In the 1980s, Young and his coworkers introduced the use of measurements of the carbon oxidation of single indispensable amino acids as indicators of adequacy of the amino acids (Young et al., 1989). This marked a major theoretical advance over the nitrogen balance and plasma amino acid response methods. The theoretical basis of the direct amino acid oxidation (DAAO) method is that the nutritional indispensability of an amino acid is a function of its inability to synthesize its carbon skeleton.
Thus, when the test amino acid is labeled with the production of breath 13CO2 is assumed to be a good measure of the irreversible oxidative loss of the amino acid.
The method has been applied in a similar manner to the plasma amino acid response approach by designing experiments to determine a breakpoint in the relationship between the carbon catabolism of the amino acid (as measured as breath 13CO2) and its intake. Thus by analogy to the concentration method, it is assumed that below the requirement the test amino acid is conserved and that there is a low constant oxidation rate, but once the requirement is reached, the oxidation of the test amino acid increases progressively.
Although the DAAO method was an important advance beyond nitrogen balance, it also presented limitations, which have been discussed in depth (Fuller and Garlick, 1994). The most salient problem arises from the reliance on the determination of a breakpoint in the oxidation of the test amino acid. This reliance requires studies at very low intakes of the test amino acid. However, at these low dietary intakes, the intake of the infused labeled amino acid becomes significant in relation to dietary intake. This can lead to errors in the estimation of amino acid oxidation based on the production of labeled CO2. Thus, values of amino acid oxidation based on the production of 13CO2 are likely to be overestimated. The second limitation is that the DAAO method can only be used with full accuracy for those amino acids whose carboxyl group is released directly to the body bicarbonate pools. This limits its use largely to the branched chain amino acids, phenylalanine, and lysine. Other amino acids, such as threonine and tryptophan, pose particular problems (Zhao et al., 1986).
A criticism of this method has been that measurements were only made during a short period during which food was given at regular hourly intervals. This period was therefore not representative of the day as a whole. A later modification of this approach was to infuse the labeled amino acid during a period of fasting followed by a period of hourly meals, thus acknowledging the discontinuous way in which food is normally taken (Young et al., 1987). However, although this was an advance on the earlier approach, assumptions still had to be made to extrapolate the results from the short periods to a full day.
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