Digestion trials are used to estimate the absorption rate of an element by animals. There are apparent and true absorptions. Apparent absorption is defined as total intake minus total fecal excretion of the element, and that difference is often expressed as a percentage of the total intake. Fecal excretion of the element consists of the unabsorbed portion of the element and the absorbed portion that is subsequently excreted into the gastrointestinal tract. The latter portion of the total fecal excretion is designated as ''total endogenous fecal excretion.'' It can be estimated by extrapolating responses of fecal losses of the element to zero dietary intake or by monitoring fecal loss of an injected element isotope. True absorption corrects for the portion of total endogenous excretion and is calculated as follows:
True absorption = [total intake — (total fecal excretion — total endogenous fecal excretion)] /total intake x 100
The value for true absorption is greater than that for apparent absorption. True absorption is more accurate than apparent absorption for elements such as calcium, phosphorus, zinc, and copper, as the gastrointestinal tract is a major pathway of excretion for these elements. For both apparent and true absorption measures, the limitation lies in the fact that not all the absorbed elements are available for storage or physiological use by the animals.
Balance trials are used to estimate the retention of an element. It is defined as total intake minus total excretion (total fecal plus total urinary) of the element. As urine is a major pathway of excretion for minerals such as potassium, magnesium, and iodine, retention is a useful indicator of their bioavailability. However, in many situations, the mineral element excreted in the urine represents a portion that has been utilized in metabolism. In such cases, net retention is not a good indicator of bioavailability.
Both radioactive and stable isotopes can be used to determine absorption and retention of mineral elements. The procedure usually involves a single oral dose, followed by measurements of the administered radioactive or stable isotope in blood, target organs, or whole body. This approach is specific so it can sort out the unavailable portion from the endogenous loss of the testing elements. The disadvantage of this method is the need for special equipment to detect both types of isotopes and the very limited manipulating time.
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