Credit: Natazanchik on Shutterstock
Participants lost weight after swapping animal products for plant foods, even when those greener choices weren’t nutritionally ideal.
In A Nutshell
A 16-week trial found that a low-fat vegan diet led to more weight loss than a Mediterranean diet in the same people.
Plant-based diet scores rose much more on the vegan plan, mostly because animal products and added fats were removed.
The “unhealthful” plant-based score went up on the vegan diet even without more junk food, due to how the index handles animal foods and oils.
Across both diets, higher overall and “unhealthful” plant-based scores were linked with greater weight loss, even after accounting for calories eaten.
When two of the world’s most celebrated diets went head-to-head in a rigorous scientific showdown, the vegan diet came out ahead for weight loss. However, the real surprise wasn’t which diet won — it was why.
A study published in Frontiers in Nutrition compared the Mediterranean diet against a low-fat vegan diet in a 16-week crossover trial involving 62 overweight adults. Each participant followed both diets in random order, separated by a four-week washout period. While both eating plans have their devotees and proven health benefits, only the vegan diet consistently moved the needle on the scale.
Beyond pounds lost among participants, researchers calculated specialized scores measuring how plant-based each diet was, via three different indices: the plant-based dietary index (PDI), the healthful plant-based index (hPDI), and the unhealthful plant-based index (uPDI). These scoring systems award or subtract points based on whether foods come from plants or animals and whether those plant foods fall into “healthy” or “unhealthy” categories.
How Each Diet Stacked Up
Participants following the Mediterranean diet stuck to the well-established PREDIMED protocol. They consumed at least two servings of vegetables daily, two to three servings of fresh fruit, and three weekly servings each of legumes, fish or shellfish, and nuts or seeds. They used 50 grams of extra virgin olive oil as their main cooking fat each day and favored white meats over red meats when eating animal products.
The vegan group ate a low-fat diet consisting entirely of fruits, vegetables, grains, and legumes, with zero animal products allowed. Neither group received instructions about counting calories or avoiding processed foods, making the trial a real-world test of how these diets perform when people aren’t micromanaging every bite.
Despite higher “unhealthful” index scores, vegan dieters still shed pounds. (Credit: Elena Veselova on Shutterstock)
Plant-Based Scores Tell an Unexpected Story
After 16 weeks on the vegan diet, participants’ overall plant-based dietary index scores jumped by 7.6 points compared to essentially no change on the Mediterranean diet. The healthful index increased on both diets, though slightly more on the vegan plan.
But the results took an unexpected turn with the unhealthful plant-based index. This score decreased by about 5 points on the Mediterranean diet while increasing by nearly 7 points on the vegan diet. This increase wasn’t because people started eating more refined grains or potatoes—both actually decreased slightly on the vegan diet. Instead, the mathematical quirk came from how the index calculates scores. It uses reverse scoring for animal products and certain healthy fats, so removing them from the diet automatically pushes the unhealthful index higher, even when people aren’t eating more junk food.
Despite the higher “unhealthful” index scores, vegan dieters still lost weight. When researchers examined the relationship between dietary index changes and weight loss during the first 16 weeks, they found that increases in both the overall plant-based index and the unhealthful plant-based index correlated with dropping pounds. These associations held up even after accounting for differences in calorie intake.
Most of the boost in plant-based scores on the vegan diet came from one simple change: eliminating animal products entirely. This single shift added about 5.5 points to each dietary index. Reducing oil and nut consumption added another 3.7 points to the unhealthful index.
The Mediterranean diet showed its own patterns. Participants on this plan decreased their consumption of refined grains and sweets more dramatically than those on the vegan diet. They also consumed significantly more vegetable oil, particularly the recommended extra virgin olive oil. Sugar-sweetened beverage consumption dropped on both diets, with a slightly larger decrease on the vegan plan. Even with these changes, the original trial still found more weight loss on the vegan diet than on the Mediterranean plan.
