To the Editor:
Bischoff et al.(1) add usefully to the body of information showing the importance of adequate vitamin D status for the minimization of the risk of falls in the elderly. They attribute their findings to an improvement in musculoskeletal function as a result of supplementation with vitamin D. As such, this constitutes yet one more piece of evidence building the case for clinically important functions of vitamin D apart from the operation of the calcium economy.
Described in their paper, but not commented on, are some important results with respect to the classical function of vitamin D on calcium absorption. Their subjects started from a mean serum 25(OH)D concentration of 12 ng/ml, a value that would be considered deficient by essentially all standards. In their vitamin D-supplemented group, they succeeded in raising that value to 26.2 ng/ml, a value above the lower end of the reference range, even if perhaps not fully optimal.(2,3) Both groups were given 1200 mg Ca/day, but only the group that received supplemental vitamin D showed evidence of a calcium effect. Specifically, serum parathyroid hormone (PTH) did not fall in the Ca-only group, whereas it did in the Ca + D group. Similarly, bone resorption biomarkers were virtually unchanged in the Ca-only group but fell by nearly one-third in the Ca + D group. It is well established that both changes occur with calcium alone in women who are vitamin D replete.
Although measurement of calcium absorption was not reported, these results strongly suggest that the group receiving only calcium were not, at their prevailing vitamin D levels, able to achieve sufficient net absorption to influence PTH secretion or the hormone's effects on bone resorption. In a sense, this is not surprising, because it has been long recognized that vitamin D is necessary for the active transport of calcium across the intestinal mucosa. What has been lacking from this picture in humans is quantitative information with respect to absorptive performance as a function of serum 25(OH)D level. We have recently shown that absorption, directly measured, improves when 25(OH)D level rises from a mean of 20 to 35 ng/ml.(2) However, we lacked information on absorption at lower 25(OH)D levels. This paper helps indirectly to fill that gap.
Parfitt, in his heuristically important reconceptualization of vitamin D insufficiency osteopathy,(4) presumes that calcium absorptive performance would be a function of vitamin D status up to repletion, but lacked the quantitative data needed to characterize the relationship. The study by Bischoff et al., together with our own direct measurement at higher 25(OH)D levels, adds evidence that this formulation is essentially correct, and that vitamin D repletion is important for fully normal control of calcium absorption efficiency within the range of 25(OH)D values typically encountered in various populations.
Absorption, as is generally recognized, occurs by a passive route, as well as by active transport. However, passive absorption is inefficient, probably accounting for gross transfer into the blood of no more than 10–15% of ingested calcium. At the 1200 mg calcium supplement dose used by Bischoff et al., that could have resulted in absorption of perhaps as much as 150 mg of calcium. However, it happens that digestive juice calcium, that is, flux of calcium in the opposite direction (out of the body and into the gut), amounts to nearly that same quantity.(5) This means that, in the absence of active transport, net absorption from the supplement used by Bischoff et al. would have been predicted to be close to zero. Their findings in the Ca-only group are consistent with that prediction.