Quantitative and qualitative circadian variations of amino acid intestinal efflux in mixed-fed and in protein-meal-fed rats.

The magnitude and composition of amino acid intestinal efflux was followed over a light-dark cycle in rats ingesting the same daily amount of protein administered either in a mixed diet (12% casein) or as a separate meal (70% casein concentrate) fed 2 hours after the onset of the light period with a protein-free diet available at all times. Intestinal efflux was determined by an instant porto-aortic difference measured on pooled samples from six rats at time-points spaced every 3 hours over a light-dark cycle. During protein digestion (dark for the mixed-fed rats) and (light for the separately-fed ones), essential amino acid composition of intestinal output fell into line with that of the protein ingested (casein) while non-essential amino acid composition did not. The discrepancy bore on alanine and glycine which were released in excess and on aspartic and glutamic acids, glutamine and serine which were released in deficit from their content in casein. From the follow-up of individual amino acid release and uptake, we concluded that intestinal efflux reflects the composition of the dietary protein only with respect to the amino acids, mostly essential, that are not metabolized by the intestinal wall.