In this review, the terminology that is
used to describe the bioavailability and ileal digestibility
of AA in pig feed ingredients is defined. Aspects of
the methodology to establish bioavailability and ileal
digestibility values also are discussed, and recommendations
about the use of these values are provided. Two
main factors can contribute to differences between bioavailability
and ileal digestibility of AA. First, some
AA, such as Lys, may be absorbed in chemical complexes
that preclude their use for metabolism. Second,
fermentation in the upper gut may result in a net loss
or gain of AA to the animal. In addition, dietary effects
on the efficiency of using bioavailable AA intake for
tissue growth or milk production should be considered
and may be attributed to endogenous AA losses in the
hindgut and the metabolic costs associated with endogenous
gut protein synthesis and losses. Ileal digestibility
values may be expressed as apparent ileal digestibility
(AID), standardized ileal digestibility (SID), or true ileal
digestibility (TID). These terms are used to specify
how ileal endogenous AA losses are reflected in digestibility
values. Ileal endogenous AA losses may be sepa