Lysine requirement of adult males is not affected by decreasing dietary protein.

Recently, we reported the lysine requirement to be 41.2 mg.kg-1.d-1 using indicator amino acid oxidation (IAAO) in healthy adult males fed a protein intake of 1.0 g.kg-1.d-1. A lower protein intake has been hypothesized to significantly lower the requirement estimate. We tested this hypothesis using IAAO by estimating the lysine requirement at a protein intake of 0.8 g.kg-1.d-1 in five healthy adult males. Lysine requirement was determined from the rate of appearance of 13CO2 in breath (F13CO2) by using a primed 4-h continuous infusion of L-[1-(13)C]phenylalanine. Phenylalanine flux was not affected by graded increases in dietary lysine. Phenylalanine oxidation and F13CO2 decreased linearly (P < 0.05) as lysine intake increased to a breakpoint, after which the rates were not significantly different. Two-phase linear regression determined this breakpoint (mean lysine requirement) to occur at a dietary lysine intake of 45.0 mg.kg-1.d-1. We conclude that no reduction appears in the lysine requirement with a lower protein intake. Thus, lysine requirement appears not to be altered by protein intakes within the lower end of the protein requirement range.