A secondary analysis of a randomized clinical trial reports that a low-fat vegan diet was associated with greater weight loss than a Mediterranean diet, even when the vegan menu included plant foods such as refined grains and potatoes that are labeled “unhealthful” in a common plant-based diet scoring system. The benefits were linked to replacing animal products with plant foods and limiting added oils and nuts, according to researchers from the Physicians Committee for Responsible Medicine in a study published in Frontiers in Nutrition.
The new report builds on a randomized crossover trial involving 62 overweight adults, conducted by the Physicians Committee for Responsible Medicine and colleagues. Participants followed both a low-fat vegan diet and a Mediterranean diet for 16 weeks each, in random order, separated by a four-week washout period, according to the original trial and the new analysis published in Frontiers in Nutrition.
The low-fat vegan diet emphasized fruits, vegetables, grains, and legumes. The Mediterranean diet emphasized fruits, vegetables, legumes, fish, low-fat dairy products, and extra-virgin olive oil. No formal calorie limits were imposed on either diet, according to the trial protocol.
In the original crossover trial, the low-fat vegan diet led to greater reductions in body weight and body fat and improved insulin sensitivity and cholesterol levels compared with the Mediterranean diet, while weight remained essentially unchanged on the Mediterranean diet.
The new analysis examined participants’ three-day dietary records using three related scores from the plant-based diet index (PDI):
- the overall PDI, which increases with higher intake of plant foods overall;
- the healthful PDI (hPDI), which increases with more "healthful" plant foods (such as fruits, vegetables, whole grains, nuts, legumes, oils, coffee, and tea) and fewer "unhealthful" plant foods (such as fruit juice, sugar-sweetened beverages, refined grains, potatoes, and sweets);
- and the unhealthful PDI (uPDI), which increases with greater intake of "unhealthful" plant foods and lower intake of "healthful" plant foods.
According to the Frontiers in Nutrition paper, PDI scores rose significantly when participants followed the low-fat vegan diet but did not change on the Mediterranean diet. The hPDI score increased on both diets, with a trend toward a larger increase on the vegan pattern, while the uPDI score increased on the vegan diet and decreased on the Mediterranean diet.
Only the increases in PDI and uPDI—changes seen exclusively on the low-fat vegan diet—were linked to weight loss across the first 16 weeks of the trial. The rise in hPDI scores, which occurred with both diets, was not associated with changes in body weight.
The researchers report that most of the increase in PDI, hPDI, and uPDI scores on the vegan diet came from avoiding animal products. Reducing intake of added oils and nuts on the low-fat vegan diet also contributed to higher uPDI scores, since the index classifies these foods as "healthful" plant-based items.
"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," said lead author Hana Kahleova, MD, PhD, director of clinical research at the Physicians Committee, in a news release from the organization.
Taken together, the findings suggest that, in this randomized crossover study of adults with excess weight, replacing animal products with plant-based foods—even those categorized as "unhealthful" by the plant-based diet index—and cutting back on added oils and nuts were associated with greater weight loss on a low-fat vegan diet than on a Mediterranean diet.
