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ATLANTA -- Eating vegan or vegetarian may help obese patients lose more weight than dietary patterns that allow limited amounts of meat, researchers reported here.

In a randomized trial, patients assigned to a vegan diet or a vegetarian one shed significantly more pounds over 2 months than those who followed a typical omnivorous diet -- about 5% of body weight compared with a 2% loss, Brie Turner-McGrievy, PhD, MS, RD, of the University of South Carolina, and colleagues reported at Obesity Week.

Action Points Note that this study was published as an abstract and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.

After those 2 months, patients were allowed to add whatever foods they wanted back into their diets -- and still the vegan and vegetarian groups dropped more weight by 6 months than omnivores (about 8% and 6%, respectively, compared with 3%).

Turner-McGrievy said the findings "point to a potential use of plant-based eating styles in the prevention and treatment of obesity and related chronic diseases," adding that these are eating patterns -- not "diets" in the traditional sense of the word -- and they don't involve calorie restriction.

That could make it easier for patients to stick to, potentially translating into better results, she said. They simply emphasize a low-fat, low glycemic-index eating pattern that ultimately leads to improved metabolic parameters.

But some experts cautioned that any diet that limits food choices could be diminishing caloric intake -- which could be responsible for the weight-loss effects.

Reducing "the number of food choices, you will reduce food intake," said Donna Ryan, MD, professor emeritus at Pennington Biomedical Research Center in Baton Rouge, La., adding that it's hard to tell whether it is "some quality of the diet versus the restriction in choices that is having the effect."

David Katz, MD, MPH, of Yale's Prevention Research Center, agreed.

"By restricting choices the most, the vegan diet undoubtedly cut calories the most," Katz told MedPage Today. "It's pretty straight-forward: [if you] apply rules, set limits, restrict choices, and impose some discipline where there was none, food intake goes down."

Even so, several other epidemiological studies have found benefits for vegan and vegetarian diets, including a lower risk of type 2 diabetes, certain types of cancers, and all-cause mortality.

But no trial has looked at the effects of specific eating patterns on weight loss -- so Turner-McGrievy and colleagues launched the New Dietary Interventions to Enhance the Treatments for Weight Loss (New DIETs) Study, focusing on five eating patterns.

They enrolled 63 overweight and obese patients, mean age 49, mean body mass index 35 kg/m2, the majority of whom were white (79%) and female (73%).

All five groups were told to opt for low-fat, low-glycemic-index foods -- but they didn't have to restrict calories.

Omnivores could eat all meats, semivegetarians had to limit their red meat and poultry intake, pesco-vegetarians could have fish, vegetarians could eat eggs and dairy but no other animal products, and vegans had to forego all animal products, focusing instead on grains, beans, fruits, and vegetables.

During the first 2 months of the study, participants had to stick to those diets, but for the final 4 months, they could add back whatever foods they desired.

The vegan, vegetarian, pesco-vegetarian, and semivegetarian groups had weekly group meetings, while the omnivorous group had monthly meetings plus weekly newsletters and email correspondence. All groups were given diet information handouts and recipe books.

Overall, all groups lost weight at both 2 months and 6 months compared with baseline -- but vegans and vegetarians lost the most by 2 months (about 5% of body weight), which was significantly greater from the 2% lost by omnivores (P<0.05).

Pesco-vegetarians also shed significantly more pounds than omnivores at that time point, about 4% of body weight (P<0.05).

By 6 months, when patients were able to put desired foods back into their diets, the vegan and vegetarian groups had lost significantly more weight than the other three groups (8% and 6% versus 3% to 4%, P<0.05).

As for macronutrients, protein intake stayed the same across groups, but the vegan group had the greatest increase in carbohydrate intake and the biggest drop in total fat and saturated fat intake -- results that were closely mirrored by the vegetarian diet.

Vegans and vegetarians also had the greatest declines in cholesterol, Turner-McGrievy noted.

Although 90% of patients made it through the first 2-month phase of the trial and 80% the full 6 months, adherence wasn't great: 50% of patients faithfully stuck to each of their diets, with the exception being the vegetarian diet, which had about 80% adherence.

But Turner-McGrievy noted that the cheating was modest: "Our vegan group said they just wanted a little bit of parmesan cheese. The vegetarian group said they just wanted to add a little fish. The semivegetarians said they just wanted to add a little red meat."

"Everyone wanted to bump up just a little bit more," she said, noting, however, that weight loss patterns were still the same when focusing specifically on those who were nonadherent in those first two months.

That suggested to her that assigning a stricter diet to begin with could make patients more compliant with the tenets of these various plant-based diets.

"The thinking is that if you're a nonadherent semivegetarian, you'll move back over to being omnivorous," she said. "But if you're a nonadherent vegan, you may go back over to pesco-vegetarian."

Turner-McGrievy concluded that the results need to be confirmed in a larger trial for a longer period of time in a more diverse sample before any recommendations can be made.

Ryan also noted that the group should look further into the effects of their counseling programs, since the omnivores in this study had much less counseling than the other four groups: "How can you be sure it's an effect of the diet, and not the behavioral counseling?"