Utilization of Calories and Nitrogen by Rats Fed Diets Containing Purified Casein versus a Mixture of Amino Acids Simulating Casein

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Abstract
The effects were studied of varying nitrogen and energy intake levels on calorie and nitrogen storage in rats of 2 age groups. Male rats 28 or 100 days of age were fed for 4 and 6 weeks, respectively, diets containing either casein or an amino acid mixture simulating casein, at 2 levels of nitrogen and 2 levels of calorie intake. For the young rats, at the lower calorie intake and at both levels of nitrogen intake, there were no significant differences in nitrogen gains due to nitrogen source. At the higher calorie intake, and at both levels of nitrogen intake, nitrogen gains were higher for rats receiving casein than for those receiving amino acids. For the adult rats, at the lower nitrogen intake there were no significant differences in nitrogen gains (or losses) due to nitrogen source, regardless of the calorie intake. At the higher nitrogen intake and at both levels of calorie intake, animals receiving casein gained more (or lost less) nitrogen than rats receiving amino acids. It is postulated that a difference in the most limiting factor (whether energy or nitrogen) accounted for the difference in the results between the 2 age groups. For both the young and the adult rats, calories stored were unaffected by substituting an amino acid mixture for casein.