Pharmacokinetic implications of single and repeated dosage

In comparison with drugs, the postabsorptive curves of nutrients must be interpreted differently concerning their pharmacokinetic behavior: (1) Most nutrients do not behave as "nonreactives" in a strict sense. (2) For nutrients, the calculation of the retained quantity is important and not the calculation of the attainable concentration as in the case with drugs. These principles are shown for a lipid and a water soluble substance by calculated and observed cumulation curves: (1) The carotenoid canthaxanthin shows limited absorption. The primary invasion occurs via lymphatics as chylomicra. After absorption, the carotenoid shifts in part into other lipoprotein fractions with very low elimination constants, thus forming typical cumulation curves of very high plasma concentrations. (2) Vitamin B6, in contrast, is absorbed in an unlimited way up to very high doses. Nevertheless, rapid excretion prevents longlasting cumulation effects. However, chronical ingestion causes higher concentrations of binding proteins in different tissues. In this way, another type of long-lasting cumulation effect is induced.