Time course of the total and radioactive carbon dioxide production by piglets receiving dietary [14C]phenylalanine.

The influence of experimental protocol (i.e., timing of feeding and of the collection of expired carbon dioxide) upon the sensitivity of using the oxidation of a labelled amino acid as an indicator of the adequacy of the dietary amino acid balance was investigated using 2.5-kg piglets confined in ventilated chambers. In the first experiment it was shown that consumption of two meals, each of 20 g and separated by 2 h, after an overnight fast resulted in a rapid rise in carbon dioxide concentration in the air withdrawn from the chamber. The concentration peaked 12 min after the second meal and remained elevated for 50 min. In the second experiment the piglets received two meals containing three levels of either lysine (8, 10, and 12 g/kg) or tryptophan (1.3, 1.8, and 2.0 g/kg) plus 10 microCi L-[alanine-1-14C]phenylalanine (1 Ci = 37 GBq) in each meal. Radioactivity released as 14CO2 peaked in the second hour following the last meal, but there was such great variation in the mean for the three diets that they were not statistically different. However, in the third hour following the second meal both the means and their variances had decreased with the differences between the diets being statistically significant for this time period. An experiment with varying levels of tryptophan showed a similar effect of meal consumption on 14CO2 release. The third experiment confirmed the effect of time of collection upon the statistical significance of the dietary amino acid balance measured by the release of radioactivity from [14C]phenylalanine.