Objectives: Several studies have found plant-based diets (defined as vegetarian diets) to be associated with favorable weight outcomes, but they have not distinguished between high- and low-quality plant foods. We aimed to examine associations of changes in intake of variations of plant-based diets with weight change over 4-year intervals spanning more than 20 years in three prospective cohorts.
Methods: At baseline, there were 46,790 women in the Nurses’ Health Study (NHS) (1986), 59,217 women in NHS2 (1991), and 20,975 men in the Health Professionals Follow-up Study (HPFS) (1986). We created an overall plant-based diet index (PDI) from dietary data collected every 4 years using a semi-quantitative food frequency questionnaire, by assigning positive scores to plant foods and reverse scores to animal foods. We also created a healthful PDI (hPDI) where healthy plant foods (whole grains, fruits/vegetables, nuts/legumes, oils, tea/coffee) got positive scores, while less healthy plant foods (juices/sweetened drinks, refined grains, potato/fries, sweets) and animal foods got reverse scores. For the unhealthful PDI (uPDI) we assigned positive scores to less healthy plant foods, and reverse scores to healthy plant foods and animal foods.
Results: On average, participants gained weight over 4-year intervals [2.64 lb (1.20 kg) in NHS, 4.43 lb (2.01 kg) in NHS2, 1.96 lb (0.89 kg) in HPFS]. Increased intake of PDI was modestly inversely associated with weight change [-0.09 lb (-0.04 kg) less weight gain per 1-SD increase; 95% CI: -0.12, -0.05 lb (-0.05, -0.02 kg); p-trend<0.001]. This inverse association was stronger for hPDI [-1.50 lb (-0.68 kg) less weight gain per 1-SD increase; 95% CI: -1.53, -1.46 lb (-0.69, -0.66 kg); p-trend<0.001]. Conversely, uPDI was positively associated with weight change [0.79 lb (0.36 kg) more weight gain per 1-SD increase; 95% CI: 0.75, 0.82 lb (0.34, 0.37 kg); p-trend<0.001]. Associations for hPDI and uPDI were stronger among overweight/obese, less physically active, and younger participants (Fig 1).
Conclusions: Healthful plant-based diets rich in high-quality plant foods are associated with less weight gain over 4-year intervals. These findings support current guidelines to increase intake of healthy plant foods, while reducing intake of less healthy plant foods and animal foods.