Development of the indicator amino acid oxidation technique to determine the availability of amino acids from dietary protein in pigs.

Standardized ileal ("true") digestibility is currently the best estimate of amino acid digestibility, but it does not measure bioavailability. Growth assays to determine amino acid bioavailability are expensive and laborious; thus, a rapid method is needed. Applying the principle of slope-ratio assay to the indicator amino acid oxidation (IAAO) method, we hypothesized that the reduction in indicator oxidation per gram of lysine in feedstuffs relative to that per gram of free lysine represented the bioavailability of lysine, here termed "metabolic availability." Indicator oxidation in pigs was linear over increasing lysine intakes (r = 0.90, P = 0.001) when the dietary lysine contents were 2 SD below the mean lysine requirement of the pigs. Peas were treated (raw, heated to reduce lysine availability, or heated with added lysine) to test the responsiveness of the IAAO to differing lysine availability. Free lysine reduced indicator oxidation by 3.16% of dose oxidized per gram added lysine, whereas the addition of protein lysine as raw (-2.81%) and heated peas (-1.73%) reduced oxidation to a lesser degree. Adding free lysine to heated peas decreased indicator oxidation, evidence that heating had worsened the utilization of pea protein for protein synthesis by reducing the bioavailability of lysine alone. Pea diets differed only in the availability of lysine; therefore IAAO detected differences in lysine bioavailability. Because the IAAO technique responds to lysine available at the sites of protein synthesis, the metabolic availability covers all losses during digestion, absorption, and utilization of lysine. This method can determine the metabolic availability of amino acids of a feedstuff within 2 wk.