TIME-COURSE OF AMINO-ACID-ABSORPTION IN GROWING RATS ESTIMATED AFTER FEEDING OF A N-15-LABELED WHEAT/YEAST RATION

The time course of AA digestion, AA balance (sV AS), and AA absorption (wV AS) was estimated on growing rats (Wistar rats, LW= 124 g) in different sections of the intestinal tract using the combination of N-15 tracer and TiO, marker techniques. The animals received once a diet of N-15 labelled wheat and yeast as protein sources supplemented by TiO2 as a marker. Up to 6 h after feeding the amino acid composition the N-15 excess acid the TiO2 content in the digesta of stomach, small and large intestine were determinated in the relation of amino acids resp, of N-15 labelled amino acids to the marker. In addition the content of amino acids and the N-15 excess Of these amino acids were estimated in plasma. From these data the disappearance rates and the relation of exogenous to endogenous amino acids as well as the sV and the wV values of the different amino acids were calculated for the different gut sections. The following results were obtained: The relative disappearance rate for N and TiO2 marker out of the stomach went approximately parallel but with a delay for TiO2 of about 30 minutes. The AA composition of the stomach content, the small and the large intestine content did not vary in dependence of the time. The AA composition of the stomach digesta was nearly identical to that of the diet, while that of the small intestine was between exogenous AA composition (feed) and endogenous AA composition (digesta on protein free feeding). AA composition of the large intestine digesta showed quite big differences (bacterial AA break down and AA synthesis). Considering a delay time (small intestine: 1 h, large intestine: 4 h) the exogenous portion of the different AA remained constant in both of these intestinal sections during the whole experimental time. The exogenous AA part varied for small intestine digesta between 31 and 69% (mean value: 41%), and for large intestine digesta between 13 and 39% (mean value: 22%). The sV AS values in the small intestine (AA balance resp. precaecal digestibility) differed from 61% (threonine) to 86% (proline) with an average of 73.4 +/- 7.4%, those for wV AS (AA absorption) from 81% (lysine) to 94% (proline) with an average of 88.1 +/- 4.1%. There were significant differences between AA, but they are negligible for practical purposes. In the small intestine the estimated Values for postprandial absorption of the exogenous AA accounted after 1 h, 2 h, 4 h and 6 h for 21%, 33.7%, 46.5%, and 70.7% of the AA intake respectively, mean absorption rate=9.1 +/- 0.5%/h. The AA balance in the whole tract (sV AS) was 75 to 94% (on average 82 +/- 5.3%) and the wV AS (balance corrected by the N-15 method) ranged from 92 to 99% (on average 96 +/- 1.8%). These values correspond to the faecal AA digestibility in conventional experiments. In the large intestine the postileal disappearance of the AA (sV) was on average 9.7% with a maximum value of 26% for glycine, and with a minimum for methionine of -2.1% caused by bacterial synthesis of methionine. The postileal wV in the large intestine amounted to 7.5%. The time course of the disappearance rate of the N-15 labelled AA in the small intestine and the appearance rate of these AA in the plasma showed an analogous behavior. Both of them characterize the postprandial absorption. The following conclusion can be drawn: The method used (combination of N-15 tracer and TiO2 marker techniques) enables determining the time course of transit and the variation of exogenous AA: endogenous AA proportion in the different intestinal sections and estimating the faecal and precaecal digestibility of the different AA. The course of secretion and absorption of the different AA should be specified in further experiments using the more precise analysis of N-15 by GC-MS resp. GC-C-IRMS technique. An apply of this method to farm animals (pigs) seems to be possible.