Obesity and Weight Management

Key Points

• Background: Not all plant-based diets are created equal. A vegetarian/vegan diet could contain high amounts of unhealthy, energy-dense foods that promote obesity. Therefore, the diet’s composition is paramount for weight management.
  
  
• A systematic review and meta-analysis of 86 cross-sectional and 10 cohort prospective studies was unable to find any longitudinal studies examining weight as a primary outcome but did identify cross-sectional case-control studies.1 They found that those following a vegetarian diet had a body mass index (BMI) about 1.5 kg/m2 lower and vegans about 1.7 kg/m2 than omnivores.1 However, cross-sectional observational studies are subject to reverse causality and residual confounding so use caution when interpreting results. Longitudinal randomized control trials investigating the impact of a plant-based diet on weight loss/management are needed.
  
  
• A review of three prospective observation cohort studies (the Nurses’ Health Study [NHS], NHS2, and the Health Professionals Follow-up Study) which included 126,982 adults.2 They found that those following a healthy plant-based diet with whole grains, fruits/vegetables, nuts/legumes, tea/coffee, and vegetable oils had less weight gain over a 20-year period than those consuming an unhealthy plant-based diet with refined grains potatoes/fries, sweets, and sugar-sweetened beverages. However, this study relied on self-reported dietary intakes (collected every four years) and weight gain for analyses. Self-reported data are notoriously unreliable because there is a high degree of subject bias. The long periods between data collection also increase the likelihood of error. Objective and highly controlled measures of intake (e.g., biochemical analyses, calorimetry, etc.) are needed.
  
  
• A prospective cohort in Europe of 11,554 participants with normal weight (BMI<25 kg/m2) at baseline found that greater adherence to a plant-based diet (not necessarily entirely plantbased) significantly reduced the risk of developing overweight or obesity by 18% after a median follow up of 10.3 years (HR: 0.82; 95%CI: 0.72,0.95; P=0.001).3 The dietary pattern that emphasized less healthy plant foods (i.e., saturated fats, and refined grains) showed a 7%, nonsignificant increase in risk (P=0.36).
  
  
• A meta-analysis of 12 randomized control trials including a total of 1151 participants found that vegetarian diet groups without energy restriction lost about 2.0 kg more than nonvegetarian diet groups.4 Also, vegan diet groups lost about 2.5 kg more than lacto-ovo-vegetarian groups. However, weight loss attenuated in participants with follow-up beyond 1 year. Longer-lasting prospective studies are needed to gain a clearer understanding of the efficacy of plant-based diets for weight loss and management.
  
  
• Overall results: The weight loss achieved in these trials is similar to the amount achieved by other omnivorous diets that are high in fiber and whole grains.5,6

Conclusion

Calorie restriction with an emphasis on nutrient-dense foods over energy-dense foods is necessary for weight loss and management. A healthy plant-based diet primarily comprised of fiber and plant-based proteins can be lower in calories than the typical “Western” diet and therefore can reduce the risk of developing obesity and can aid in weight loss. However, evidence is lacking to support a complete restriction of animal foods for reducing obesity incidence beyond what could be achieved by a modest reduction of animal foods coupled with high-fiber plant foods. Nonetheless, a plant-based diet is not associated with weight gain. Therefore, a plant-based diet, including one with modest restrictions of animal foods, can be beneficial to curb obesity incidence and promote healthy weight management practices.

 References

1. Dinu M, Abbate R, Gensini GF, Casini A, Sofi F. Vegetarian, vegan diets and multiple health outcomes: A systematic review with meta-analysis of observational studies. Crit Rev Food Sci Nutr. 2017;57(17):3640-3649. doi:10.1080/10408398.2016.1138447
   
   
2. Satija, Ambika, et al. “Changes in Intake of Plant-Based Diets and Weight Change: Results from 3 Prospective Cohort Studies.” The American Journal of Clinical Nutrition, vol. 110, no. 3, 2019, pp. 574–82, https://doi.org/10.1093/ajcn/nqz049.
   
   
3. Gómez-Donoso, Clara, et al. “Healthful and Unhealthful Provegetarian Food Patterns and the Incidence of Overweight/Obesity in the Seguimiento Universidad De Navarra (SUN) Cohort (OR33-05-19).” Current Developments in Nutrition, vol. 3, no. Suppl 1, 2019, p. nzz039.OR33-05-19-, https://doi.org/10.1093/cdn/nzz039.OR33-05-19.
   
   
4. Huang, Ru-Yi, et al. “Vegetarian Diets and Weight Reduction: A Meta-Analysis of Randomized Controlled Trials.” Journal of General Internal Medicine : JGIM, vol. 31, no. 1, 2016, pp. 109–16, https://doi.org/10.1007/s11606-015-3390-7.
   
