Table 1.

Relation of vitamin D intake, leisure-time physical activity, and season to plasma 25(OH)D in premenopausal women

Vitamin D sourcesβ*P
Vitamin D intake (IU/d)
    From food0.028<0.001
    From supplements0.014<0.001
Leisure-time physical activity (MET-hour/wk)0.168<0.001
    Winter (reference)
  • * β values are estimated from linear regression analysis [25(OH)D is treated as a continuous variable]. β values represent absolute mean difference in nmol/L of plasma 25(OH)D for increments of 1 IU vitamin D intake, 1 MET-hour/wk of physical activity, or for blood collected in spring, summer, or fall compared with winter (adjusted mean in winter, the season of reference, is 57.0 nmol/L). β values are estimated from a model that included age (y), body mass index (kg/m2), and all variables in the table. Pseason < 0.0001 (3 degrees of freedom).

  • The number of women who took vitamin D supplements was 190. The number recruited in spring, summer, fall, and winter was 249, 184, 218, and 90, respectively.