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 amine 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 requirements estimate. We tested this hypothesis using IAAO be estimating the lysine requirement was determined from the rate of appearance of (CO2)-C-13 in breath ((FCO2)-C-13) by using a primed 4-h continuous infusion of L-[1-C-13]phenylalanine. Phenylalanine flux was not affected by graded increases in dietary lysine. Phenylalanine oxidation and (FCO2)-C-13 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.