Duration of Physical Activity and Serum 25-hydroxyvitamin D Status of Postmenopausal Women

Purpose: To investigate whether the association between physical activity and serum 25-hydroxyvitamin D (25(OH)D) concentrations is independent of sun exposure, body size, and other potential explanatory variables. Methods: By using data from a sample of 1343 postmenopausal women from the Women's Health Initiative, we used linear regression to examine the associations of duration (minutes/week) of recreational activity and of yard work with 25(OH)D concentrations (nmol/L). Results: In age-adjusted analyses, positive associations were observed between 25(OH)D concentrations and both duration of recreational physical activity (β = 0.71, SE [0.09], p <001) and yard work (β = 0.36, SE [0.10], p = .004). After further adjustment for vitamin D intake, self-reported sunlight exposure, waist circumference, and season of blood draw, 25(OH)D was significantly associated with recreational activity (β = 0.21, SE [0.09], p = .014) but not with yard work (β = 0.18, SE [0.09], p = .061). Interactions were observed between season and both recreational activity (P_interaction = .082) and yard work (P_interaction = .038) such that these activity-25(OH)D associations were greater during summer/fall compared with winter/spring. Self-reported sunlight exposure and measures of body size did not modify the associations. Conclusions: The observed age-adjusted activity-25(OH)D associations were attenuated after adjusting for explanatory variables and were modified by season of blood draw. Adopting a lifestyle that incorporates outdoor physical activity during summer/fall, consuming recommended amounts of vitamin D, and maintaining a healthy weight may improve or maintain vitamin D status in postmenopausal women.