Comparative effectiveness of plant-based diets for weight loss: A randomized controlled trial of five different diets

Objective: The aim of this study was to determine the effect of plant-based diets on weight loss.

Methods: Participants were enrolled in a 6-mo, five-arm, randomized controlled trial in 2013 in South Carolina. Participants attended weekly group meetings, with the exception of the omnivorous group, which served as the control and attended monthly meetings augmented with weekly e-mail lessons. All groups attended monthly meetings for the last 4 mo of the study. Diets did not emphasize caloric restriction.

Results: Overweight adults (body mass index 25–49.9 kg/m2; age 18–65 y, 19% non-white, and 27% men) were randomized to a low-fat, low-glycemic index diet: vegan (n = 12), vegetarian (n = 13), pesco-vegetarian (n = 13), semi-vegetarian (n = 13), or omnivorous (n = 12). Fifty (79%) participants completed the study. In intention-to-treat analysis, the linear trend for weight loss across the five groups was significant at both 2 (P< 0.01) and 6 mo (P < 0.01). At 6 mo, the weight loss in the vegan group (−7.5% ± 4.5%) was significantly different from the omnivorous (−3.1% ± 3.6%; P = 0.03), semi-vegetarian (−3.2% ± 3.8%; P = 0.03), and pesco-vegetarian (−3.2% ± 3.4%; P = 0.03) groups. Vegan participants decreased their fat and saturated fat more than the pesco-vegetarian, semi-vegetarian, and omnivorous groups at both 2 and 6 mo (P < 0.05).

Conclusions: Vegan diets may result in greater weight loss than more modest recommendations.


Alex’s Notes: Nearly all diets contain some amount of plants. On this spectrum, anchoring one end are vegan diets that exclude all animal-based products, and anchoring the other end are omnivorous diets that do not exclude any foods. Along the way from one end to the other we come across various vegetarian diets. Epidemiological evidence suggests that vegans and vegetarians have less weight gain and a reduced incidence of type-2 diabetes compared to omnivores. However, no study has yet compared the various diets under experimental conditions.

The current study thus set out to determine how these diets fared in a two-month weight loss intervention and four-month follow-up period. Overweight and obese adults interested in losing weight were eligible for inclusion, and ultimately 62 middle-aged men and women were randomized to five different diets. The average BMI ranged from 32 to 35 kg/m2 between the groups, but there was no significant statistical difference. The five diet categories are shown in the table below.

Dietary Group Definitions of diet patterns
Vegan Does not contain any animal products (meat, fish, poultry, eggs, or dairy) but emphasizes plant-based foods, such as fruits, vegetables, whole grains, and legumes/beans.
Vegetarian Does not contain meat, fish, or poultry but does contain eggs and dairy, in addition to plant-based foods, such as fruits, vegetables, whole grains, and legumes/beans.
Pesco-vegetarian Does not contain meat or poultry but does contain fish and shellfish, eggs, and dairy, in addition to plant-based foods, such as fruits, vegetables, whole grains, and legumes/beans.
Semi-vegetarian Contains all foods, including meat, poultry, fish and shellfish, eggs, and dairy, in addition to plant-based foods, such as fruits, vegetables, whole grains, and legumes/beans. However, red meat is limited to once per week and poultry is limited to ≤5 times per week.
Omnivorous Contains all food groups.

After randomization to the diet groups, the participants received a handout that provided details about their assigned diet. Self-monitoring dietary intake was not required and was never discussed with the participants. In fact, that subjects were told they could eat whatever they wanted so long as they adhered to their dietary templates, but that they should limit fast foods and processed foods. All groups attended weekly one-hour meetings for the two-month weight loss period, with the exception of the omni-group, which only received weekly emails and attended a one-hour meeting after four and eight weeks in order to serve as a “minimal interaction” group. Of course when one is trying to compare the effectiveness of various diets, this hardly seems fair.

Questionnaires assessed dietary intake from two days of unannounced 24-h dietary recalls, and physical activity at baseline, after the weight-loss intervention (2-months), and after the follow-up period (6-months).Dietary adherence was measured as the absence of any proscribed foods from the dietary recalls (e.g. absence of meat, dairy, and eggs from vegan participants' food records). Participants in the omni-group were considered adherent if their percent energy from fat was ≤40%.

Focusing on weight

When looking solely at the weight of the participants, it becomes apparent that the vegan group had the largest reductions in weight at both two and six months. At the two month mark, the vegan weight loss (-4.8%) was significantly different from the omni-group (-2.2%) only. The vegan weight had continued to decline and by the six month mark their weight lost (-7.5%) was significantly different from the pesco-veg (-3.2%), semi-veg (-3.2%), and omni-group (-3.1%) weight lost. The vegetarian group followed a similar trend as the vegan group, with identical weight loss at two months and just over 6% weight loss at six months, but the study was not powered to detect statistical differences between this group and the vegans.

