Indicator Amino Acid Oxidation Is Not Affected by Period of Adaptation to a Wide Range of Lysine Intake in Healthy Young Men

The number of days of adaptation to a specific amino acid intake required prior to the determination of amino acid requirements using the indicator amino acid oxidation method (IAAO) is still in debate. In this study, our objective was to determine whether adaptation for 8 h, 3 d, and 7 d to a wide range of lysine intakes had any effect on the oxidation of the indicator amino acid, L-[1-C-13]phenylalanine, to (CO2)-C-13 ((FCO2)-C-13). Five healthy young men randomly received each of 4 levels of lysine (5, 20, 35, and 70 mg . kg(-1) . d(-1)) along with an amino acid mixture to achieve a protein intake of 1.0 g . kg(-1) . d(-1) and energy intake of 1.5x resting energy expenditure during 4 separate 7-d study periods. IAAO studies were conducted on d 1, 3, and 7. During each study day, oral consumption of L-[1-C-13]phenylalanine was followed by collection of breath for (FCO2)-C-13 and plasma for measurement of phenylalanine enrichment. (FCO2)-C-13 was affected by lysine intake but did not differ among adaptation periods of 8 h, 3 d, or 7 d. Phenylalanine flux was not significantly affected by period of adaptation. These results suggest that the minimally invasive IAAO model, where participants are adapted prior to protein intake for 2 d followed by study day adaptation to the test amino acid intake for 8 h, may be sufficient to estimate individual amino acid requirements in healthy young men. J. Nutr. 139: 1082-1087, 2009.