Utilization of Gelatin, Casein and Zein by Adult Rats

Abstract

Nitrogen balance experiments were carried out with adult male rats, using casein, gelatin and zein. The percentage retention calculated by McCollum's ('12) method averaged 74 for casein, 23 for gelatin and 57 for zein. Studies were made of the nitrogen balance data, first, to determine possible errors inherent in this type of experiment, and, second, to find significant correlations. With the large adult male rats used it was found unnecessary to correct the urinary nitrogen for the loss in weight during the experiment. A fall in calorie intake somewhat below the theoretical requirements during the ‘nitrogen free’ feeding period was of no apparent significance. There was no apparent supplementary effect of the nitrogenous compounds of the yeast extract and the dextrin on the inadequate proteins used. The percentage retention increased as less protein was ingested, even though it was never fed above the endogenous level. When the same animals were used in experiments with more than one protein, there was no significant correlation between the individual utilization of the different proteins. Gelatin and casein were so well digested that there was no significant difference between the percentage retention calculated by McCollum's ('12) method and the biological value calculated by Mitchell's ('23-'24a) method. However, zein showed a poor and variable digestibility, the exogenous fecal nitrogen ranging from 4 to 36 per cent of the ingested zein. The biological values varied widely, from 18 to 72, the average being 45. This wide spread in results led to the discovery of a highly significant correlation between the exogenous fecal nitrogen expressed as the percentage of nitrogen ingested and the exogenous urinary nitrogen expressed as the percentage of nitrogen resorbed. Further experiments with rats of known efficiency index for growth showed no relation between the inherent efficiency of the animal and the digestibility or utilization of the zein. Analysis of undigested zein residues for nitrogen distribution furnished no explanation of the highly significant correlation between the food wastage in the feces and that in the urine.