The digestion and absorption of protein by normal man

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Abstract
Yeast N15-labelled protein and yeast N15-iabelled protein hydrolysate were fed in amounts equivalent to 0.9-0.4 mg of N15 /kg body wt. to healthy human subjects. The proportions of fed N15 in the urine 72 hr. after feeding either whole protein or protein hydrolysate were similar, and less than when a single N15-labelled amino acid was fed. Values after 24 hr. periods, anf for fecal elimination of the isotope, were more variable. Comparison between the N15 content of urinary urea and blood urea in the initial stages of the experiments showed consistently higher values for the former. Both showed almost maximum labelling within 1 hr. of ingestion of whole protein and protein hydrolysate. The N15 contents of urinary ammonia in both types of experiment rose sharply to maximum values 30-60 min. after ingestion and fell rapidly thereafter. Plasma amino N showed similar time relationships; urinary amino N values were less consistent. Under the conditions of the experiment hydrolysis of protein within the gut was not a major rate-determining factor in its absorption. in experiments at pH 7.2 and 37[degree] in vitro with high concentrations of trypsin and chymotrypsin yeast protein broke down rapidly to trichloroacetic acid-soluble material. The significance of these findings is discussed in relation to the problems of digestion and absorption of protein in the human subject.