There are several strengths to the present study. The study was delivered with modest contact with study participants in the four plant-based groups, who received eight weekly classes, followed by four additional classes and online support via Facebook groups over 4 mo (total of 12 one-hour class sessions in
6 mo). In behavioral weight loss treatment research, weight regain is common when contact time in a study is decreased [42,
43]. In the present study, weight loss continued to occur in the vegan and veg groups despite transitioning to monthly meetings. In addition to the modest contact, other aspects of the study also make the findings applicable outside the research setting, including participants preparing all their own foods or finding meals to eat at restaurants. The study also had a low attrition rate of 21%, particularly considering that no incentive was provided at
6 mo. Most behavioral weight loss studies have attrition rates greater than 30% [44,45]. Greater weight loss occurred in the vegan group despite equal diet adherence among the groups. The present study was also conducted in the southern United States where traditional southern food preferences might result in high obesity rates greater challenges to adoption of more plant-based eating styles than other regions [46].
There are also some limitations. Although weight change was an objectively measured outcome, diet and physical activity were both self-reported, and changes in body composition, including changes in lean mass, were not assessed. The dietary data were collected by two unannounced, 24-h recalls, which is considered to be an accurate way to measure overall dietary intake [47–49]. For energy expenditure, the Paffenbarger Physical Activity Questionnaire was used and has been shown to be both valid and reliable [50,51]. Other limitations include the short duration and a sample that was mostly white and educated. The sample had a higher percentage of men, however, than is typical for behavioral weight loss programs [52–55]. Additionally, neither participants nor study personnel were blinded to diet assignment. The study was not powered to detect differences in weight loss and dietary intake between the vegan and vegetarian groups. Future research with a larger sample size will be needed to examine differences between vegan and vegetarian. Finally, the present study had reduced contact time for the omni group, which met monthly during the initial 8 wk of the study, receiving their di- etary information mainly by e-mail during that time. All groups received the same intensity of the intervention for the majority of the study (months 3–6), however, and even without the omni group included, weight loss among vegan participants was significantly greater than pesco-veg or semi-veg participants. This indicates that, even without inclusion of the omni group, there is evidence toward greater weight loss with the vegan diet compared with plant-based diets, which include some fish or meat.