WASHINGTON, D.C. — Eating more meat, fish, eggs, and dairy products was associated with weight gain even when those foods were minimally processed, according to a new study from the Physicians Committee for Responsible Medicine published in Nutrition Research . Minimally processed plant foods, such as fruits, vegetables, whole grains, and beans, showed the opposite pattern.
"Ultra-processed foods get most of the blame for weight gain, but our results suggest we also need to ask whether a food comes from a plant or an animal," says lead author Hana Kahleova, MD, PhD, director of clinical research at the Physicians Committee. "A chicken breast is minimally processed. So is an apple. In our study, they did not move the scale in the same direction."
The researchers analyzed data from a randomized crossover trial in which 62 adults with overweight followed a Mediterranean diet, rich in olive oil, fish, and nuts, and a low-fat vegan diet for 16 weeks each, separated by a four-week washout. Participants kept three-day diet records, and every food was classified by degree of processing, using the NOVA system, and by whether it came from a plant or an animal.
Compared with the Mediterranean diet, the vegan diet cut minimally processed animal foods by 135 grams per day more and raised minimally processed plant foods by 202 grams per day more. Processed animal and plant foods also fell more on the vegan diet. Across both diets, eating more minimally processed animal foods was linked to weight gain; this was the strongest association in the analysis and the only one to remain statistically significant after correction for multiple comparisons. Processed and ultra-processed animal foods tended to track with weight gain, and minimally processed plant foods with weight loss, although these associations were weaker. Ultra-processed plant foods were not associated with weight change.
In the main trial, published in 2022, participants lost an average of 6 kilograms (about 13 pounds) on the vegan diet, with no change on the Mediterranean diet. Minimally processed plant foods are high in fiber and water and low in calorie density, which helps people feel full on fewer calories.
"If you want to lose weight, fill your plate with beans, vegetables, fruits, and whole grains, and leave the animal products off," says Dr. Kahleova. "The degree of processing still matters, but it is not the whole story."
Founded in 1985, the Physicians Committee for Responsible Medicine is a nonprofit organization that promotes preventive medicine, conducts clinical research, and encourages higher standards for ethics and effectiveness in education and research.
