(P14-078-23) Relationships Between Vegetarian or Vegan Dietary Patterns and Body Mass Index in the General Population: An Umbrella Review of Systematic Reviews

Objectives: To determine the association between a vegetarian or vegan dietary pattern and body mass index (BMI) in adults in the general population.

Methods: In this umbrella review, five databases were searched to identify systematic reviews (SRs) published from 2018 to January 2023 that reported the relationship between a vegetarian or vegan dietary pattern, compared to a mixed or non-vegetarian diet, and BMI. SRs were excluded if they targeted adults with specific risk factors or disease. Abstracts and full-text articles were screened in duplicate. SRs that assessed risk of bias (ROB), graded certainty of evidence (COE) and conducted meta-analysis were prioritized when interpreting evidence. If not available in included SRs, COE was determined using the GRADE method. Quantitative results are reported as mean difference (95% confidence interval).

Results: A total of 534 abstracts were identified and screened and 73 full-texts were reviewed. In preliminary results, five SRs met eligibility criteria, all of which examined the relationship between a vegan diet and BMI. No SRs assessed COE and two SRs conducted meta-analyses for the outcome of BMI. Three SRs focused on trials (n=11) and two SRs focused on observational studies (n=67). In the SR that synthesized RCTs quantitatively and assessed ROB of included studies, a vegan diet reduced BMI compared to no intervention (no pooled data; mean reduction from -1.03 to -3.90 kg/m²) or compared to another health-promoting diet [-0.5 kg/m² (-0.76, -0.27)], and other SRs focusing on trials reported similar findings. In the SR synthesizing observational studies quantitatively and assessing ROB, vegans had a lower BMI than omnivores [-1.72 kg/m² (-2.52, -1.32)]. The other SR analyzing observational studies reported similar findings and ROB assessment. COE  was preliminarily graded as low for the outcome of BMI due to the risk of bias of included studies, small sample sizes in trials, and observational study designs.

Conclusions: Evidence from recent SRs suggests that vegans have a lower BMI compared to individuals following mixed or non-vegetarian diets (evidence certainty: low). Evidence was not available to evaluate the impact of vegetarian diets that were not vegan.

Funding Sources: Academy of Nutrition and Dietetics and Foundation and the Vegetarian Nutrition Dietetics Practice Group