual (rstudent) of À2.46 for one study 22 (Data S6), which can be explained by the small RR.Leave-one-out diagnostics of the included studies showed no relevant changes in summary risk estimates, showing a range from RR = 0.57 (95% CI = 0.44-0.74) to RR = 0.66 (95% CI = 0.54-0.82)for obesity (Data S6).
The certainty of the evidence was rated as "low," particularly because of risk of bias (see Data S7).
Our sensitivity analysis of unmeasured confounding showed that an unobserved confounder would have to be associated with both sustainable diets and overweight with an RR of at least 2.26 and with obesity with an RR of at least 2.66 to fully explain away the mean RRs of 0.69 and 0.61, respectively.To render the risk estimates statistically nonsignificant, unobserved confounding strength associated with sustainable diets and overweight and obesity with RRs of 1.96 and 1.88 would be necessary to move the upper confidence limits of 0.76 and 0.78, respectively, to include the null.
Stratified subanalyses indicated that the relation of high versus low levels of sustainable diet and weight change was not modified by sample size or different adjustment factors, including adjustments for physical activity, total energy intake, educational level, and social status or income (all p for difference >0.05) (Table 2).Only study design showed a significant p for difference of 0.0007.Comparing studies that assessed primarily organic dietary habits (N = 5) with studies assessing nonorganic sustainable diet (N = 3) showed no relevant differences in risk estimates (RR 0.64 [95% CI = 0.48-0.86]vs. RR 0.53 [95% CI = 0.27-1.01]with a p for difference of 0.58; Table 2).
We excluded two additional risk estimates given in the study by Cacau et al., 21 which utilized waist circumference as an additional measure of "increased abdominal obesity" and "substantially increased abdominal obesity," which did not meaningfully alter the results.

| DISCUSSION
This systematic review and meta-analysis summarized the existing evidence on the association between sustainable diets and risk of overweight and obesity.Pooling 15 risk estimates from eight studies showed that sustainable diets are associated with a significantly lower risk of overweight and obesity.
The studies included in our meta-analysis assessed plant-based or organic diets which provided additional information on the sustainability of such diets.In general, plant-based diets are more environmentally friendly than meat-rich diets as they are related to less water and land use and less GHG emissions. 38We furthermore included studies focusing on organic foods as exposure variable because such foods have repeatedly been considered ecologically beneficial and sustainable, 16,38,39 although they do not always meet all aspects of sustainable diets (especially in terms of monetary costs).1][42] By avoiding monoculture and providing habitats for various species, organic food production can help promote biodiversity 43,44 and therefore help maintain ecological balance.6][47] Those are fundamental aspects of sustainability and it therefore seems reasonable to include diets with a high percentage of organic food in our review.F I G U R E 2 Forest plot of random-effects (RE) meta-analysis of adjusted risk estimates of high versus low adherence to sustainable diets in relation to overweight.The black square and the respective line represent the risk estimate and corresponding 95% confidence interval (CI) for each study.The diamond represents the summary relative risk (RR) with the corresponding CI for risk of overweight based on all studies combined.I 2 , heterogeneity among studies; p, p-value (statistical significance).
To address climate and environmental challenges comprehensively, a combination of sustainable practices, including organic farming, along with policies, technology, and consumer choices is essential. 48[51] Our research provides a basis for emphasizing the beneficial aspects of transforming food systems and diets from a public and planetary health perspective.The biologic mechanisms through which sustainable diets protect against adiposity include their large content of fruits and vegetables and their richness in fiber, which enhances satiety. 52Furthermore, the nutritional value of sustainable and organic food is possibly higher due to its higher content of vitamins, minerals, antioxidants, and anti-inflammatory compounds, which may prevent weight gain. 534][55] Finally, proponents of a sustainable diet possibly live more health-consciously in general and therefore benefit their overall health, independent of the specific ingredients contained in organically produced foods. 56While most studies included in our analysis adjusted for lifestyle factors such as physical activity or total energy intake, the results of our stratified subanalyses indicated that the relation of high versus low levels of sustainable diet and weight change was not modified by adjustments for physical activity or total energy intake.
There are some limitations to our study.First, our systematic literature research resulted in a relatively small number of studies included, with European countries being predominantly represented.
Also, the number of prospective cohort studies in this area is currently still relatively small, and findings indicated weaker associations between sustainable diets and overweight or obesity in cross-sectional than cohort studies.To confirm our findings, further high-quality research considering all regions of the world equally using different study designs is required.
Second, all studies assessed dietary habits primarily using selfadministered questionnaires (e.g., food frequency questionnaires and 24HR) or interviews, instruments potentially susceptible to recall bias, selective reporting, and socially desirable behavior.
T A B L E 2 Stratification criteria, number of included relative risks (RRs), point estimates and 95% confidence intervals (CI) of RRs, difference between included studies, and results of random-effects meta-regression meta-analysis for each subgroup.Third, the identified studies that assessed obesity showed a moderate to high overall risk of bias, mainly due to potential confounding, known limitations of questionnaire-based dietary assessments as mentioned above, and bias due to missing data without further consideration sensitivity analyses.Given the moderate to high risk of bias, the overall certainty of the evidence according to GRADE was rated as "low." Fourth, sustainability itself lacks a universally agreed-upon definition.Therefore, every study included utilized a different method to classify sustainable diet, and organic agriculture was assumed to be sustainable, potentially resulting in high between-study heterogeneity.
Although organic food consumption seems to be associated with more sustainable diets, 16 the higher monetary costs of organic food is not in line with the Food and Agriculture Organization definition of sustainable diets.Furthermore, the sustainability of organic food can vary depending on factors such as location, crop type, and farming methods.Water and arable land requirements might even be higher than conventional farming methods. 57,58fth, dietary patterns change over time, so a single measurement of diet at baseline may not fully reflect possible changes of an individual's diet during follow-up or the study period.
Furthermore, our influence diagnostics showed that the Seconda et al. study 22 exerted a modestly disproportionate influence on the pooled obesity risk.That impact was partially explained by the study's comparatively small risk estimate, even though the overall weight of that study was small as indicated by the forest plot.
Our systematic review and meta-analysis has numerous important strengths.We conducted a systematic search in two large databases with a priori defined search terms and extraction of relevant information from included studies.This resulted in a large number of participants included.
To our knowledge, the present study is the first meta-analysis on the association of sustainable diets and the risk of overweight and obesity that utilized the E-value as a criterion for possible undetected confounding.All included studies addressed potential confounding by adjusting their risk estimates for relevant variables.Also, an unobserved confounder needed to be associated both with sustainable diets and the risk of overweight and obesity with a risk ratio of 2.26 and 2.66, respectively, which is rather strong compared with the measured confounding variables.Even if the observed E-values support a true exposure-outcome association, the presence of an unmeasured confounder linked to sustainable diets and overweight and obesity cannot be completely ruled out. 31e previous meta-analysis examined the association between organic food consumption and obesity and showed a modest 11% reduction in the risk of obesity.This meta-analysis included one study for which no full-text is available. 59To calculate a reasonable risk of bias analysis and to obtain valid results, we decided to exclude this study in our analyses.Also, that meta-analysis contained only four obesity risk estimates, whereas we added studies addressing planetary health and sustainability and organic food, resulting in a total of nine obesity risk estimates.Additionally, we did not restrict our outcome to obesity but, rather, included six risk estimates for overweight, which is far more prevalent than obesity.That resulted in a more comprehensive and meaningful analysis of the association between sustainable diets and adiposity risk.
Promoting sustainable and healthy diets is key to combatting the global syndemic or worldwide extent of obesity, undernutrition, and climate change.Not only does the co-occurrence of malnutrition and climate change but also their bidirectional relationship leads to mutual amplification. 7Specifically, climate change can affect food production and availability, which can impact food choices and dietary patterns through changes in temperature and precipitation patterns.
Conversely, obesity and consumption of high-energy, processed foods can cause substantial GHG emissions. 60ansition toward more sustainable diets is complex as nutritional behavior heavily depends on food environments, including economic and political factors, resulting in circumstances where unhealthy and nonsustainable foods are often cheaper and more accessible than healthier options. 61Also, national food-based dietary guidelines widely lack sustainability or planetary health factors, with currently only 17% of the world's population being covered by food-based dietary guidelines that address environmental sustainability. 62The actual application of sustainable diets is therefore dependent on transformation and improvements in food environments as part of a comprehensive strategy to ensure a healthier and more sustainable future. 7 conclusion, the current systematic review and meta-analysis Overall, we pooled 15 risk estimates (N = 6 for overweight and N = 9 for obesity) from eight studies and found a statistically significantly reduced risk of overweight or obesity for high versus low adherence to sustainable diets.The summary risk estimates were RR = 0.69; 95% CI, 0.62-0.76for overweight and RR = 0.61; 95% CI, 0.47-0.78for obesity.There was large heterogeneity between studies (I 2 = 67.3%for overweight; I 2 = 98.1% for obesity) (Figures2 and 3 ).

