Long-term dietary intervention influence on physical activity in the Women's Health Initiative Dietary Modification randomized trial

Purpose: In the Women's Health Initiative (WHI) Dietary Modification (DM) randomized trial, dietary intervention significantly reduced breast cancer mortality (P = 0.02). In observational studies, physical activity is associated with lower breast cancer incidence. Currently, dietary intervention influence on other health-related behaviors is unknown. Therefore, we evaluated whether the WHI dietary intervention influenced self-directed physical activity.

Methods: Of 48,835 postmenopausal women, 19,541 were randomized to dietary intervention (18 nutritionist-led group sessions first year, then quarterly sessions throughout 8.5 years [median] intervention) and 29,294 to a usual diet comparison (written health-related materials only). Neither randomization group received specific or ongoing instructions to increase physical activity. Episodes per week of moderate or vigorous recreational physical activity (MVPA) were serially reported. Marginal longitudinal logistic regression models were used to assess physically inactive (MVPA = 0) or physically active (MVPA > 0) participants by randomization group. Marginal Poisson regression models estimated mean weekly MVPA.

Results: At entry, 45.6% of all participants reported physical inactivity (MVPA = 0). In 43,760 women with MVPA information, throughout 15.9 years (median) cumulative follow-up, dietary intervention group participation was associated with 7% lower physical inactivity rate (odds ratio [OR] 0.93, 95% confidence interval [CI] 0.91, 0.95, P < 0.001) and a 4% higher mean MVPA (ratio of means [RM] 1.04 95% CI 1.02, 1.06, P < 0.001), relative to the comparison group.

Conclusion: In a randomized trial setting, a low-fat dietary pattern intervention was associated with a long-term, favorable influence on self-directed recreational physical activity.

Trial registration: NCT00000611.