   
5. Hjorth, Mads F., et al. “Personalized Dietary Management of Overweight and Obesity Based on Measures of Insulin and Glucose.” Annual Review of Nutrition, vol. 38, no. 1, 2018, pp. 245–72, https://doi.org/10.1146/annurev-nutr-082117-051606.
   
   
6. Suhr, J., et al. “Wholegrain Rye, but Not Wholegrain Wheat, Lowers Body Weight and Fat Mass Compared with Refined Wheat: A 6-Week Randomized Study.” European Journal of Clinical Nutrition, vol. 71, no. 8, 2017, pp. 959–67, https://doi.org/10.1038/ejcn.2017.12.

Key Points

• Objective: The purpose of this review was to evaluate intervention studies assessing the effects of different plant-based diets on BMI and weight.
  
  
• Methods: Studies were included if they implemented an intervention, outcomes included changes in weight and/or BMI, calories were not restricted in both study groups, a vegetarian or vegan diet was part of the intervention, the study duration was at least 4 months and only included adults >18 years. Twenty-two publications from 19 studies were included, only 3 were not randomized controlled trials.
  
  
• Results: Eight of the 22 articles reported a significant reduction in body weight and/or BMI in the vegan/vegetarian group alone. Nine other articles revealed a significantly greater reduction of one of the outcomes in the vegan/vegetarian groups compared to a control group.

Conclusion

The authors concluded that changing to a plant-based diet may be beneficial toward weight and BMI in people with overweight, type 2 diabetes, cardiovascular disease, or rheumatoid arthritis. Weight loss was attributed to increased intakes of fiber, polyunsaturated fats, and plant proteins in addition to reduced energy intake, saturated fats, and animal proteins. Many of the studies in this systematic review restricted fat and/or energy intake, and therefore, the observed improvements in weight/BMI cannot be entirely attributed to the plant-based diet mechanisms. The varying macronutrient composition and energy content likely also contributed to the observed effects.

Editor's Note: In general, there is a positive effect of plant-based diets on weight however, several studies found that calorie-restricted diets are just as effective as vegan/vegetarian diets. Increased dietary fiber is known to slow gastric emptying and thereby promote satiety. A plant-based diet that is high in fiber and limited in unsaturated fat and refined grains can aid in reducing caloric intake and thereby promote weight loss.

Key Points

• Objective: The purpose was to conduct a systematic review of prospective cohort studies among adults to examine associations between different plant-based diets and obesity risk. They further aimed to explore the characteristics of a plant-based diet that are most protective for healthy body composition.
  
  
• Methods: Studies were eligible for inclusion if they included only adults >18 years, primary outcomes were weight and/or BMI or other indicators of adiposity (i.e., body fat %, waist circumference), and the study examined the effects of a plant-based diet compared to another diet with less plant-based food consumption. Nine cohort studies were included.
  
  
• Results: Adiposity changes were assessed by either body weight (5 cohorts), body mass index (BMI, 2 cohorts) waist circumference (3 cohorts), or body fat (2 cohorts). The authors concluded that high-quality plant-based diets that emphasize fiber, whole grains, and unsaturated fats and are minimally processed are favorably associated with weight management. However, some of the studies relied on self-reported dietary intake measures which are unreliable and introduce
  measurement error.

Conclusion

A healthy plant-based diet is more easily adopted across different cultures, socioeconomic statuses, knowledge, and preferences than other well-known healthy diets e.g., Mediterranean and dietary approaches to stop hypertension (DASH) diets. Therefore, it may be easier to adhere to and more feasible for widespread implementation. Future research should focus on the acceptability, feasibility, and adherence to a plant-based diet in more diverse populations and geographical locations. Genetic and metabolic phenotypic interactions should also be explored to elucidate associations with weight management.

Future Directions

Based on the findings from this topical review, researchers may consider the following suggestions for future research.

Study Duration: Longitudinal randomized control trials investigating the impact of plant-based diets on weight loss and weight maintenance are needed.

Dietary Intake Measures: More studies using objective and highly controlled measures of intakes (i.e., biochemical analyses, calorimetry, etc.) are needed.

Prospective Studies: Longer-lasting (>6 months) prospective studies are needed to gain a clearer understanding of the efficacy of plant-based diets for weight loss and maintenance.

Long-Term Adherence: Studies lasting >6 months should be conducted to investigate the effects of long-term adherence on obesity risk and incidence.

Diverse Populations: Racial, ethnic, age, and gender differences in obesity risk and incidence should be explored in western and non-western populations.

Nutrigenomics and Metabolomics: Genetic and metabolic phenotypic interactions should also be explored to elucidate associations between plant-based diets and obesity, weight loss, and weight maintenance.