Determination of indispensable amino acid (IAA) requirements necessitates a range of intakes of the test IAA and monitoring of the physiological response. Short-term methods are the most feasible for studying multiple intake levels in the same individual. Carbon oxidation methods measure the excretion of 13CO2 in breath from a labelled amino acid (AA) in response to varying intakes of the test AA following a period of adaptation. However, the length of adaptation to each AA intake level has been a source of debate and disagreement among researchers. The assertion of the minimally invasive indicator amino acid oxidation (IAAO) technique is that IAA requirements can be estimated after only a few hours (8 h) of adaptation to each test AA intake, suggesting that adaptation occurs rapidly in response to dietary adjustments. On the contrary, the assertion of most other techniques is that 6–7 d of adaptation is required when determining IAA needs. It has even been argued that a minimum of two weeks is needed to achieve complete adaptation. This review explores evidence regarding AA oxidation methods and whether long periods of adaptation to test IAA levels are necessary when estimating IAA requirements. It was found that the consumption of experimental diets containing lower test IAA intake for greater than 7 d violates the terms of a successful adaptive response. While there is some evidence that short-term 8 h IAAO is not different among different test amino acid intakes up to 7 d, it is unclear whether it impacts assessment of IAA requirements.

The effect of frequency of feeding (once daily or six equal portions at intervals of 3 h) of diets containing lysine (6.4 g/kg) supplied as either protein-bound or free lysine on the metabolism of essential amino acids was investigated in pigs over the 23–34 kg weight range. The amounts of radioactivity from 14CO2 produced by the pigs given oral doses of [14C]lysine or [14C]phenylalanine were determined. [14C]Lysine was confirmed to be unsuitable presumably because the oral dose was diluted by the dietary free lysine or was absorbed ahead of the protein-bound lysine, or both. With [14C]phenylalanine addition, there was a significantly (P < 0.01) greater production of 14CO2 by pigs fed once daily on the diet containing free lysine. However, with frequent feeding, there was no significant difference (P > 0.05) in the amount of 14CO2 produced by pigs from [14C]phenylalanine for either diet. Growth responses of the pigs were lower when the diet containing free lysine was given once daily (P < 0.01) but were similar when the diets were given frequently. It is concluded that the more rapid absorption of free lysine by growing pigs fed once daily results in an imbalance of amino acids at the sites of metabolism. This leads to greater oxidation of essential amino acids than occurs when all the amino acids are supplied in the protein-bound form. With frequent feeding, a better balanced supply of amino acids is absorbed, leading to similar rates of oxidation of excess essential amino acids from diets containing either free or protein-bound lysine.