Objective: The aim of the present study was to check the relationship between seasonal variations of vitamin D status and parathyroid function explored both in basal conditions and after oral calcium load.
Design: The calcium and parathyroid hormone response to the intake of calcium load was studied at two different seasons, before winter (November) and after winter (March-April), corresponding to different vitamin D status.
Subjects: Eighteen healthy young male adults (age: 25 +/- 3 y) were studied. All were medical students who were selected as having no disorders known to affect calcium metabolism.
Intervention: At each period an oral calcium load (1 g of elemental calcium as calcium carbonate) was administered. Blood samples were collected before and 1 h, 2 h, 3 h and 4 h after the intake of calcium. Serum ionized calcium (Ca2+) and intact parathormone (PTH1-84) were measured at each time point and 25-hydroxyvitamin D (25(OH)D) was measured before each calcium test.
Results: After winter, basal 25(OH)D concentrations were decreased (from 16.4 +/- 6.6 to 11.5 +/- 4.4 micrograms/l) and basal PTH concentrations were increased (from 24.1 +/- 6.5 to 31.7 +/- 9.1 pg/ ml), and the difference between pre- and post-winter basal concentrations were statistically significant for both variables (P < 0.001). A statistically significant negative correlation between PTH and 25(OH)D was obtained both before (r = -0.63; P = 0.005) and after (r = -0.64; P = 0.004) winter. The maximum decrement in PTH (delta PTHmax) was not different before (13.92 +/- 4.58 pg/ml) and after (14.14 +/- 7.79 pg/ml) winter, but as a consequence of post-winter higher basal levels of PTH, at all time points after oral calcium load, concentrations of PTH after winter were significantly higher than before.
Conclusions: The present results show that PTH concentrations are physiologically linked to 25(OH)D concentrations, and emphasize the need of taking into account the vitamin D status of each subject to predict the effect of an oral calcium load on absolute concentrations of PTH.