13.2, 26.4, 39.7, 52.9% of the mean tryptophan requirement of 3.78 mg/kg/d will be given to subjects.

Tryptophan will be supplied at 13.2, 26.4, 39.7, or 52.9% of requirement; sourced from crystalline amino acid and white cornmeal maize.

The quality of corn protein will be assessed by measuring the amount of its tryptophan available for protein synthesis (metabolic availability) using the slope ration method. The oxidation of the indicator amino acid phenylalanine will be measured in response to feeding graded intakes of tryptophan in a reference protein patterned after egg protein, and provided as a crystalline amino acid mixture. The pattern of oxidation of phylalanine in response to tryptophan provided in corn will be compared to the pattern of oxidation obtained from feeding the reference protein. On each study day, subjects will be fed 8 hourly meals and breath samples will be taken to measure the oxidation of phenylalanine. Samples will be collected at baseline after the fourth meal, and half hourly, beginning 2.5 hrs after the 5th meal. Breath samples will be analyzed for 13CO2 enrichment.