The study, “How Plant-Based Do We Need to Be to Achieve Weight Loss?” divided overweight participants into five dietary groups: vegan, vegetarian, pesco-vegetarian, semi-vegetarian, and omnivorous.

All of the diets emphasized low fat and low –glycemic-index foods but did not require participants to restrict their calorie intake. Here’s how the groups compared in terms of mean percent loss of body weight:

• Vegan: -4.8 percent
• Vegetarian: -4.8 percent
• Pesco-Vegetarian: -4.3 percent
• Semi-Vegetarian: -3.7 percent
• Omnivore: -2.2 percent

Following up after six months, the vegans and vegetarians had lost 8 percent and 6 percent total body weight respectively compared to 3 percent for the dieting omnivores. A semi-vegetarian, also called a “flexitarian,” is someone who eats mostly plants and dairy but will occasionally eat meat.

Lead researcher Brie Turner-McGrievy, PhD, said that because the nonmeat meals were already less caloric (and contain less saturated fat), some dieters may find vegan and vegetarian eating patterns easier to adopt for weight loss without the burden of obsessively counting calories.

Dr. Turner-McGrievy noted that only 50 percent of the participants in the study were able to strictly adhere to their designated diets, but she described the “cheating” as minimal.

“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 semi-vegetarians said they just wanted to add a little red meat," she said. “Everyone wanted to bump up just a little bit more.”

Dr. Turner-McGrievy hypothesized that initially aiming for a strict vegan or vegetarian lifestyle will ultimately result in more plant consumption than before—even if there is cheating.

"The thinking is that if you're a nonadherent semi-vegetarian, 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."