Abstract
Vitamin D functions in many body systems, but perhaps the best attested of the nutrient's actions-and certainly the one first associated with human disease-is its role in transferring calcium (and phosphorus) from ingested food into the body fluids. In this capacity, vitamin D functions as a part of a control system that operates to maintain constancy of the calcium ion concentrations in the extracellular fluid against the demands of obligatory excretory losses and skeletal mineralization. In both transfers, vitamin D works in concert with parathyroid hormone. Quantitative analysis of the inputs and drains of the calcium economy reveals that, at contemporary calcium intakes, D-mediated absorptive enhancement only partially mitigates the impact of low calcium intake or large calcium losses. While 1,25(OH)2D is clearly the most potent form of the vitamin, 25(OH) D exerts significant vitamin-D-like activity in its own right at physiological serum levels. Optimal vitamin D status is operationally defined as a level of D intake (or production) high enough to ensure that the D-mediated transfers are not limited by D availability. However, at intakes closer to those prevailing during hominid evolution, minor shifts in vitamin D mediated absorption are fully adequate to compensate for stresses on the calcium economy.