The Quantitative Amino Acid Requirements of Young Women I. Threonine

Abstract

The general plan and methods of studies to determine the quantitative requirements of young women for the amino acids threonine, valine, tryptophan, phenylalanine and leucine have been described. For the study of each amino acid there was an experimental period of 6 to 7 weeks during which a semi-purified diet was used. This included a few foods low in nitrogen, mineral and vitamin supplements, and chemically pure amino acids in the L form (except for DL-isoleucine). The intake of the amino acid being measured was reduced stepwise until negative nitrogen balance resulted. Usually the intake was then increased until nitrogen equilibrium was established. Nitrogen equilibrium was defined as the zone in which the excretion of nitrogen was within 95 to 105% of the intake, rather than a point at which excretion and intake were numerically equal. The smallest intake of an amino acid which permitted all of the subjects to remain in nitrogen equilibrium was considered the minimum requirement. After the minimum requirement of each of these amino acids had been determined separately, its adequacy was tested in the presence of minimum amounts of the other 4 amino acids. The results with respect to threonine are reported also. The first group of 6 subjects was studied on several different intakes of threonine. A daily intake of 214 mg of threonine was sufficient to maintain two subjects in nitrogen equilibrium and 103 mg was sufficient for two other subjects. However, in a second group of 5 subjects who were on an intake of 214 mg of threonine daily, two were in negative balance. A third group of 8 additional subjects was then used to test the adequacy of 214 mg of threonine for maintaining nitrogen equilibrium when valine, tryptophan, phenylalanine, and leucine were in the ration also in the minimum amounts. Six of these subjects remained in equilibrium; the other two subjects were in negative balance until their intakes of threonine were increased to 305 mg daily. The smallest intake of threonine which permitted all of the subjects to be in nitrogen equilibrium was 305 mg daily. This amount is rounded to 310 mg and suggested as a tentative minimum daily threonine requirement of normal young women similar to those studied here.