Stimulation of intestinal calcium absorption by orally administrated vitamin D3 compounds: a prospective open-label randomized trial in osteoporosis

SUMMARY: Intestinal fractional calcium absorption (FCA) was assessed before and after vitamin D3 treatment. Serum 1,25(OH)(2)D concentration was significantly increased by plain vitamin D3 and reduced by eldecalcitol. The 1α hydroxyl calcidiol and eldecalcitol treatments increased FCA, which may be induced through direct stimulation of vitamin D receptors in the intestine. INTRODUCTION: To assess the effects of vitamin D3 compounds on intestinal FCA and calcium-regulating hormones in post-menopausal osteoporosis, a randomized open-label prospective study was conducted. METHODS: Forty eligible patients were allocated randomly into four groups: eldecalcitol (ELD; 0.75 μg/day), 1α hydroxyl calcidiol (ALF; 1 μg/day), plain vitamin D3 (800 IU/day), and control. Before and after the 4-week treatment, intestinal FCA was estimated by using a double isotope method, and serum concentrations of calcium-regulating hormones and a bone turnover marker were measured. RESULTS: The baseline FCA value of the participants was 21.5 ± 7.9% (mean ± SD) and was significantly correlated with serum 1,25(OH)(2)D (calcitriol) concentration. After the treatment, the FCA significantly increased by 59.5% (95% CI, 41.6 to 77.4%) in the ELD group and by 45.9% (27.9 to 63.8%) in the ALF group, whereas no significant change in the plain vitamin D3 group was found. Unlike the baseline FCA, post-treatment FCA exhibited no significant correlation with serum calcitriol concentration. Parathyroid hormone levels were suppressed by ALF and plain vitamin D3 but were sustained in the ELD and control groups. Serum calcitriol tended to be suppressed by ELD, whereas plain vitamin D3 treatment increased both serum 25(OH)D and calcitriol concentrations. CONCLUSION: These findings suggest that oral administration of vitamin D3 analogues (ALF and ELD) stimulates FCA but plain vitamin D3 does not. Those effects of vitamin D3 compounds on FCA were independent of serum calcitriol concentration, suggesting that ALF and ELD may directly stimulate intestinal vitamin D receptors.