Overview of zinc absorption and excretion in the human gastrointestinal tract

Zinc homeostasis is primarily maintained via the gastrointestinal system by the processes of absorption of exogenous zinc and gastrointestinal secreti on and excretion of endogenous zinc. Although these processes modulate net absorption and the size of the readily exchangeable zinc pools, there are l imits to the effectiveness of the homeostatic mechanisms of these and other systems, As a result of the interplay of the subcellular regulation of the se mechanisms and host, dietary and environmental factors, zinc deficiency is not uncommon, especially on a global basis. This overview briefly review s current understanding about the subcellular mechanisms of zinc absorption and transport, Factors recognized to affect zinc absorption at the whole b ody level are reviewed and include the amount and form of zinc consumed; di etary promoters, such as animal protein and low-molecular-weight organic co mpounds; dietary inhibitors, such as phytate and possibly iron and calcium when consumed as supplements; and physiologic states, such as pregnancy, la ctation and early infancy, all of which increase the demand for absorbed zi nc. The control of endogenously secreted zinc is less well understood. Avai lable data suggest that the quantity of secreted zinc with each meal may be considerable and that efficient reabsorption is critical to the maintenanc e of normal zinc balance. Factors that have been proposed to interfere with the normal reabsorption of endogenous zinc include phytate and unabsorbed fat. Understanding of the dietary, physiologic, pathologic and environmenta l factors that may adversely affect these processes, and therefore zinc hom eostasis, will be critical to preventing and treating zinc deficiency in hu man populations.