Effect of vitamin D intake on seasonal variations in parathyroid hormone secretion in postmenopausal women.

Vitamin D intake should be sufficient to maintain calcium absorption and prevent increased parathyroid secretion throughout the year. To determine the level of intake that achieved the latter in elderly women, we studied the interrelations among vitamin D intake, serum 25-hydroxyvitamin D (25(OH)D) levels, and parathyroid hormone concentrations in a cross-sectional study of 333 healthy, white, postmenopausal women with low median calcium (408 mg a day) and vitamin D (112 IU a day) intakes who lived in Massachusetts. The overall inverse relation between serum parathyroid hormone and 25(OH)D levels was found to be dependent on vitamin D intake. In women whose estimated intake of vitamin D was less than or equal to 220 IU a day, the mean (+/- SD) serum parathyroid hormone values were lowest in those studied between August and October (30 +/- 11 ng per liter; n = 72) and highest in those studied between March and May (37 +/- 16 ng per liter; n = 54); the respective serum 25(OH)D levels were 93 +/- 32 and 63 +/- 21 nmol per liter. At vitamin D intakes of more than 220 IU a day, the mean serum parathyroid hormone and 25(OH)D levels did not vary with the season. The correlation between vitamin D intake and serum 25(OH)D concentration, although significant in all women (r = 0.29; P less than 0.001), was highest in those studied between March and May (r = 0.65; P less than 0.001) and lowest in those studied between August and October (r = 0.13; P greater than 0.10). The estimated serum 25(OH)D level associated with a vitamin D intake of 220 IU a day between March and May was 95 nmol per liter. Mean serum calcium values were similar at all times in both groups. We conclude that the dietary intake of more than 220 IU of vitamin D a day by postmenopausal women in Massachusetts may be sufficient to maintain constant serum 25(OH)D and parathyroid hormone concentrations throughout the year. Such an intake prevents a seasonal increase in parathyroid hormone secretion, with its possible deleterious skeletal effects.