NUTRIENT BIOAVAILABILITY, WITH SPECIAL REFERENCE TO CALCIUM

The term ''bioavailability'' attempts to include in a single concept t he effect of a sequence of metabolic events, i.e., digestibility, solu bilization, absorption, organ uptake and release, enzymatic transforma tion, secretion and excretion. Each of these events is difficult to me asure experimentally, and, with the possible exception of digestibilit y and solubilization, all are age-dependent and subject to nutritional and hormonal controls. In the case of calcium, the body's demand incr eases and then decreases with age; it also increases with pregnancy an d lactation. Age, pregnancy and lactation each affect the regulatable component of the intestinal absorption of calcium. The passive compone nt of calcium absorption is a function of the amount of calcium solubi lized and of intestinal transit time. Inasmuch as digestibility and so lubilization of calcium are very difficult to determine separately, ev en a reliable measure of calcium absorption includes a measure of unce rtainty. If one wishes to include in the term ''calcium bioavailabilit y'' rates of net deposition in bone, as well as rates of excretion fro m the body, quantitative information on calcium pool size, turnover an d the effects thereon of age, sex, endocrine and nutritional status ar e needed. In the case of other nutrients, rates of enzymatic transform ation and organ utilization need also to be taken into account. It wil l therefore require major research programs before the term '' bioavai lability'' of a nutrient can become a quantitative concept useful for clinical, nutritional or managerial evaluation and counseling.