[Circannual rhythm of plasma 25-hydroxycholecalciferol in normal man]

Circannual rhythm of serum 25OH-cholecalciferol has been evaluated in 11 normal adult subjects. Blood samples were collected monthly. The pattern of serum levels of the metabolite was biphasic with the lower average values between January and June and maximal mean levels between July and October. Mean values were higher than in North-European countries. The estimated dietary intake of cholecalciferol was in the normal range. Therefore diet does not seem to account for the relatively elevated serum levels of the metabolite. On the contrary sunlight exposure appeared to have marked influence both on mean 250HD levels and on the seasonal rhythm. In spite of seasonal wide variation of serum 250HD levels, blood calcium and phosphate values were stable. The results of serum PTH assay favour the hypothesis that the maintenance of steady blood calcium levels may be due to seasonal variation of parathyroid secretion.