Suppression of C-terminal telopeptide in hypovitaminosis D requires calcium as well as vitamin D.

We compared the effects of oral calcium and vitamin D separately and together on relevant variables in 22 postmenopausal volunteers with initial serum 25OHD levels below 60 nmol/L. Subjects were allocated randomly to two regimens: group 1 received 1 week of calcium 1,000 mg, followed by 7 weeks with additional vitamin D3 1,000 i.u. daily; group 2 received 7 weeks of D3 1,000 i.u. daily, followed by 1 week with additional calcium 1,000 mg. We measured serum calcium, phosphate, PTH, 25OHD, CTX, and ALP at baseline and after 1 and 8 weeks in group 1 and after 7 and 8 weeks in group 2. There were no significant changes in ALP from either vitamin D or calcium. Calcium caused significant elevation of serum 25OHD as well as major suppression of serum CTX, which could not easily be accounted for by suppression of PTH. Vitamin D caused no significant change in any variable except elevation of serum 25OHD. The suppressive effect of calcium (whether given first or second) on serum CTX was threefold greater than that of vitamin D (whether given first or second) (P < 0.001), although their suppressive effects on serum PTH were the same. Calcium and vitamin D yielded greater and more significant effects on all variables (except ALP) than either treatment alone. We suggest that calcium may elevate serum 25OHD by prolonging its half-life and that it may have an inhibitory effect on bone resorption independent of, or in addition to, its suppression of PTH.