Purified Amino Acids as a Source of Nitrogen for the Growing Rat

Abstract

A study was made of the growth of weanling rats receiving rations containing mixtures of purified amino acids, acid hydrolyzed casein and intact casein. Daily food consumption was recorded and data on the protein efficiency of the different rations are presented. The growth rate of young rats fed 20% acid hydrolyzed casein supplemented with 0.2% DL-tryptophan and 0.2% L-cystine was not significantly less than that of rats receiving an equivalent amount of intact casein, offering no support for the presumption that the rat requires a source of strepogenin under these conditions. Improved rations containing 18 amino acids to provide a nitrogen level of 2.5% gave average growth rates of 4.1 and 4.4 gm per day under conditions of ad libitum and forced, paired feeding, respectively. These values represent 80 to 90% of the growth rate of rats receiving ad libitum a ration containing casein at an isonitrogenous level. Evidence presented indicates that the improved growth rate was the result of increasing the level of arginine hydrochloride to 0.75% and adding 2% L-glutamic acid and 1.8% L-tyrosine to a ration containing other “non-essential” amino acids and the minimum levels of the 10 essential amino acids. The probable reasons for the slight inferiority of the amino acid-containing rations to the casein-containing ration are discussed.