Applied nutritional investigationComparative effectiveness of plant-based diets for weight loss: A randomized controlled trial of five different diets
Introduction
Well-planned vegan and vegetarian diets can provide adequate nutrition, and have demonstrated health benefits in disease prevention and treatment [1]. Vegan and vegetarian diets have been used effectively for weight loss and maintenance [2], [3]. Anchoring the two ends of the plant-based dietary spectrum are vegan diets (exclude all animal products) and omnivorous diets (omni: no foods excluded). Between these two diets are other plant-based diets, such as semi-vegetarian (semi-veg: occasional meat intake), pesco-vegetarian (pesco-veg: excludes meat except seafood), and vegetarian (veg: excludes all meat and seafood, but contains eggs and dairy products). Several epidemiologic studies have examined differences in weight-related outcomes among these diets, finding lower body weights [4] and less weight gain over time among vegans compared with other groups [5].
These prospective cohort studies [4], [5] examining the five diets along the plant-based dietary spectrum have categorized participants according to their preexisting dietary patterns, making it difficult to account for the inherent differences that may exist among individuals who self-select different patterns. To date, there have been no randomized trials comparing the effectiveness of these five different diets on weight loss. Therefore, the goal of this study was to examine the differences in weight loss among participants randomized to adopt an omnivorous, semi-vegetarian, pesco-vegetarian, vegetarian, or vegan diet. Transitioning to plant-based diets may lead to greater increases in fiber [6], [7], [8], with high-fiber diets being associated with lower body weights in epidemiologic studies [9], and greater decreases in dietary fat [6], [7], [8], with studies showing that low-fat diets are associated with weight loss [10]. Because of these potential changes in nutrients among groups, we hypothesized that differences in weight loss would follow similar patterns seen in epidemiologic studies with weight loss being incrementally greater along the plant-based dietary spectrum from omni to semi-veg to pesco-veg to veg to vegan diets. Furthermore, we hypothesized that the vegan group would have significantly greater weight loss compared with the pesco-veg, semi-veg, and omni groups.
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Materials and methods
The New DIETs (New Dietary Interventions to Enhance the Treatments) for weight loss study was a 2-mo weight loss intervention with a 4-mo follow-up period. Recruitment and exclusion criteria are described elsewhere [11]. Briefly, overweight or obese (body mass index [BMI] 25–49.9 kg/m2) adults, who were interested in losing weight, were between the ages of 18 and 65 y with a stable medical status (e.g., no uncontrolled thyroid conditions or diabetes), and were willing to accept random
Results
Participants were screened in February 2013 and the trial was completed by August 2013. Of 219 participants who were screened, 63 (29%) were randomly assigned to a diet. At the 2-mo assessment time point, 57 (90%) of those assigned to a diet completed the body weight assessment and questionnaires, and 56 (89%) completed 2 d of dietary recalls. At the 6-mo time point, 50 (79%) completed the study (i.e., provided a body weight measurement at 6 mo), 46 completed the questionnaires (73%), and 49
Discussion
This randomized trial examined the effect of differing levels of plant-based diets on body weight and intake of macronutrients, fiber, and cholesterol. This is the first study to go beyond observational trials to intervention research by randomizing participants to adopt these five different plant-based eating styles. This randomized design allowed for a more rigorous control of factors that may affect body weight, such as exercise and education level, than can be used in observational designs.
Conclusions
This study provides evidence for greater short-term weight loss and improved macronutrient, fiber, and cholesterol intake among individuals randomly assigned to follow plant-based diets that do not include meat (vegan) compared with other plant-based approaches with limited meat (pesco-veg and semi-veg) or unrestricted meat intake (omni). Studies examining the effect of plant-based diets on long-term weight loss maintenance are needed. Diets excluding food groups have not been the norm in
Acknowledgments
The authors acknowledge the University of South Carolina's Office of Public Health Practice for assistance with survey design.
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GTM and SW designed the research study. GTM, EEW, and CRD conducted the research. EAF performed statistical analysis. GTM, SW, and EAF wrote the paper. GTM had primary responsibility for final content. The authors have no conflicts of interest to declare.