Certain amino acids, including tryptophan, are essential for the growth of young animals and for maintenance in the adult. The necessity for the essential amino acids is not limited to protein for formation of new tissue; the metabolic use of methionine for detoxication and for the formation of such physiologically important substances as choline and creatine exemplifies this fact. Current experimental studies reveal metabolic functions of comparable importance for tryptophan.
Support for the view that the daily requirement for tryptophan may reflect other physiologic functions than formation of tissue protein is found in a recent observation in adult human subjects1 that only approximately 1 per cent of ingested tryptophan from dietary proteins appears in the urine as such. Since the fecal loss should be small according to the available evidence, a large part of ingested tryptophan must be accounted for by some other disposition. A portion of ingested tryptophan
THE METABOLISM OF TRYPTOPHAN. JAMA. 1947;134(2):147. doi:10.1001/jama.1947.02880190035011