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

For non-metabolizable supplemental nutrients, bioavailability is effectively 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 intrinsic labeling of a source is possible, tracer methods are generally the most accurate and precise, as well as often the least expensive. Factors influencing the measured end points of the various methods are described briefly. These include source factors such as pharmaceutic formulation, subject factors such as mucosal mass and the need status of the absorbing subject, and co-ingested factors such as other foods or food constituents. Extensive experience has shown that absorbability is difficult to predict from knowledge 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.