Dose dependence of zinc and manganese absorption in man.

Intestinal absorption and excretion are the main regulatory mechanisms for maintenance of body homeostasis of most trace elements. In spite of considerable research on trace element metabolism in man and experimental animals, the knowledge about the mechanisms involved in these processes is still limited. The characteristics of trace element metabolism, such as low fractional absorption, the presence of the elements in intestinal juices and a rapid plasma turn-over, complicate specific studies of intestinal trace element absorption and excretion in the intact animal or man. Several in vivo and in vitro experimental models have, therefore, been developed, e.g. everted intestinal sacs, in situ loops, brush-border-membrane vesicle preparations and intestinal perfusion techniques, to study the kinetics of trace element absorption, usually administering aqueous solutions of the element at different concentrations or with the addition of ligands. However, the chemical environment at the site for intestinal absorption after ingestion of food is much more complex with the presence of partly digested food components, amino acids, organic acids, competing ions, etc. Therefore, the results from experimental models have to be judged with caution.