Utilization of protein hydrolysates by normal and protein-depleted animals

Abstract

1. 1. The protein stores of the animal are the labile cytoplasmic proteins in the tissues and body fluids, cellular nitrogen being drawn upon to maintain the nitrogen integrity of essential tissues. 2. 2. The amount of nitrogen excreted from the protein stores during the feeding of a protein-free ration decreases as the stores are depleted. Less dietary nitrogen is needed to maintain nitrogen equilibrium in a depleted than in a normal animal. 3. 3. The relationship between absorbed nitrogen and nitrogen balance is linear in the region of negative nitrogen balance becoming curvilinear in the region of positive balance. The slope of this line, called the nitrogen balance index, is a function of the retention of nitrogen in the body of the animal, the index increasing as the retention increases. The index evaluates the retention of dietary nitrogen for the overall filling of the protein stores of the animal. 4. 4. As the protein stores decrease the linear relationship between nitrogen balance and absorbed nitrogen extends further and further into the region of positive nitrogen balance, the greater the positive balance the greater the degree of depletion in proteins. Different patterns of amino acids fill the stores at different rates. Feeding a protein source above an amount which produces maximum filling, is inefficient. 5. 5. Supplementation of proteins and protein hydrolysates with amino acids can improve the indexes but imbalances produced by the addition of abnormal amounts of amino acids may cause loss in body nitrogen. It is possible under conditions of imbalance to build up one tissue while another is being torn down. 6. 6. Hydrolysates or amino acid mixtures fed intravenously have a lower index than when fed orally. The higher oral index is due, in part, to the effects of digestive and absorptive processes in the gut and more direct action of the liver on the products absorbed from the intestine. The increased urinary excretion of amino acids and peptides following intravenous feeding does not usually affect the index significantly. 7. 7. The retention of dietary nitrogen is a function, too, of caloric intake. There is a critical level of caloric intake (approximately 50 per cent of adequate) below which the retention of nitrogen is markedly reduced. 8. 8. The retention of nitrogen fed orally is not affected by the degree of enzymatic hydrolysis of the protein. 9. 9. Depletion in proteins results in reduction in protein stores of the body, labile liver cytoplasm, plasma albumin, hemoglobin, plasma gamma globulin and in the nitrogen of the other tissues of the body. Thirty times as much nitrogen is lost from the body tissues as from the plasma. A shift in body fluids accompanies the depletion, resulting in a nutritional edema accompanied usually by a fall in plasma volume. The decrease in plasma volume can be so great that hypoproteinemia does not develop. 10. 10. When the animal is repleted, the nitrogen is retained in the body tissue proteins and plasma proteins in the ratio of 30:1. Restoration of tissue nitrogen is a function of the patterns of amino acids which are given, different patterns producing different rates of repletion of the various protein stores.