Submitted to: Journal of Nutrition
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/10/1997
Publication Date: N/A
Citation: Interpretive Summary: We wanted to investigate the way the body uses the protein human beings take in through the diet; how much is available for growth and development, and how much is used up in the process of digestion, in breaking down the protein in the intestine. We studied seven pigs and implanted them with catheters in different places in order to take blood samples. We fed them a milk-based diet and infused through the stomach a solution of labeled algal protein to trace its path through the body. On average, only about 60 percent of the dietary intake of essential amino acids was absorbed, because the intestinal cells broke the remaining 40 percent down for catabolism. Since the piglet is a model for the human infant, this work has important implications for the way we assess human dietary intake. The results suggest that the amount of protein we eat in our diets is substantially less than the amount that can be used by the body for growth and development.
Technical Abstract: To investigate the extent and pathways of the first-pass intestinal metabolism of dietary amino acids, seven female pigs (28 d old, 8.0 kg) were implanted with arterial, venous, portal and gastric catheters and with an ultrasonic portal blood flow probe. The pigs were fed a milk-based diet once hourly and infused intragastrically with (U-**13C)algal protein. On average, 56% of the essential amino acid (EAA)**3 intake appeared in the portal blood. However, the net portal balance of methionine (48% of intake) and threonine (38% of intake) tended (P=0.08) to be lower than the mean of all EAA. The net portal balance (expressed as a percent of intake) of alanine (205%), tyrosine (167%) and arginine (137%) exceeded their intake. Net portal outflow of ammonia accounted for 18% of total amino acid nitrogen intake. As a percent of the enteral tracer input, there was substantial first-pass metabolism of lysine (35%), leucine (32%), phenylalanine (35%), and threonine (61%). However, only 12%, 21%, 18% and 18% of the total first-pass metabolism of threonine, leucine, lysine and phenylalanine, respectively, were recovered in mucosal protein. We conclude that roughly one-third of dietary intake of EAA is consumed in first-pass metabolism by the intestine and that amino acid catabolism by the mucosal cells is quantitatively greater than amino acid incorporation into mucosal protein.