Calculation of Zinc Absorption in Humans Using Tracers by Fecal Monitoring and a Compartmental Approach

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Abstract
Zinc absorption is often determined following oral administration of tracer by fecal monitoring as dose minus tracer excreted in feces. The value obtained for absorption with this method is influenced by excretion of absorbed tracer into feces during the fecal collection period and by any incomplete elimination of unabsorbed tracer. In the present study a published, physiologically based compartmental model of zinc metabolism has been used to calculate, after oral tracer administration, the fecal appearance of unabsorbed and absorbed tracer and the appearance when fractional rates of endogenous excretion and elimination were varied. Absorption was determined by fecal monitoring and compared to the value determined using parameter values of the compartmental model. The value calculated for absorption using fecal monitoring varied with length of fecal collection, rate of excretion of absorbed tracer (secretion) and rate of elimination of unabsorbed tracer. Absorption was determined correctly by fecal monitoring only when the amount of absorbed tracer excreted was equivalent to the amount of unabsorbed tracer remaining in the gut. Fecal monitoring determines a parameter representing the combined processes of absorption, endogenous excretion (or secretion) and fecal elimination.