Main Uses of Fecal Markers

The fecal output (FO) of an animal depends on its intake (I) and the proportion of this that remains undigested. This proportion can be calculated as (1 — D), where D is the digestibility of the diet (D). In mathematical terms:

Rearranging this relationship provides the major approach to estimating intake:

External markers have been used for many years to determine FO. They are given as an oral dose once or twice daily, or in the form of a controlled-release device, which is dosed once and then resides in the digestive tract, releasing a known daily dose of marker. The FO is estimated from the dilution of the marker dose in feces.

Indigestible substances in the diet can also be used as fecal markers. In this case, they are functioning as internal markers. The increase in the fecal concentration of marker relative to the concentration in the diet provides an estimate of digestibility (D), which, in the previous equation, also allows the estimation of intake.

Many dietary substances have been evaluated as digestibility markers, and none has proved wholly successful. Consequently, digestibility has routinely been determined using laboratory procedures imitating the process of digestion (in vitro digestibility).

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