ACUTE PTH RESPONSE TO ORAL CALCIUM LOAD AND SEASONAL-VARIATION OF VITAMIN-D STATUS IN HEALTHY-YOUNG ADULT SUBJECTS

Objective: The aim of the present study was to check the relationship between seasonal variations of vitamin D status and parathyroid functi on explored both in basal conditions and after oral calcium load. Desi gn: The calcium and parathyroid hormone response to the intake of calc ium 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 (Ig of elemental calcium as calcium carbo nate) 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 (Ca 2+) and intact parathormone (PTH1-84) were measured at each time point and 25-hydroxyvitamin D (25(OH)D) was measured before each calcium te st. Results: After winter, basal 25(OH)D concentrations were decreased (from 16.4 +/- 6.6 to 11.5 +/- 4.4 mu g/l) and basal PTH concentratio ns were increased (from 24.1 +/- 6.5 to 31.7 +/- 9.1 pg/ml), and the d ifference between pre- and post-winter basal concentrations were stati stically significant for both variables (P < 0.001). A statistically s ignificant negative correlation between PTH and 25(OH)D was obtained b oth before (r = -0.63; P = 0.005) and after (r = -0.64; P = 0.004) win ter. The maximum decrement in PTH (Delta PTHmax) was not different bef ore (13.92 +/- 4.58 pg/ml) and after (14.14 +/- 7.79 pg/ml) winter, bu t as a consequence of post-winter higher basal levels of PTH, at all t ime points after oral calcium load, concentrations of PTH after winter were significantly higher than before. Conclusions: The present resul ts show that PTH concentrations are physiologically linked to 25(OH)D concentrations, and emphasize the need of taking into account the vita min D status of each subject to predict the effect of an oral calcium load on absolute concentrations of PTH.