How do plant-based diets impact body weight?

In a recent study published in the European Journal of Clinical Nutrition, researchers analyzed randomized controlled trial (RCT) data to determine the association between plant-based diet index (PDI), unhealthful PDI (uPDI), and healthful PDI (hPDI) and weight loss among overweight adults.

​​​​​​​Study: Does diet quality matter? A secondary analysis of a randomized clinical trial. Image Credit: Wild As Light/​​​​​​​Study: Does diet quality matter? A secondary analysis of a randomized clinical trial. Image Credit: Wild As Light/


Plant-origin diets are related to lower weight and improved health outcomes. A prospective cohort study conducted in 2021 in the United States (US) showed that increased PDI and hPDI were related to a lower risk of diabetes mellitus type 2; however, uPDI alterations were not.

Serum metabolome analysis verified the findings. Observational studies have advocated measuring the nutritional value of plant-based food consumption patterns, resulting in the formulation of PDI, uPDI, and hPDI indices.

However, the link between such categorization and body weight needs confirmation by randomized controlled trials.

About the study

In the present study, researchers evaluated the association between changes in body weight and changes in the three PDI indices following a vegan-type diet.

The primary RCT was conducted in Washington, District of Columbia, from January 2017 to February 2019. The secondary study included 244 individuals randomized to the vegan diet group (intervention, n = 122) or the control group (n = 122) for 16 weeks.

The intervention group followed a diet low in fatty foods and high in vegetables, fruits, legumes, and grains, whereas the controls made no dietary changes. Registered dieticians analyzed three-day dietary records (one weekend and two weekdays) to calculate the PDI, uPDI, and hPDI indices.

A repeated measure analysis of variance (ANOVA) was performed to analyze the statistical data. Multiple regression modeling was performed with all dietary components as candidate estimators to identify dietary constituents that could independently predict body weight changes.

Individual food components investigated included fruits, vegetables, whole grains, nuts, legumes, vegetable oils, tea and coffee, fruit juice, sugar-sweetened beverages, refined grains, potatoes, sweets, animal fats, dairy, eggs, animal-based products, meat, and seafood.


In total, 223 (91%) individuals completed the study. Self-documented calorie intake significantly decreased in both study groups, more so among those following the vegan diet (368 kcal/day reduction). The macronutrient percentage was not significantly altered among controls.

In contrast, in the intervention group, individuals significantly increased their carbohydrate intake (23% increase in daily calories) and fiber (12 g/day increase) and reduced fat intake (18% reduction of daily calories), protein (5.0% reduction in daily calories), and cholesterol (215 mg/day reduction).

Over 16 weeks, among vegan diet participants, there was an increase in the intake of fruits, vegetables, whole grains, legumes, fruit juice, animal fats, dairy, eggs, meat, and seafood by 0.8 points, 0.8 points, 1.2 points, 1.6 points, 0.4 points, 1.6 points, 1.9 points, 1.8 points, 1.9 points, and 1.5 points, respectively.

On the other hand, there was a decrease in the consumption of nuts, vegetable oils, tea and coffee, sugar-sweetened beverages, refined grains, and sweets by 0.8 points, 1.3 points, 0.2 points, 0.5 points, 1.3 points, and 0.1 points, respectively.

Body weight was significantly reduced in the intervention group (a 5.9 kg reduction) due to a decrease in visceral fat (209 cm3 reduction) and fat mass (4.1 kg reduction).

All three PDI indices were significantly higher in the intervention group, with effect sizes of 11, 11, and 5.4 for PDI, hPDI, and uPDI, respectively. The changes in all PDI indices were significantly associated with body weight changes.

Multiple regression modeling indicated that the food components independently predictive of weight loss included whole grains, legumes, meat, vegetable oils, and sweets. Consuming vegetables, fruits, whole grains, legumes, fruit juices, and potatoes was negatively correlated with weight changes.

On the other hand, consuming nuts, vegetable oils, tea and coffee, sugar-sweetened beverages, refined grains, sweets, animal fats, dairy, eggs, meat, and seafood were positively correlated with weight changes.


Overall, the study findings showed inverse associations between plant-based dietary indices and body weight alterations.

The findings indicated that minimizing the consumption of animal-based products, vegetable oils, refined grains, sweets, tea and coffee, meat, seafood, eggs, and dairy could lead to an effective reduction in weight among overweight adults.

Weight loss strategies must be aimed at increasing the intake of food components such as vegetables, fruits, legumes, and whole grains.

The findings inform weight management programs and aid in reducing the health burden of obesity. However, further research, including randomized controlled trials with more diverse populations, is required to increase the generalizability of the study findings.

Future studies must include objective measures of dietary data collection to improve the standardization of the results.

Journal reference:
Pooja Toshniwal Paharia

Written by

Pooja Toshniwal Paharia

Dr. based clinical-radiological diagnosis and management of oral lesions and conditions and associated maxillofacial disorders.


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