Factors influencing the measurement of bioavailability, taking calcium as a model

For non-metabolizable supplemental nutrients, bioavailability is effectivel y equivalent to absorbability. Methods for measuring absorbability (balance , pharmacokinetic, tracer, urine increment, evoked physiological responses, and in vitro) are briefly characterized and their utility compared. When i ntrinsic labeling of a source is possible, tracer methods are generally the most accurate and precise, as well as often the least expensive. Factors i nfluencing the measured end points of the various methods are described bri efly. These include source factors such as pharmaceutic formulation, subjec t factors such as mucosal mass and the need status of the absorbing subject , and co-ingested factors such as other foods or food constituents. Extensi ve experience has shown that absorbability is difficult to predict from kno wledge of the chemistry of the source, or even from the results of in vitro testing. Hence direct measurement of absorbability is essential to assure regulators and the general public that the source delivers what it promises .