Fecal Balance Studies Single Isotopic Label

Fecal Balance Studies (Single Isotopic Label) 

The crudest approach to study element absorption is by taking the human body as a black box. As for chemical balance studies, element absorption can then be assessed as the difference between dietary intake of an isotopic label and its fecal recovery. This approach, however, does not deliver the fraction of element that has truly been absorbed, that is, the fraction of element in the digested food moiety that has passed the intestinal mucosa. Digestion can only occur by action of 

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True absorption, as the element fraction that is taken up by the intestinal mucosa and released into circulation, cannot be assessed by fecal balance techniques using a single oral isotopic label only. A second label needs to be administered intravenously to determine the fraction of absorbed element that re-enters the intestine by secretory pathways. Intravenous isotopic labels in metabolic studies are usually intended to mimic freshly absorbed element from the diet. This entails that labels must be injected slowly over several minutes about 1 h after test meal administration or, ideally, as an infusion over 20-30 min in physiological saline. 

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