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Nutritional Studies with the Guinea Pig

VI. Tryptophan (with Ample Dietary Niacin)

National Institute of Arthritis and Metabolic Diseases and National Institute of Neurological Diseases and Blindness, Public Health Service, U. S. Department of Health, Education, and Welfare, Bethesda, Maryland

The requirement of the guinea pig for tryptophan was studied using a diet containing protein and which was adequate in all essential amino acids other than tryptophan.

Feeding the basal diet containing 0.108% of tryptophan, growth was slightly retarded and some fatalities occurred; after 6 to 8 weeks on the diet the eyes of all of the animals showed advanced cataractous changes and some corneal vascularization which could be seen on histological examination. Most of the animals showed alopecia, all had swollen abdomens and excreted soft, unformed feces. No discoloration of the teeth and no definite lowering of hemoglobin values were observed.

Maximum growth was obtained by the addition of 0.03% of L-tryptophan to the diet. At this level a definite improvement in the condition of the eyes occurred. Protection was not complete with the addition of 0.06% but it was with 0.1%. The requirement for complete eye protection and for good growth, therefore, is more than 0.16% but possibly somewhat less than 0.2%.

The D-isomer appeared to have from one-fourth to one-third the growth-promoting activity of the L-form. An addition of 0.06% to the diet was necessary to obtain 100% survival. Whether the D-form was at all protective to the eyes is doubtful. The presence of the D-isomer did not appear to suppress the protective action of the L-form on the eyes and at the higher supplemental levels it may possibly have had a slight additional effect.

The most outstanding finding in these studies is that the requirement for tryptophan by one specific organ, the eye, is greater than that for maximum growth. It is probably the first observation of a higher amino acid requirement of a specific organ in comparison to the body as a whole.