A new analysis from the Physicians Committee for Responsible Medicine, published in Frontiers in Nutrition, reports that people who follow a vegan diet tend to eat more plant-based foods, even those labeled as “unhealthy” by the plant-based diet index. This shift in eating patterns was associated with more weight loss compared to individuals following the Mediterranean diet.
Weight loss was linked to several factors: avoiding animal products; eating items such as potatoes and refined grains, which are categorized as “unhealthy” by the plant-based diet index; and limiting added oils and nuts, which the same index classifies as “healthy.”
“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.
How the Diets Were Compared
This analysis builds on a previous Physicians Committee study that directly compared a low-fat vegan diet with a Mediterranean diet. In that trial, 62 adults with excess weight were randomly assigned to one of the two eating patterns for 16 weeks. The vegan plan included fruits, vegetables, grains, and beans, while the Mediterranean plan featured fruits, vegetables, legumes, fish, low-fat dairy, and extra-virgin olive oil. No calorie restrictions were placed on either group.
After the first 16 weeks, participants returned to their usual diets for a four-week break and then switched to the opposite plan for another 16 weeks. Results from the original study showed that the vegan diet led to greater weight loss and improvements in body composition, insulin sensitivity, and cholesterol levels.
Understanding the Plant-Based Diet Index
In the secondary analysis, researchers examined participants’ dietary logs to explore how three scoring systems within the plant-based diet index (PDI, hPDI, and uPDI) related to weight changes on both diets. The PDI system labels foods in the following way:
“Healthful” plant-based foods 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 index 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.
What the Scores Reveal About Weight Loss
The analysis showed that PDI scores rose significantly among participants on the vegan diet but stayed the same among those on the Mediterranean diet. The hPDI score increased in both groups, while the uPDI score rose in the vegan group and decreased among those eating the Mediterranean diet.
Only the increases in PDI and uPDI scores, seen exclusively on the low-fat vegan diet, were linked to weight loss. The rise in hPDI scores did not correspond with changes in body weight in either group.
Why the Vegan Diet Produced These Effects
Most of the increase in PDI, hPDI, and uPDI scores on the vegan diet came from removing animal products. Cutting back on oils and nuts also contributed to higher uPDI scores. Together, these patterns indicate that replacing animal-based foods with plant-based options, along with reducing oils and nuts, may be effective strategies for weight management.
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