Abstract
Seasonal variation of serum vitamin D metabolites, PTH, bone turnover markers, and bone mineral density (BMD), adjusted for confounding variables, was studied in a cross-sectional population of 251 ambulatory elderly women aged 65-77 yr. A significant (P <0.05) seasonal change was observed in serum 25 hydroxyvitamin D (25OHD), bone resorption marker (urine N-telopeptide), and BMD of the spine, total body, and mid-radius. Serum 25OHD was significantly lower (P <0.05) in winter (December, January, February, March) compared with summer (June, July, August, September), with the nadir in February (68.4 ± 6.74 nmol/liter) and the zenith in August (85.6 ± 5.12 nmol/liter). Mean serum PTH levels were higher in winter when serum 25OHD was low, and mean serum PTH was lower in summer when serum 25OHD was high, although the seasonal change in serum PTH was not significant. The change in serum 1,25-dihydroxy vitamin Da paralleled that of serum 25OHD levels, but the seasonal effect was not significant. Mean 24-h urine N-telopeptide showed a significant seasonal change (P <0.05); it was about 24% higher in February (zenith) compared with that in August (nadir). The zenith month of urine N-telopeptide levels corresponded to the nadir month of serum 25OHD levels and vice versa. A significant (P <0.05) inverse correlation was observed between 24-h urine N-telopeptides and serum 25OHD levels. There was a significant (P <0.05) seasonal change in mean BMD of spine, total body, and mid-radius. These changes paralleled those in serum 25OHD levels. Spine BMD was 8.4% higher in August (zenith) compared with that in February (nadir), whereas total body BMD and mid-radius BMD were 6.1 and 7.6% higher, respectively, in July (zenith) compared with that in January (nadir). There was a nonsignificant increase of 3.6% in total hip BMD. In summary (see Fig. 5), the seasonal changes in vitamin D metabolism in elderly women are closely associated with small changes in serum PTH, changes in bone resorption, and BMD.