Plasma calcidiol, season, and serum parathyroid hormone concentrations in healthy elderly men and women.

Wintertime declines in vitamin D lead to increased concentrations of parathyroid hormone (PTH) and accelerated bone loss in postmenopausal women. We conducted this study to compare calcidiol (25-hydroxyvitamin D) concentrations of men and women, to examine the influence of season, travel, vitamin D intake, and other variables on these concentrations, and to compare associations between calcidiol and PTH concentrations in elderly men and women. In this cross-sectional study of 182 men and 209 women aged > 65 y, mean calcidiol concentrations were higher in men than in women overall (mean +/- SD: 82.4 +/- 35.8 compared with 68.9 +/- 32.1 nmol/L, P < 0.001). In the subset measured in winter (February-May), plasma calcidiol concentrations were lower and not significantly different in men (59.4 +/- 21.8, n = 52) and women (57.7 +/- 23.4 nmol/L, n = 83). In a multiple-regression model, calcidiol concentrations were positively influenced by wintertime travel (P = 0.012), vitamin D intake (P = 0.002), and time spent outdoors (P = 0.096) and negatively influenced by weight (P < 0.001) and age (P = 0.039). Plasma calcidiol and serum PTH concentrations were inversely related, with PTH rising slowly as calcidiol concentrations declined below 110 nmol/L (95 CI: 60, 168 nmol/L). More than 90% of the men and women had calcidiol concentrations below this value in the wintertime. The high prevalence of lower wintertime calcidiol values may increase risk of bone loss in elderly men and women.