People choose to become vegetarian or vegan for many reasons, most notably for ethical or environmental reasons, or as part of cultural or spiritual practices. Another growing trend is adopting these dietary patterns as part of weight loss efforts. Population research indicates that non-meat eaters tend to be leaner than omnivourous peers, and anecdotally many people report (significant) weight loss when moving to a plant-based diet. Thus the vegetarian diet-weight loss connection was born. Some recent meta-analyses, like this one or this one, have shown that there is merit to these claims, but the effect size is small and the quality and quantity of studies analyzed was poor. The recommendation is that yes, these diets are healthful and could be recommended for weight loss but we cannot draw any broad or strong conclusions as to their true effectiveness yet.
In these sorts of scenarios, there are several mechanisms at play that may encourage weight loss. First, the intervention effect. Simply being part of a trial can change a person’s behaviour, whether or not they receive any specific instruction or dietary advice. The participants may be more willing to adhere to a vegetarian diet than they would be if left to their own devices. Knowing that their food habits and such will be tracked, they may be more likely to strive to eat well than if no one was watching.
Second, energy density and fibre. By nature, most plant foods are lower in calories and higher in fibre than animal foods; for the same volume of food, whole-food, plant-based diets provide less energy while providing as much, if not more satiety. This ends up resulting in a calorie deficit and some weight loss for a lot of people (unless you’ve matched the calories from your previous diet).
Third, cutting the empty calories. Vegetarian diets, and particularly vegan diets, are not often conducive to grab-and-go eating as most such offerings are made with animal products and almost always covered in bacon. By nature, most fast foods and convenience foods, fried foods, and snack foods are limited. Vegan weight loss diets take these restrictions one step further by prohibiting refined foods, desserts, and sugar-sweetened drinks as well. Unfortunately there really is no innovation here, as avoiding processed and sugar-laden foods is a core tenet of just about every weight loss diet out there. These items often provide a substantial portion of daily energy intake in North America while providing little to no vitamins, minerals, protein, antioxidants, or fibre; we could all stand to eat less of these, less often.
Just like omnivores, vegetarians and vegans come in all shapes and sizes; vegetarian diet patterns are no magic bullet for weight loss, or even for weight maintenance. It’s important to remember that honey roasted peanuts, coconut ice cream, Oreos, potato chips, soda pop, french fries and onion rings are all vegan. Yes, they are occasional treats to enjoy, but they should not be the basis of one’s diet. All in all, if you are looking to lose weight, weak evidence shows that a vegetarian diet may be helpful but only if it is well planned. Likely a lot of the effects noted are not due to the virtuosity of vegan eating, but rather returning to highly nutritious, minimally processed foods.