Researchers comparing two popular dietary patterns found that one produced unexpectedly strong effects on body weight and metabolic markers. The findings suggest that the specific types of plant-based foods chosen may influence outcomes in surprising ways. Credit: Shutterstock
A new analysis finds that a low-fat vegan diet leads to greater weight loss than the Mediterranean diet, even when it includes foods labeled “unhealthful” in plant-based diet scoring systems.
Eating a vegan diet leads people to consume a wider range of plant-based foods, including items labeled as “unhealthy” in the plant-based diet index, and this pattern was linked to greater weight loss than the Mediterranean diet, according to a new analysis from the Physicians Committee for Responsible Medicine published in Frontiers in Nutrition.
The study found that several factors supported weight loss: excluding animal products; eating foods such as potatoes and refined grains (both categorized as “unhealthy” in the plant-based diet index); and limiting added oils and nuts (classified as “healthy” in the same index).
Study Background
“Our research shows that even when a low-fat vegan diet includes so-called unhealthy plant-based foods—as defined by the plant-based diet index—like refined grains and potatoes, it’s better than the Mediterranean diet for weight loss, because it avoids animal products and added oils,” says Hana Kahleova, MD, PhD, director of clinical research at the Physicians Committee for Responsible Medicine and lead author of the study.
The new findings come from a follow-up assessment of an earlier Physicians Committee study that compared a low-fat vegan diet with a Mediterranean diet. In the original trial, 62 adults with excess weight were randomly placed on one of the two eating patterns for 16 weeks. The vegan group ate fruits, vegetables, grains, and beans, while the Mediterranean group focused on fruits, vegetables, legumes, fish, low-fat dairy, and extra-virgin olive oil. Neither group was asked to restrict calories.
After the initial phase, everyone returned to their usual diets for a four-week washout period, then moved to the opposite diet for another 16 weeks. Results from the study showed that the vegan approach led to greater weight loss and also produced more favorable changes in body composition, insulin sensitivity, and cholesterol levels compared with the Mediterranean diet.
What the Secondary Analysis Examined
In this secondary analysis, the participants’ dietary records were used to assess the relationship of a plant-based diet index (PDI), healthful PDI (hPDI), and unhealthful PDI (uPDI) with weight loss on both the vegan diet and Mediterranean diet. “Healthful” plant-based foods, as defined by the PDI system, include fruits, vegetables, whole grains, nuts, legumes, oils, coffee, and tea. “Unhealthful” plant-based foods include fruit juice, sugar-sweetened beverages, refined grains, potatoes, and sweets. The PDI system assigns scores as follows:
PDI: Score increases with more plant-based foods overall.
hPDI: Score increases with more “healthful” plant-based foods and fewer “unhealthful” plant-based foods.
uPDI: Score increases with more “unhealthful” plant-based foods and fewer “healthful” plant-based foods.
In the analysis, the PDI score increased significantly on the vegan diet and did not change on the Mediterranean diet; the hPDI score increased on both diets; and uPDI increased on the vegan diet and decreased on the Mediterranean diet.
The increases in the PDI and uPDI scores, which were seen only the low-fat vegan diet, were associated with weight loss. The changes in hPDI, which were seen on both diets, were not associated with changes in body weight.
The majority of the increases in the PDI, hPDI, and uPDI scores came from avoiding animal foods on a vegan diet. Reducing the consumption of oils and nuts further increased the uPDI score by points on a vegan diet. These findings suggest that replacing animal products with plant-based foods, and reducing the consumption of oil and nuts, may be successful strategies for weight loss.
Reference: “Plant-based dietary index on the Mediterranean and a vegan diet: a secondary analysis of a randomized, cross-over trial” by Hana Kahleova, Reagan Smith, Ilana Fischer, Haley Brennan, Tatiana Znayenko-Miller, Richard Holubkov and Neal D. Barnard, 30 October 2025, Frontiers in Nutrition.
DOI: 10.3389/fnut.2025.1666807
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