ATLANTA – Vegetarian and vegan diets that did not emphasize counting calories led to more weight loss over 8 weeks and at 6 months than did diets that included meat, according to data presented at Obesity Week.
"We’re not trying to get people to lose weight just so they can look attractive," said Gabrielle Turner-McGrievy, Ph.D., in an interview. "We’re hoping they will choose a healthy body weight to prevent diabetes and cancer."
The results are from the first randomized study to directly compare the effect of vegan, vegetarian, and omnivorous diets, without caloric restrictions, on body weight, said Dr. Turner-McGrievy, of the department of health promotion, education, and behavior at the Arnold School of Public Health at the University of South Carolina in Columbia.
In the New DIETs (New Dietary Interventions to Enhance the Treatments for Weight Loss) study, 63 adults (73% female, 79% white) with a mean age of 49 years and a mean body mass index of 35 kg/m2 were randomly assigned to one of five diets: vegan, vegetarian, pesco-vegetarian, semi-vegetarian, and omnivorous*. All five dietary approaches emphasized low-fat, low-glycemic-index foods but did not require caloric restriction. All participants attended weekly meetings to learn more about the type of diet they had been assigned. All diets stressed consumption of foods that were as unprocessed as possible. Participants who screened positive for eating disorder–like behavior were not included in the study.
An intent-to-treat analysis showed that at 8 weeks, the groups that followed a plant-based diet had lost an average of 8-10 pounds, while those consuming some meat lost an average of 5 pounds.
At 6 months, the vegans had lost about 7% of their body weight, the semi-vegetarian group had lost about 4% of their body weight, the pesco-vegetarian group had lost about 3%, and the omnivorous group had lost about 3%, Dr. Turner-McGrievy reported.
"All groups lost a significant amount of weight without having to count calories, but it was greatest in the vegan group," she said. Compliance in the vegan group at 8 weeks averaged about half; at 6 months, it had dropped to 30%.
"Our message here is that adherence may not be completely necessary," she said. "We shifted people so far down the dietary spectrum that even if they added a little cheese or meat here or there, they still weren’t going back to where they were before."
The exact reason for the greater weight loss in the plant-based diet groups was not clear, said Dr. Turner-McGrievy, but she and her colleagues theorized that changes in macronutrient content, which were found to be significantly different across groups (P < .05), may be a factor. "When we looked at the nutritive changes of these five different diets, we saw a much greater decrease of saturated fat among the vegan diet group," she said. "Vegetarians and vegans also decreased their cholesterol more than the other groups."
When Dr. Turner-McGrievy was asked whether any groups consumed more food, on average, than other groups, she said her team had not determined if there were different levels of hunger between the groups. Study participants were enthusiastic about the "freedom" they had from calorie counting: "On calorie-restricted diets, if at the end of the day you’ve reached your calorie allotment, but you’re still hungry, then you go to bed hungry," she said.
Despite the limited sample size and duration of the NEW DIETs study, Dr. Turner-McGrievy said one of the study’s implications is that when it comes to treating obesity and its attendant comorbidities, there can be a focus on nutrition. "It moves this area of science forward and shows there is another approach that can be used instead of calorie restriction," she said. People with obesity could also be placed on a "stepwise" system, where they start out eating slightly less meat, and eventually move to a more plant-based diet, she noted.
Obesity Week was presented by the Obesity Society and the American Society for Metabolic and Bariatric Surgery.
Dr. Turner-McGrievy did not have any relevant disclosures.
*Correction, 12/11/2013: An earlier version of this article misstated one of the diets included in the New DIETs study.