In a six-month study, people who ate a vegan or vegetarian diet lost about twice as much weight as those following modified vegetarian or omnivorous diets, according to research published in Nutrition.
Overweight and obese adults who wanted to lose weight were randomly assigned to one of five low-fat and low-glycemic index diets: vegan (no animal products), vegetarian (dairy products included), pesco-vegetarian (dairy products and seafood included), semi-vegetarian (all food included, but red meat no more than once a week and poultry no more than five times a week), or omnivorous (no restrictions on food type and frequency).
Participants were told they could eat small amounts of nuts and nut butters, avocados, seeds, and olives in their diets but were encouraged to focus on lower-fat food options. The dieters were not given goals for limiting the number of kilojoules they ate. As the researchers put it, “participants were free to eat until they were satisfied.”
After six months, those in the vegan group had lost the most weight, an average of 3.4 kilograms. The vegetarian group was not far behind, with an average loss of 2.8 kilograms. Those in the other groups lost only half as much weight (an average of 1.4 kilograms for the pesco-vegetarian, semi-vegetarian groups and the omnivores). There was no significant difference in reported activity level among the five groups.
The design of this study included elements with high potential relevance to sustained weight-loss efforts in the real world. As the researchers noted, “The weight loss achieved in this study occurred without the need for dietary self-monitoring. Dietary self-monitoring is considered the cornerstone of behavioural treatment for weight loss; however, dietary self-monitoring requires daily recording of all foods/drinks consumed, which can be burdensome, time-consuming, and tedious. Adherence to self monitoring can be low and may decrease over time.”
The researchers said that the relative simplicity of the diets (no kilojoule counting, no food logging) along with initial weight loss was probably motivational for those in the vegan and vegetarian groups, and made sticking with the diets easier.