People assigned to a low-fat vegan diet ate fewer calories without reporting a meaningful reduction in the total weight of food they consumed, according to a secondary analysis of a randomized clinical trial.
The findings, published in JAMA Network Open, suggest that dietary energy density, or the number of calories packed into a given amount of food, may help explain why participants lost weight without being instructed to count calories or limit portions. But the analysis does not prove that lower energy density caused the weight loss, and the results apply to the particular low-fat vegan diet tested rather than every vegan eating pattern.
The study was funded by the Physicians Committee for Responsible Medicine, an organization that promotes plant-based nutrition. Most of the researchers were affiliated with the organization. The authors reported that the funder had no role in the study’s design, analysis or publication decisions.
The original trial included 244 adults with overweight who were randomly assigned to follow either a low-fat vegan diet or continue their usual diets for 16 weeks. Participants in the vegan group were encouraged to eat vegetables, fruit, grains and legumes while avoiding animal products and minimizing higher-fat foods. Neither group was given a calorie limit.
The new paper was a secondary analysis of data from 223 participants who completed the trial. Researchers examined whether changes in dietary energy density might help explain the reductions in calorie intake and body weight previously observed in the vegan group.
Energy density describes how many calories a food provides relative to its weight. Foods that contain a lot of water or fiber, including many vegetables, fruits, beans and broth-based dishes, tend to provide fewer calories per bite than foods rich in fat or added sugar.
After 16 weeks, energy density had fallen by about 30% in the vegan group, while it remained largely unchanged among people continuing their usual diets.
Participants assigned to the vegan diet also reported eating about 357 fewer calories per day relative to the change in the control group. At the same time, the total weight of food they ate did not differ significantly between groups.
In practical terms, the participants may have been eating a similar physical amount of food while getting fewer calories from it.
The changes were driven by more than simply removing meat. Participants in the vegan group reported eating more vegetables, fruit and legumes and less meat, dairy, eggs, sweets, nuts and added fats. Those shifts changed the overall balance of water, fiber, fat and calories in their diets.
The results help illustrate why a meal’s size does not always reflect its calorie content. A large bowl containing beans, vegetables and a broth-based sauce may weigh as much as a smaller serving of a richer dish while providing fewer calories.
That does not mean people can eat unlimited amounts without affecting their weight. It also does not mean calorie-dense foods are automatically unhealthy. Nuts, seeds, avocado and olive oil provide valuable nutrients even though they contain more calories in a smaller amount.
The study instead suggests that meals built around more water-rich and fiber-rich foods may help some people feel that they are eating a satisfying amount while lowering total calorie intake.
However, researchers cannot conclude that reduced energy density was the reason participants lost weight. The vegan group changed many aspects of its diet at once, including fat intake, food choices, fiber intake and the balance of plant and animal foods.
Participants were randomly assigned to the dietary groups, but they were not randomly assigned to specific energy-density levels. That means the study can show that lower energy density and weight loss occurred together, but not that one directly caused the other.
The analysis was also not the original trial’s primary purpose. It was conducted years after the dietary data were collected, making it an exploratory look at a possible mechanism behind the earlier findings.
Food intake was measured using three-day dietary records completed at the beginning and end of the study. Such records can provide useful estimates, but people may forget foods, misjudge portions or change how accurately they report their intake during a weight loss study.
That uncertainty means the reported calorie reduction should not be interpreted as an exact measure of what every participant ate.
The comparison group also continued its usual diet rather than receiving another structured eating plan. The trial therefore cannot show whether a low-fat vegan diet works better than a Mediterranean, vegetarian or omnivorous diet designed to reduce energy density in similar ways.
People do not need to eliminate all animal products to apply the basic principle. Meals that emphasize vegetables, fruit, beans, whole grains and broth-based foods can have a lower energy density even when they include some meat, dairy or other animal foods.
The quality of a vegan diet matters as well. A pattern centered on beans, vegetables, fruit and whole grains differs substantially from one dominated by refined grains, sweets and highly processed foods.
The findings provide a plausible explanation for why the specific vegan intervention helped participants lower their calorie intake without eating a smaller total amount of food. More research would be needed to determine how much energy density contributed to the weight loss and whether the same effect occurs with other dietary patterns.
The study was funded by the Physicians Committee for Responsible Medicine. Hana Kahleova and several other authors were affiliated with the organization. Neal Barnard serves without compensation as president of the Physicians Committee for Responsible Medicine and the Barnard Medical Center and has received royalties and honoraria from nutrition-related books and lectures. Richard Holubkov received personal and statistical consulting fees from the Physicians Committee for Responsible Medicine.