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Well-planned vegan and vegetarian diets can provide adequate nutrition, and have demonstrated health benefits in disease prevention and treatment [1]. Vegan and vegetarian di- ets have been used effectively for weight loss and maintenance [2,3]. Anchoring the two ends of the plant-based dietary spec- trum are vegan diets (exclude all animal products) and omniv- orous 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 epide- miologic 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. There- fore, 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