Indicator amino acid oxidation responds rapidly to changes in lysine or protein intake in growing and adult pigs.

There is disagreement about the adaptation time required when using the indicator amino acid oxidation (IAAO) technique. Our objective was to establish the adaptation time required to obtain a plateau in indicator (L-[1-(14)C]-phenylalanine) oxidation in response to a test diet using growing and adult pigs. Four barrows (20 kg) and 4 sows (240 kg) were surgically implanted with venous catheters for isotope infusion. Growing Pigs: After 7 d of adaptation to an adequate lysine intake of 8.8 g/d, phenylalanine oxidation in growing pigs was 9.38 +/- 1.25% of the infused dose. At 2, 3, 4, or 6 d after reducing lysine intake to 3.8 g/d, and then increasing it back to 8.8 g/d, phenylalanine oxidation was 16.94 +/- 0.84% (P < 0.05) and 9.70 +/- 0.80% (P < 0.05), respectively, with no significant effect of days of adaptation to diet. Adult Pigs: After 14 d of adaptation to an intake of 200% of the amino acid maintenance requirement, phenylalanine oxidation in sows was 4.23 +/- 0.45% of dose. Changing the intake to 100 and 50% of the maintenance requirement, increased (P < 0.05) phenylalanine oxidation to 5.95 +/- 0.26 and 7.90 +/- 0.26%, respectively, with no significant effect of time (1, 2, 5, 6, 9, and 10 d) after diet change. The CV for repeated phenylalanine oxidation measurements within pigs and diets was 13.5% for growing and 8.8% for adult pigs. This demonstrates that the IAAO requires <2 d of adaptation regardless of age, dietary challenge (individual amino acid or total protein) or direction (increase or decrease) of change, and that the measured oxidation rate (% of dose) is highly repeatable.