BIOAVAILABILITY OF ORAL VITAMINS, MINERALS, AND TRACE-ELEMENTS IN PERSPECTIVE

Bioavailability of orally administered vitamins, minerals, and trace e lements is subject to a complex set of influences. Still, administrati ve regulation is necessary on how to quantify it. The most common appr oach to this problem is to determine the fraction of an oral dose that reaches the systemic circulation. For micronutrients, however, this a pproach has to consider the physiological plasma concentration as well as the mechanisms that regulate intestinal absorption and distributio n of micronutrients between functional and storage compartments in res ponse to the demand. The rate of exchange between these compartments h as an impact on the delivery of such compounds into the plasma compart ment as well as on the plasma clearance. Monitoring the area under the plasma concentration time curve after oral administration is an inade quate tool for bioavailability determination if there are substantial impacts of homeostatic mechanisms on the plasma concentration of a mic ronutrient. In nutritional science the term ''bioavailability'' encomp asses the sum of impacts that may reduce or foster the metabolic utili sation of a nutrient. Bioavailability in this sense can be quantified by the rate by which deficiency symptoms are cured or by the weight ga in during growth. Both of these endpoints, again, are influenced by ho meostatic mechanisms. To exemplify the scope of impacts on parameters that are commonly used to quantify the bioavailability of oral micronu trient preparations the basic traits of homeostatic regulation are sum marised and compared for iron, magnesium, vitamin A, folic acid, and v itamin B-12. The mechanisms that adapt absorption, distribution, and e xcretion of these five micronutrients to the demand differ to such an extent that no common approach can be derived to consider these impact s in bioavailability determination. In consequence, therefore, we reco mmend to define and regulate individual strategies for bioavailability testing for each micronutrient with regulated kinetics.