LEUCINE OXIDATION CHANGES RAPIDLY AFTER DIETARY-PROTEIN INTAKE IS ALTERED IN ADULT WOMEN BUT LYSINE FLUX IS UNCHANGED AS IS LYSINE INCORPORATION INTO VLDL-APOLIPOPROTEIN B-100

We measured the C-13 enrichments of expired CO2 and deuterium enrichme nts of plasma free lysine and VLDL-apolipoprotein B-100 in five nullip arous women who received an oral bolus dose of [1-C-13]leucine and a p rimed, constant infusion of [H-2(4)]lysine on d 2 and 6 while consumin g protein diets of 1.5, 1.0 and 0.4 g.kg(-1).d(-1) Peak enrichments an d cumulative percent recoveries of C-13 in expired CO2 increased durin g the high, and decreased during the low protein periods within 24 h o f altered intakes; these changes averaged 89% of that on d 6 of the hi gh and low protein diets. The early changes in leucine oxidation showe d significant relationships with urinary nitrogen excretion on d 6 of the dietary periods. The ratio of the isotopic enrichment of lysine in VLDL-apolipoprotein B-100 to that in plasma was unaltered by the leve l of protein intake. Thus, amino acid oxidation adapts rapidly to alte red protein intakes in adult women and is useful to determine protein needs during rapidly changing physiologic conditions. However, 6-d per iods of protein intake over the range of 1.5 to 0.4 g.kg(-1).d(-1) do not affect the proportional use of dietary amino acids for hepatic sec retory protein synthesis.