3. 4 |
Sensitivity and stratified analysesFindings of the risk of bias assessment are summarized in Data S4.Two of the eight studies measuring obesity as outcome showed high overall risk of bias due to confounding (domain 1), limitations in measuring dietary habits (domain 2), and handling of missing data (domain 5).

F I G U R E 1
Study selection illustrated on PRISMA 2020 flow diagram.23

F I G U R E 3
Forest plot of random-effects (RE) meta-analysis of adjusted risk estimates of high versus low adherence to sustainable diets in relation to obesity.The black square and the respective line represent the risk estimate and corresponding 95% confidence interval (CI) for each study.The diamond represents the summary relative risk (RR) with the corresponding CI for risk of obesity based on all studies combined.I 2 , heterogeneity among studies; p, p-value (statistical significance).
show a decreased risk of overweight and obesity with high adherence to sustainable diets.Our findings make a potentially important contribution to the collective improvement of planetary health and public health through diet.Given the potential cobenefits of sustainable diets for health, climate protection, and sustainable development, more high-quality research is needed to strengthen the evidence, particularly for the PHD.Sustainable food systems are crucial for sustainable development and for fostering human and planetary health.AUTHOR CONTRIBUTIONSChristophReger, Anja M. Sedlmeier and Carmen Jochem conceived and designed the study and interpreted the data.Christoph Reger, Anja M. Sedlmeier and Carmen Jochem did the statistical analyses and accessed and verified the underlying study data.Christoph Reger wrote the first draft with input from Anja M. Sedlmeier and Carmen Jochem All authors critically revised the manuscript for intellectual content.All authors had full access to the data in the study and had final responsibility for the decision to submit for publication.
cross-sectional studies;(2)were carried out in generally healthy participants; (3) defined sustainable diets as exposure in a reasonable and reproducible way using a sustainability measure and considered BMI-defined overweight/obesity as the primary outcome; (4) provided a relative risk (RR) or odds ratio (OR) and 95% confidence intervals (CI) for the highest versus the lowest levels of sustainable diet/organic food consumption;(5)were published before November23, 2023; and (6)were written in English language.Studies assessing solely plant-based (such as vegetarian, Study characteristics of four cross-sectional studies and four cohort studies of sustainable diets and the risk of overweight and obesity.
T A B L E 1