Definition of apparent, true, and standardized ileal digestibility of amino acids in pigs

Measures of ileal digestibility (ID) are used routinely as estimates of amino acid (AA) bio-availability in pig feed ingredients. Values for ID may be expressed as apparent (AID), standardized (SID), or true (TID). Values for AID are calculated by deducting the total ileal outflow of AA (the sum of endogenous losses (IAAend) and non-digested dietary AA) from dietary AA intake. The IAAend may be separated into basal losses, which are not influenced by feed ingredient composition, and specific losses induced by feed ingredient characteristics such as anti-nutritional factors and dietary fiber. If the AID values are corrected for total IAAend, then values for TID are calculated. Lack of additivity of AID values in feed formulation may be overcome by correcting AID values for basal IAAend only, which yields SID values. Until reliable procedures for the routine measurement of specific IAAend become available, it is suggested that SID values are used for feed formulation. It is advisable that basal IAAend are measured in digestibility experiments and that these losses are reported with SID values.