According to WHO, obesity, as of 2016, is a worldwide health problem, with more than 1.9 billion (39%) adults overweight and of these, 650 million (13%) obese. Higher body mass index (BMI) is a major risk factor for cardiovascular disease, diabetes, musculoskeletal disorders, and some cancers.
Various diets have been proposed to reduce obesity and its associated health problems, including the lacto-ovo vegetarian diet (milk and eggs allowed) and the vegan diet (no animal products). As of 2016, randomized controlled trials that had looked into vegetarian diets and its effect on weight were mixed.
Researchers affiliated with Harvard University performed a meta-analysis, a study of studies, of randomized clinical trials where patients were assigned to lacto-ovo vegetarian/vegan diets or non-vegetarian diets. The authors’ systematic search yielded 1,513 studies, of which 12 met their inclusion criteria. Combined, the 12 clinical trials included 1,151 adults with ages ranging from 18 to 82 and with mean baseline BMI ranging from 25 (overweight) to 53 (grade 3 obese). Follow-up periods ranged from 8 weeks to 2 years, with a median of a little over 4 months.
The authors found that overall, those assigned to vegetarian (including vegan) diets lost significantly more weight (about 4.4 pounds) than those assigned to non-vegetarian diets. Additional analyses revealed that those assigned to vegan diets lost more (5.5 pounds compared to controls) than those assigned to lacto-ovo vegetarian diets (3.3 pounds compared to controls).
A related NPR article noted that none of the studies did a head-to-head comparison of vegetarian diets with other diets, such as low-carb diets. As such, Dr. Frank Hu, a co-author, stated that
…we can’t conclude that vegetarian diets are superior to either low-fat or low-carb diets in terms of weight loss…
The study authors stated that a possible reason for the weight loss was due to the intake of vegetables, fruits, and whole grains – all of which are high in fiber. They also noted that the long-term effects of a vegetarian diet on weight loss remain “unsettled” as most of the included trials ran for less than 12 months.
Huang, R. Y., Huang, C. C., Hu, F. B., & Chavarro, J. E. (2016). Vegetarian diets and weight reduction: a meta-analysis of randomized controlled trials. Journal of general internal medicine, 31(1), 109-116.
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