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 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.