Abstract Vitamin D is suggested to have a role in the coupling of bone resorption and formation. Compared with women, men are believed to have more stable bone remodelling, and thus, are considered less susceptible to the seasonal variation of calcitropic hormones. We examinated whether seasonal variation exists in calcitropic hormones, bone remodelling markers and BMD in healthy men. Furthermore, we determined which vitamin D intake is required to prevent this variation. Subjects (N=48) comprised healthy Caucasian men aged 21-49 years, from the Helsinki area with a mean (SD) habitual dietary intake of vitamin D of 6.6 (5.1) mug/d. This was a 6-month double-blinded vitamin D intervention study, in which subjects were allocated to three groups of 20 mug (800 IU), 10 mug (400 IU), or placebo. Fasting blood samplings were collected six times for analyses of serum 25-OHD, iPTH, BALP and TRACP. Radial volumetric BMD was measured at the beginning and end of the study with peripheral QCT. Wintertime variation was noted in S-25-OHD, S-PTH and S-TRACP (p<0.001, p=0.012 and p<0.05, respectively), but not in S-BALP or vBMD in the placebo group. Supplementation inhibited the winter elevation of PTH (p=0.035), decreased the S-BALP concentration (p<0.05), but benefited the cortical BMD (p=0.09) only slightly. Healthy men are exposed to wintertime decrease in vitamin D status that impacts PTH concentration. Vitamin D supplementation improved vitamin D status and inhibited the winter elevation of PTH, and also decreased BALP concentration. The ratio of TRACP to BALP illustrates the coupling of bone remodelling in a robust way. A stable ratio was observed among those retaining a stable PTH throughout the study. A daily intake of vitamin D in the range of 17.5-20 mug (700-800 IU) appears to be required to prevent winter seasonal increases in PTH and maintain stable bone turnover in young, healthy Caucasian men.

Key words: Bone Health - Vitamin D, Calcitropic Hormones