Amino acid imbalance and nitrogen retention.

It is 12 years since Krehl and coworkers (1, 2) reported that the growth of young rats fed on low protein diets deficient in niacin and tryptophan was retarded when a tryptophan-deficient protein supplement such as gelatin or zein was added to the diet, and that the growth retardation was prevented if additional tryptophan was provided. In the interval, amino acid imbalances involving practically all of the essential amino acids have been demonstrated in several species of animals raised on variously supplemented diets containing cereals or purified proteins (3-7). Growth retardations have generally served as a means of detecting amino acid imbalances; however, in some instances other metabolic defects such as an accumulation of fat in the liver (8, 9) and changes in the activities of certain liver enzyme systems (10, 11) were found. Observations on the relationship between amino acid imbalance and liver fat deposition suggest that the proportions of amino acids required for certain specific physiological processes may differ from those required for growth. When the rate of gain is increased by providing the amino acid most limiting for growth, more of the other amino acids are probably used for tissue protein synthesis and less may be available for specific functions such as the control of liver fat deposition (9). With regard to the growth-retarding effect of amino acid imbalance, it seems likely that, when additional amino acids which cannot be used for tissue protein synthesis are excreted, there may also be some increase in the loss of the amino acid which is most limiting for growth (12) ; such a loss would depress the growth rate. It seems likely that a general increase in amino acid. catabolism might result from the creation of an amino acid imbalance which retards growth. The present investigation on the effect of amino acid imbalance on nitrogen retention in rats fed a low protein diet containing 6 per cent of fibrin