Influence of daily calcium and vitamin D supplementation on parathyroid hormone secretion

Abstract

Calcium and vitamin D supplementation have been shown to reduce secondary hyperparathyroidism and play a role in age-related osteoporosis. In order to define the optimal regimen of calcium and vitamin Dsupplementation to produce the maximal inhibition of parathyroid hormone secretion ,we compared the administration of a calcium-vitamin D supplement as a single morning dose with the administration of twodivided doses at 6-hour intervals.Twelve healthy male volunteers were assigned to three investigational procedures ,which were alternated at weekly intervals. After a 'blank' control procedure ,whenthey were not exposed to any supplements ,they received one of two calcium-vitamin D supplement regimens: either two doses of Orocal® D3 (500 mg calcium and 400 IU vitamin D3) with a 6-hourinterval between doses ,or one water-soluble effervescent powder pack of Cacit® vitamin D3 ,taken in the morning (1000 mg calcium and 880 IU vitamin D3).During the three procedures (controland the two calcium-vitamin D supplementation protocols) ,veinous blood was drawn every 60 minutes for up to 9 hours ,for serum calcium and parathyroid hormone measurements. The order of administrationof the two calcium and vitamin D supplementation regimens was allocated by randomization.No significant changes in serum calcium were observed during the study. During the first 6 hours following calcium-vitaminD supplementation ,a statistically significant decrease in serum parathyroid hormone was observed with both regimens ,compared with baseline and the control procedure. During this first period ,no differenceswere observed between the two treatment regimens. However ,between the 6th and the 9th hour ,serum parathyroid hormone levels remained significantly decreased compared to baseline with the twice-daily OrocalD3 administration ,while they returned to baseline values with the once-daily Cacit D3 preparation. During this period ,the percentage decrease in serum parathyroid hormone relative to baseline was significantlygreater with Orocal D3 than Cacit D3 (p = 0.0021).We therefore conclude that the twice-daily administration of 500 mg calcium and 400 IU vitamin D3 at 6-hour intervals provides a more prolongeddecrease in serum parathyroid hormone levels than the administration of the same total amount of calcium and vitamin D ,as a single morning dose in young healthy volunteers. These results might have implicationsin terms of protection of the skeleton against secondary hyperparathyroidism and increased bone resorption and turnover in elderly subjects.