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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-100^1,2,

Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030

We measured the ¹³C enrichments of expired CO₂ and deuterium enrichments of plasma free lysine and VLDL-apolipoprotein B-100 in five nulliparous women who received an oral bolus dose of [1-¹³C]leucine and a primed, constant infusion of [²H₄]lysine on d 2 and 6 while consuming protein diets of 1.5, 1.0 and 0.4 g·kg^-1·d^-1. Peak enrichments and cumulative percent recoveries of ¹³C in expired CO₂ increased during the high, and decreased during the low protein periods within 24 h of altered intakes; these changes averaged 89% of that on d 6 of the high and low protein diets. The early changes in leucine oxidation showed 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 level of protein intake. Thus, amino acid oxidation adapts rapidly to altered protein intakes in adult women and is useful to determine protein needs during rapidly changing physiologic conditions. However, 6-d periods 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 secretory protein synthesis.

KEY WORDS: • oxidation • apolipoproteins • protein synthesis • amino acids • humans

¹ This work is a publication of the USDA, ARS Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX, and has been funded with federal funds from the U.S. Department of Agriculture, Agricultural Research Service, under Cooperative Agreement no. 38-6250-1-003. The contents of this publication do not necessarily reflect the views or policies of the U.S. Department of Agriculture, nor does mention of trade names, commercial products or organizations imply endorsement by the United States government.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence should be addressed.

Manuscript received 8 January 1993. Revision accepted 10 August 1993.

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