Another interesting point is that the vegan, vegetarian, and omnivore groups showed a continuous decline in weight throughout the entire six months, whereas the semi- and pesco-vegetarian groups had an increase in weight from two to six months, ending right around the omni-group. I have no explanation for this other than perhaps physical activity. Adjusting for baseline levels, there was no significant difference between the self-reported intentional physical activities of the groups (represented as kcal/day) at two or six months. However, from two to six months there was an increase in the reported physical activity of the vegan, vegetarian, and omnivore groups, but a reduction in activity for the semi- and pesco-vegetarian groups, which could explain the weight rebound.

Changes to the diet

The changes in weight can also be explained by changes in the diet. According to baseline dietary questionnaires, the participants went into their interventions with a diet composed of 16-17% energy as protein, 35-40% fat, 40-45% carbohydrates, and 15-20g of fiber daily. At six months, the vegan group (-903 kcal) had a significantly greater reduction in caloric intake than the omni-group (-194 kcal) and trended for a difference from the pesco- (-327 kcal; p=0.08) and semi-vegetarian (-397 kcal; p=0.12) groups. At the two month mark, changes between the groups was not significant despite the vegan group (-786 kcal) having a 72% greater caloric reduction than the omni-group (-455 kcal).

Some of the weight loss in the vegan group may also be attributable to a loss of lean body mass, as evidenced by the significant reduction of protein intake. This is of course only speculation on my part, as the researchers did not perform any testing of body composition. At two months, the vegans were consuming a significantly lower percentage of energy as protein (~13.5%) compared to the other groups. Worse yet, the protein is entirely plant-based, making it highly likely that it is lacking in several essential amino acids (unless they consumed a ton of soy). There was actually a statistically significant trend among the group for protein intake to increase as consumption of greater amounts of animal products was increased, ending with the omnivorous diet consuming about 19.5% their calories as protein.

Some may think that this goes against the beneficial effects of a higher protein diet for weight loss, but I would argue otherwise. It seems reasonable to assume that the vegan group lost more weight because of a greater caloric deficit (both dietary and through greater physical activity) coupled with lower protein intake that may have led to losses of lean body mass. Conversely, the omnivore group increased their protein intake and demonstrated sustained weight loss through the entire six month period, albeit at a slower rate than the vegan group. If body composition testing were performed, I would bet that fat loss between the groups would have been similar, with the difference in weight coming from a loss of lean body mass in the vegan group.

Other significant trends between the groups was a decline in total fat and saturated fat intake and an increase in carbohydrate and fiber consumption as more animal products were removed (omni à semi-veg à pesco-veg à veg à vegan).

The best diet is the one you can stick with

There were no significant differences in dietary adherence at two or six months between the groups. At two months, adherence was lowest in the omnivore group (33%) followed by the vegans (50%), pesco-vegetarians (54%), semi-vegetarians (62%), and vegetarians (77%). By six months, adherence in the omnivore group actually increased (42%), whereas the vegans (33%), pesco-vegetarians (39%), semi-vegetarians (46%), and vegetarians (39%) all decreased. It is hard to discern how this may have influenced the results, as adherence was measured through unannounced 24-hour recalls of the participants, which could have simply been a bad day for someone who was otherwise very strict on their diet. Nonetheless, it does demonstrate a potential difficulty in maintaining the vegan and vegetarian diets in the long-term.

Putting it all together

This is the first study to go beyond observational trials by randomizing participants to adopt five different plant-based eating styles under experimental conditions. The weight loss achieved in this study occurred without the need for dietary self-monitoring, which can be burdensome to some individuals. Additionally, the dietary changes focus on removal of foods groups, which does not necessarily require complex processing (a food is either okay to eat or not okay). Thus, it appears that adopting more general dietary templates that focus on food groups can lead to beneficial weight outcomes without the need for more tedious food tracking.

The authors zone in on the vegan diet, suggesting that it may have a potential use in the treatment of obesity. They reference the greater weight loss and what they deem “favorable” changes in macronutrient consumption. This conclusion warrants skepticism. As mentioned earlier, body composition was not measured and it is entirely possible for the vegans to have lost considerable lean body mass. Similarly, since food consumption was not reported, we have no way of knowing if the vegan lifestyle led to greater consumption of fruits and vegetables, or simply an elimination of animal products and increased reliance on “vegan” cookies and crackers. Weight is only one aspect of health.

Nonetheless, the use of plant-based dietary approaches for weight loss has public health appeal. It must be emphasized again that there was no restriction on energy intake recommended to any of the groups in the study and participants were free to eat until they were satisfied.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 over the next four months. Additionally, plant foods are awesome for health, so having persons adopt a plant-based diet (at least, temporarily) would have benefits that extend beyond weight loss.


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