A new analysis suggests that a low-fat vegan diet can lead to greater weight loss than a Mediterranean diet, even when it includes so-called “unhealthy” plant foods like potatoes and refined grains.
The key ingredient seems to be cutting out animal products and added oils, rather than having a perfectly “clean” plant-based plate.
Researchers from the Physicians Committee for Responsible Medicine ran a randomised cross-over trial with 62 adults with overweight.
Each participant:
Followed a low-fat vegan diet (fruits, vegetables, grains and beans) for 16 weeks, thenWent back to their usual diet for four weeks, thenSwitched to a Mediterranean diet (fruits, vegetables, legumes, fish, low-fat dairy and extra-virgin olive oil) for 16 weeks
There was no calorie counting in either phase.
The original study found that the vegan diet led to greater weight loss and better changes in body fat, insulin sensitivity and cholesterol than the Mediterranean diet.
What the new analysis asked
In this secondary analysis, the team looked at what people actually ate and scored their diets using a plant-based diet index (PDI) with three components:
PDI – higher scores mean more plant foods overallhPDI – higher scores mean more “healthful” plant foods like wholegrains, vegetables, nuts and legumes, and fewer “unhealthful” onesuPDI – higher scores mean more “unhealthful” plant foods such as refined grains, potatoes, sweets and sugary drinks, and fewer “healthful” ones
They then matched these scores to changes in body weight.
What they found
On the vegan diet:
Overall plant-based scores (PDI) went upHealthful plant scores (hPDI) went upUnhealthful plant scores (uPDI) also went up
On the Mediterranean diet:
Overall plant-based scores hardly changedHealthful plant scores went upUnhealthful plant scores went down
Here is the twist:
Increases in PDI and uPDI on the vegan diet were linked to weight lossChanges in hPDI on either diet were not clearly linked to weight change
In plain English: moving towards a low-fat vegan pattern that cuts out animal foods and oils promoted weight loss, even when people still ate things like potatoes and refined grains.
Simply improving the “quality” of plant foods within a Mediterranean pattern did not have the same effect on weight.
Why might this matter
The biggest drivers of the improved scores on the vegan diet were:
Avoiding animal productsReducing oil and nut intake
This combination lowers the energy density of meals, which makes it easier for some people to eat fewer calories without actively restricting portion sizes.
For people with type 2 diabetes or those at risk, a low-fat vegan pattern can improve insulin sensitivity and cholesterol alongside weight.
However, large amounts of refined carbohydrate can still spike blood glucose, so personal monitoring and medication adjustment are important.
Study: Kahleova H et al. Frontiers in Nutrition 2025.
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