Why Plant Foods Promote Weight Loss
Several biological factors may explain why plant foods support weight loss. Plant-based foods pack more fiber per calorie than animal products, and fiber fills people up with fewer calories consumed. Plant foods also have lower energy density overall, meaning people can eat satisfying portions without overloading on calories.
In earlier research by the same team, a single plant-based meal led to more GLP-1 release than a standard meal in people with type 2 diabetes. GLP-1, or glucagon-like peptide-1, has gained fame recently as the target of popular weight-loss medications, but the body produces it naturally in response to certain foods. The hormone helps regulate appetite and increases feelings of fullness after eating.
Testing Diets in Real-World Conditions
The study recruited 62 overweight adults in Washington, D.C., between February and October 2019. Participants tracked their food intake through detailed three-day records at the study’s start, middle, and end points, capturing two weekdays and one weekend day each time. A registered dietitian certified in nutrition analysis software examined these records and calculated plant-based index scores across 17 different food categories.
Physical activity levels remained similar on both diets, measured through the International Physical Activity Questionnaire. This consistency ruled out exercise as a reason for any weight differences between the two eating plans.
The crossover design strengthened the findings considerably. Each participant essentially served as their own control, trying both diets and allowing researchers to compare results within the same person rather than between different groups of people. This approach eliminates many variables that can muddy results in diet studies.
On the vegan diet, consumption of legumes increased substantially while animal products dropped to zero. Vegetable oil intake plummeted compared to the Mediterranean diet. Refined grain consumption dropped modestly on both diets, though more dramatically on the Mediterranean plan. Potato consumption also decreased slightly on the vegan diet.
Lead author Dr. Hana Kahleova from the Physicians Committee for Responsible Medicine noted that these findings raise questions about using plant-based dietary indices as clinical tools for vegan diets. Scientists originally developed these indices by identifying dietary factors associated with diabetes risk in large observational studies of people eating omnivorous diets.
While these scoring systems may help characterize how healthy different omnivorous eating patterns are, they appear less useful for fully plant-based diets. The mathematical structure of the unhealthful plant-based index means it can increase on a vegan diet simply because animal products are removed, not because people are eating more unhealthy foods. This means the index doesn’t capture what actually matters for metabolic outcomes in the absence of animal products.
Previous research supports these latest findings. An earlier randomized trial in overweight adults showed that replacing animal products with plant foods from both healthful and unhealthful categories led to weight loss. Another tightly controlled metabolic ward study confirmed these results under carefully monitored conditions.
In this trial and related studies, people lost weight when they swapped animal products for plant foods, even when some of those foods weren’t nutritionally perfect. That suggests cutting meat, dairy, and eggs can help many people lose weight, even if their plant choices aren’t ideal every time.
Paper Summary
Study Limitations
This research relied on self-reported dietary records, which can contain inaccuracies in portion sizes and memory recall. Participants were volunteers who may not represent the broader population, though they likely reflect individuals actively seeking weight loss. Because participants prepared their own meals at home or ate at restaurants, the study offers high real-world applicability but less control over food intake than metabolic ward studies provide.
Funding and Disclosures
The Physicians Committee for Responsible Medicine provided financial support for this research. The authors reported no commercial or financial conflicts of interest related to the study.
Publication Details
Kahleova H, Smith R, Fischer I, Brennan H, Znayenko-Miller T, Holubkov R and Barnard ND (2025). “Plant-based dietary index on the Mediterranean and a vegan diet: a secondary analysis of a randomized, cross-over trial,” was published November 18, 2025 in Frontiers in Nutrition. DOI: 10.3389/fnut.2025.1666807. The study was registered at ClinicalTrials.gov under identifier NCT03698955. Lead author Dr. Hana Kahleova is affiliated with the Physicians Committee for Responsible Medicine in Washington, DC. Co-authors include researchers from the University of Utah School of Medicine and George Washington University School of Medicine and Health Sciences.
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