Changes in intake of plant-based diets and weight change: results from 3 prospective cohort studies

Background: Studies have found beneficial effects of plant-based diets on weight. However, not all plant foods are necessarily beneficial. Objectives: The aim of this study was to examine associations of changes in intake of 3 variations of plant-based diet indices (overall, healthful, and unhealthful) with weight change over 4-y intervals spanning >20 y. Methods: Data from 3 ongoing prospective observational cohort studies in the United States were used, namely the Nurses' Health Study (NHS), NHS2, and the Health Professionals Follow-up Study (HPFS), with 126,982 adult men and women. Self-reported diet data were collected every 4 y, and self-reported weight data were used to compute weight change every 4 y over >20 y of follow-up. Results: On average, participants gained a mean of 0.90 kg (HPFS) to 1.98 kg (NHS2) over 4-y intervals. Different types of plant-based diet indices were associated with different amounts of weight gain. After adjusting for several potential confounders, including concomitant changes in other lifestyle factors, a 1-SD increase in intake of an overall plant-based diet index was associated with 0.04 kg less weight gain over 4-y periods (95% CI: 0.05, 0.02 kg; P < 0.001). A 1-SD increase in intake of a healthful version of a plant-based diet index (emphasizing whole grains, fruits/vegetables, nuts/legumes, vegetable oils, tea/coffee) was associated with 0.68 kg less weight gain over 4-y periods (95% CI: 0.69, 0.66 kg; P < 0.001). Conversely, a 1-SD increase in an unhealthful version of a plant-based diet index (emphasizing refined grains, potato/fries, sweets, sweetened drinks/juices) was associated with 0.36 kg more weight gain (95% CI: 0.34, 0.37 kg, P < 0.001). Conclusion: Plant-based diets, especially when rich in healthier plant foods, are associated with less weight gain over 4-y intervals. This supports current recommendations to increase intake of healthy plant foods, and reducing intake of less-healthy plant foods and animal foods, for improved health outcomes.