The association between physical activity and serum 25(OH)D status among postmenopausal women
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Objective . To investigate the association between duration of physical activity (minutes/week) and serum 25(OH)D (nmol/L) status and determine if the association is modified by sunlight exposure or explained by other extraneous factors (e.g., anthropometric measures). Methods . Data was obtained from 1,343 postmenopausal women that participated in an ancillary study of the Women's Health Initiative Observational Study (WHI-OS), the Carotenoids in Age-Related Eye Disease Study (CAREDS; 2001-2004). Multiple linear regression was used to examine the effects of duration of physical activity (recreational and yard work) on 25(OH)D levels with and without adjustment for vitamin D intake, waist circumference, and self-reported sunlight exposure. Interactions between measures of sunlight exposure and duration of physical activity were also examined. Results . There was a significant positive association between duration of recreational activity and yard work, combined and separately, and serum 25(OH)D status in crude and adjusted models (β (SE) for adjusted total physical activity (recreational plus yard work) = 0.27 (0.09), p<0.003). This relationship was significantly modified by season of blood draw ( p for interaction =0.032), but not for self-reported sunlight exposure. The adjusted relationship between duration of total physical activity and serum 25(OH)D status was greatest for women with blood drawn during summer/fall (β(SE) = 0.38 (0.11), p<0.001), when ultraviolet B (UVB) light exposure is greatest at Northern latitudes, compared to women with blood drawn during winter/spring (β(SE)=0.03 (0.13), p=0.809), when UVB light is not strong enough for vitamin D production at Northern latitudes. In exploratory analyses the positive associations between duration of walking, mild, moderate, and strenuous physical activity, and serum 25(OH)D status were similar in magnitude. The season of blood draw interaction was only significant for walking ( p for interaction =0.068), with no significant relationship between duration of walking and serum 25(OH)D observed for women with blood drawn during the winter/spring, but there was a statistically significant positive relationship observed during summer/fall which was no longer statistically significant after adjustment for confounders. Discussion . There was a positive association between physical activity and serum 25(OH)D levels among postmenopausal women that was modified by season of blood draw, with significant positive associations only observed in summer/fall not winter/spring, and explained further by self-reported sunlight exposure, waist circumference, and vitamin D intake.