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

Abstract

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.

Keywords: dietary pattern; epidemiology; obesity; plant-based diets; prospective cohort studies; weight change.

FIGURE 1

Weight change over 4-y periods according to quintiles of change in animal foods and healthy and less-healthy plant foods. Values are weight change in lb (95% CI) pooled across the cohorts through the use of a fixed-effects model. Given that most participants gained weight over 4-y intervals, inverse associations of the diet indices with weight change (i.e., negative values) should be interpreted as “less weight gain,” not as “weight loss.” We used multivariable generalized linear regression models (with unstructured correlation matrix and robust variance) to conduct this analysis. Adjusted for age, questionnaire cycle, baseline BMI, change in smoking status, baseline and change in physical activity, hours of sleep, hours of sitting and watching television, change in alcohol consumption, change in margarine intake, and for women only, baseline parity, menopausal status, postmenopausal hormone use, and oral contraceptive use (NHS2 only). All 3 food categories were entered simultaneously into the fully adjusted model. All P values <0.001. 1 lb = 0.45 kg. NHS, Nurses’ Health Study. ²Median change in each quintile (averaged across the 3 cohorts).

FIGURE 2

Joint classification of changes in the hPDI and changes in physical activity (compared with no change in both) in relation to 4-y weight change. Values are weight change in lb (95% CI) pooled across the cohorts using a fixed-effects model. Given that most participants gained weight over 4-y intervals, inverse associations of the diet indices with weight change (i.e., negative values) should be interpreted as “less weight gain,” not as “weight loss.” We used multivariable generalized linear regression models (with unstructured correlation matrix and robust variance) to conduct this analysis. Adjusted for age, questionnaire cycle, baseline BMI, change in smoking status, baseline and change in physical activity, hours of sleep, hours of sitting and watching television, change in alcohol consumption, change in margarine intake, and for women only, baseline parity, menopausal status, postmenopausal hormone use, and oral contraceptive use (NHS2 only). The reference group (no change in hPDI or physical activity) experienced a mean 4-y weight gain of 3.2 lb. 1 lb = 0.45 kg. hPDI, healthful plant-based diet index; MET, metabolic equivalent task; NHS, Nurses’ Health Study.