Vitamin D Intake Determines Vitamin D Status of Postmenopausal Women, Particularly Those with Limited Sun Exposure

Few detailed data are available on the wide range of determinants of vitamin D status among postmenopausal women, and it is also unclear whether there may be undiscovered determinants. The objective of this study was to comprehensively evaluate determinants of serum 25-hydroxyvitamin D [25(OH)D] concentrations in a large cohort of postmenopausal women. Data from a subset of the Women's Health Initiative Observational Study were analyzed (50-79 y; n = 3345). Information on diet, lifestyle behaviors, secondhand smoke, use of dietary supplements and medication, chronic diseases, and anthropometry was collected at baseline (1993-1998) and on sun exposure at year 4 follow-up. Linear regression was performed to estimate regression coefficients (beta). Significant determinants were total vitamin D intake (food plus supplements per 100 IU/d, beta = 2.08), years of supplemental vitamin D use (beta = 0.15), total fat intake (grams per day, beta = -0.03), smoking status (beta = -2.64, current vs. never), regional solar irradiance (beta = 6.26, 475-500 vs. 300-325 Langleys), daylight time spent outdoors in summer (beta = 5.15, >2 h vs. <30 min/d), recreational physical activity (metabolic equivalent task per hour per week, beta = 0.13), waist circumference (centimeters, beta = -0.26), and race/ethnicity (beta = -11.94, black vs. white). Total vitamin D intake (partial R-2 = 0.09) explained the most variance in serum 25(OH)D concentrations (total R-2 = 0.29). The association between total vitamin D intake and serum 25(OH)D concentrations was stronger among participants who spent less rather than more daylight time outdoors in summer (P-interaction = 0.026). History and medications for hypertension; hyperlipidemia, and type 2 diabetes and secondhand smoke exposure were not associated with serum 25(OH)D. In conclusion, dietary factors and sun exposure remain important determinants of vitamin D status in postmenopausal women. Vitamin D intake should be emphasized for those with limited sun exposure.