The animal’s amino acid requirement and the potential of feedstuffs to supply those amino acids in metabolically available form (amino acid bioavailability) are intertwined. Although standardized ileal amino acid digestibility is currently most widely used as an estimate of dietary protein quality, numerous factors influence these estimates. Slope ratio assays are considered the standard against which other methods of amino acid availability are judged because they measure the effect of protein quality on the entire metabolism of the animal. However, practical use of slope ratio growth assays is limited. Recently, the slope ratio assay utilizing the indicator amino acid oxidation technique has been developed to determine the
metabolic availability (MA) of amino acids in pigs and humans This review will discuss how the indicator amino acid oxidation (IAAO) technique can: 1) address some of the problems associated with the slope ratio assay; 2) be used to determine the metabolic availability (MA) of amino acids in animal feedstuffs; 3) become a valuable measure of amino acid bioavailability; and 4) be applied in practical swine nutrition.