Digestible Amino Acids: Determination and Use

For a dietary amino acid to be retained in tissue protein in an animal, the amino acid needs to be ingested by the animal and absorbed from the intestinal tract. While the absorption of amino acids is a concept that can easily be defined, it is difficult to measure. Instead, the digestibility of amino acids is measured. Digestibility is defined as the difference between the amount of a certain amino acid ingested by the animal and the amount that is excreted in the feces or ileal fluids of the animal divided by the amount that is ingested (Sauer and Ozimek, 1986). By multiplying the fraction calculated this way by 100, the digestibility coefficient is calculated. Thus, digestibility coefficients are calculated by measuring the undigested quantity of dietary amino acids rather than the portion that was digested. It is assumed that the digestible amount of dietary amino acids equals the amount that was absorbed.