Relative Potency of Diets Containing Amino Acids and Proteins for Promoting Growth of Rats

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Abstract
The necessity for inclusion of certain nonessential amino acids and of the high levels of essential amino acids sometimes used in diets for rats was investigated. The rate of weight gain of rats produced by the best amino acid diets was compared with that produced by diets containing intact protein or combinations of intact protein and amino acids. Data are presented on weight gains of young rats subjected to 23 different dietary treatments for 8 weeks. Accumulated weight gains for the first 4-weeks appeared to be somewhat more dependable than for the first 2-week period; there was no advantage in extending the test over an 8-week period. A mean weight gain of 44+ g/rat/week for a 4-week period was supported by a protein-free diet containing 14 amino acids and 2.16% total nitrogen; of this 0.66% was supplied by L-glutamic acid. A diet containing 19 amino acids and 3.03% total nitrogen was less effective. Similar diets containing 20% of casein, lactalbumen or protein equivalent of dried lean beef supported mean weight gains of 50+ g/rat/week over the same period. This was a significantly higher (P < 0.005) rate of gain than was produced by the best amino acid diet. When adequately supplemented with amino acids, 25% of peanut meal, 20% of wheat gluten or 10% of casein in the diet likewise produced mean weight gains of 50+ g/rat/week over a 4-week period. Thus, it was shown that there was a growth-stimulating effect of the intact proteins not observed in any of the amino acid combinations tested.