Comparison of Net Protein Utilization of Milk Protein Mild Enzymatic Hydrolysates and Free Amino Acid Mixtures With a Close Pattern in the Rat

The nutritive value of two nitrogen‐containing mixtures, one formed from small peptides (milk protein mild enzymatic hydrolysates) and the other consisting of a mixture of free amino acids having the same pattern except for glutamine, was measured in rats with and without experimental liver and exogenous pancreas dysfunction. For this purpose, 30 animals (group N) were fitted with an indwelling duodenal catheter; 36 animals (group L) also underwent ligation and resection of the biliopancreatic duct. After a 3‐day recovery period, the animals in each group, divided into three equivalent batches, were given, for 10 days, a protein‐free diet ad libitum and a twice‐daily duodenal infusion (5 mL) of either saline, the small peptides, or the free amino acid mixture. The nitrogen‐containing mixtures provided 0.32 g of nitrogen per day. A blood sample (left side of the heart) was collected 1 hour before (five to six animals per batch) and after (five to six animals per batch) the last infusion for determination of glucose, insulin, and amino acids. After the animals were killed, their carcasses were freeze‐dried, ground, and analyzed for nitrogen content. Under these conditions, the net protein utilization (the gain in body nitrogen in the animals infused with one of the two nitrogen‐containing solutions in comparison with the animals infused with saline only divided by the nitrogen ingested) was calculated. This was found to be greater for the mixtures of small peptides than for the mixtures of free amino acids (group N: p <.001; group L: p <.05) and greater under conditions of normal liver and pancreatic function in comparison with dysfunction ( p < 0.005). In the animals with normal hepatopancreatic function, the blood amino acid peaks 1 hour after infusion were higher for certain amino acids (especially arginine, leucine, and tyrosine) when milk protein mild enzymatic hydrolysate solution was infused than when the free amino acid solution was infused; the same was true for the blood insulin levels. It thus appears that under conditions of discontinuous enteral feeding, a mixture of small peptides is of greater nutritive value than a mixture of free amino acids having a close composition, probably because some amino acids from peptides are more rapidly absorbed and may produce greater insulin reaction. (Journal of Parenteral and Enteral Nutrition 17:355–363, 1993)