(Getty Images/iStock)

For those hoping to shed some wintertime weight gain, research suggests that going vegetarian — or even vegan — can help.

When scientists looked at the body mass index of more than 37,000 Britons of all ages in 2003, they found that while the average male meat-eater had a BMI of about 24.4, just shy of being overweight, the average vegan had a BMI of 22.4. Among women, the patterns were similar. A 2009 study of Seventh-day Adventist church members across North America showed an even more striking difference in BMI: more than five points between those on an omnivorous diet (28.8) and those eating only plant-based foods (23.6).

You could easily explain such findings by citing general differences in lifestyle choices between vegans and meat-lovers. Scientists have found that people on plant-based diets tend to be more health-oriented than the rest of the society: Vegans drink less, smoke less and tend to exercise more.

Yet experiments in which people are randomly assigned to different diets suggest that vegetarianism can be helpful in dropping extra pounds, no matter your life philosophy or your attitude toward treadmills.

However, most such studies were done on rather small samples — some on as few as 16 people. “The randomized controlled trials were not very conclusive. But when you do a meta-analysis you can look at the evidence base as a whole and really understand what’s going on,” says Wendy Bennett, a professor of medicine at the Johns Hopkins Bloomberg School of Public Health.

A meta-analysis that took into account 12 trials involving well over 1,000 adults found last year that people on vegetarian diets lost, on average, 4.4 pounds more than those following other nutritional plans. Those who ate exclusively plant-based foods and eschewed all dairy and eggs — a vegan diet — slimmed down the most — on average, 5.5 pounds.

Fighting excess pounds with plant-based eating may lead to better health. Studies have shown that following vegetarian diets lowers the risk of diabetes, heart disease, even cancer.

The question, of course, is: Why would plant-based diets be better at keeping us slender than other weight-loss regimens, including well-known ones such as the low-carb Atkins diet?

“Nothing is as important, in my opinion, as energy density and the fiber content of such diets,” says Jim Mann, a professor of nutrition at the University of Otago in New Zealand.“A strict vegetarian has to eat a mountain of food in order to have enough calories.” Experiments that focused specifically on fiber have pointed at that nutrient’s propensity for keeping unwanted pounds away.

“There are two different types of fiber — insoluble fiber and soluble fiber,” says University of South Carolina nutritionist Brie Turner-McGrievy . “Insoluble fiber is just bulk, so it physically fills you up, whereas soluble fiber has low glycemic index, so it doesn’t spike your blood sugar really high. It’s digested slowly, and so you don’t have this ‘I feel full’ and then all of the sudden 30 minutes later, ‘I’m hungry again.’ It keeps you satiated.”

On the other hand, when compared with what many American typically eat (think burgers, steak, chicken), vegan diets aren’t loaded with protein, which is often touted as a great aid in weight loss because it can help you feel full longer.

Yet according to Mann, protein has “little long-term significance” in shedding extra pounds and keeping them off. In experiments, high-protein diets do help with faster weight loss and with keeping appetite under control, but these effects tend to disappear after a year.

“In high-protein diets,” Turner-­McGrievy says, “you are depleting your glycogen stores and the water that goes along with them, so you are going to lose a lot of weight. For some, that’s very motivating — but that’s not really body fat you are losing, that’s water.” That’s not the case with vegetarian diets, which always “contain adequate carbohydrate so they wouldn’t produce the glycogen and water loss.”

Some small studies have suggested that proteins derived from such plant foods as fava beans and split peas may provide a longer feeling of fullness than animal proteins; another suggested that those on a vegetarian diet may burn more calories during a post-exercise rest. But Mann cautions against obsessing with mechanisms by which vegetarian and vegan diets might help you lose unwanted weight.

“It’s very important not to try to identify just one factor,” he says. “Because then people say, ‘Oh, I will just have that one factor.’ They will just take a fiber pill. It’s a great risk to oversimplify nutrition.”

Sometimes the most important thing in a diet is being able to stick with it, and the idea of going meatless may be off-putting or scary to those who have never tried it. “We usually tell people to try the vegetarian diet for three weeks. It’s less scary that way — you are not doing it forever, you are doing it for three weeks. But that gives you a chance to try it out and get your taste buds adjusted,” Turner says.

For those intimidated by the prospect of a vegan diet, new, familiar-looking products on the market can help the switch. There is already a plethora of vegan cheeses, Hellmann’s vegan mayo, vegan Ben & Jerry’s ice cream and vegan pizza. And Wendy’s offers a vegetarian burger in some locations.

On the other hand, as the extent of vegan offerings increases, it’s worth noting that they may not all be good for you — or your waistline. “There is a fine line between having enough choices versus making food that was once an unhealthy animal-based food into a similarly unhealthy plant-based version,” Turner-McGrievy says.