Ditching Meat & Dairy: Even ‘Unhealthy’ Vegan Foods Can Drive Weight Loss, Study Finds
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A new analysis challenges conventional wisdom about weight loss, suggesting that eliminating animal products—even while consuming vegan foods often considered “unhealthy”—can be more effective than a traditional Mediterranean diet. The research, published in Frontiers in Nutrition in January 2025, indicates that a focus on plant-based eating, regardless of the specific plant foods chosen, can lead to weight loss without strict calorie counting.
Nearly half of U.S. adults plan to start a new diet this year, yet fewer than one in ten intend to adopt a plant-based or vegan approach—despite mounting evidence supporting its efficacy for both weight loss and long-term maintenance. This disconnect highlights a potential gap in public understanding of effective dietary strategies.
The Trial Design: Vegan vs. Mediterranean
The findings stem from a secondary analysis of a previous randomized trial involving 62 overweight adults. Participants were assigned to follow either a low-fat vegan diet – centered around fruits, vegetables, grains, and beans – or a Mediterranean diet emphasizing fruits, vegetables, legumes, fish, low-fat dairy, and extra-virgin olive oil.
For 16 weeks, participants adhered to their assigned diet without calorie restrictions. This was followed by a four-week “washout” period where they resumed their usual eating habits, before switching to the opposite diet for another 16 weeks. Previous results from the initial trial demonstrated the vegan diet’s superiority in promoting weight loss and improving markers of metabolic health, including body composition, insulin sensitivity, and cholesterol levels.
Decoding the Plant-Based Diet Index
To delve deeper into the nuances of plant-based eating, researchers utilized a Plant-Based Diet Index (PDI), a system that scores food intake based on plant food consumption. The PDI breaks down into three key components:
- PDI: Reflects the overall quantity of plant foods consumed.
- hPDI: Measures intake of “healthful” plant foods – fruits, vegetables, whole grains, nuts, legumes, oils, coffee, and tea – while penalizing “unhealthful” options.
- uPDI: Scores intake of “unhealthful” plant foods – fruit juice, sugar-sweetened beverages, refined grains, potatoes, and sweets – and subtracts points for “healthful” choices.
The analysis revealed a significant increase in the overall PDI score among those following the vegan diet, while the Mediterranean diet showed no change. Both diets saw an increase in hPDI scores. Notably, the uPDI score rose significantly on the vegan diet but decreased on the Mediterranean diet.
Why ‘Unhealthy’ Vegan Foods Still Work
This pattern suggests that participants on the vegan diet didn’t simply eat more plants; they also increased their consumption of plant staples often labeled as “unhealthy,” such as potatoes and refined grains. Despite this categorization, replacing animal products with these foods was still associated with weight loss.
Higher PDI and uPDI scores – changes observed exclusively in the vegan group – were linked to weight reduction. Improvements in hPDI scores, however, occurred on both diets and did not correlate with changes in body weight.
The Role of Oils and Nuts
A key driver of the increased uPDI score on the vegan diet was a reduction in the consumption of oils and nuts, which the index classifies as “healthful” plant foods. This reduction pushed uPDI scores higher and was directly linked to greater weight loss.
“Oils and nuts are considered ‘healthful’ by the plant-based diet index,” explained Dr. Hana Kahleova, director of clinical research at the Physicians Committee for Responsible Medicine, to Newsweek. “Consuming oils and nuts increases the score in the hPDI, but decreases the score in the uPDI. Our research showed that people following the low-fat vegan diet reduced consumption of oils and nuts, which increased the uPDI score and led to weight loss.”
Implications for Weight-Loss Strategies
The study underscores the powerful impact of eliminating animal products, with most of the index shifts stemming from this single change. Researchers also highlighted the reduction in oils and nuts as a contributing factor to the vegan diet’s effectiveness.
Rimas Geige, a medical doctor, registered dietician, and clinic nutritionist, offered a practical perspective. “As a dietician, I see this daily. A client can eat a significantly larger volume of whole plant foods, like a huge bowl of potatoes and beans (the ‘unhealthy’ plant foods in this context), for fewer calories than a small serving of olive oil-rich cheese and fish,” Geige, owner of It’s Me & You Clinic in Surrey, England, told Newsweek. “Based on this, I’d say that it’s the volume to calorie ratio that wins the initial weight battle.”
Ultimately, the findings suggest that prioritizing plant-based foods – even those categorized as “unhealthful” – and limiting high-fat plant foods like oils and nuts may be a successful strategy for weight loss, at least in the short term.
