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Estimates of the Effect of Feeding on Whole-Body Protein Degradation in Women Vary with the Amino Acid Used as Tracer^1,2,

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

We measured how feeding status affects the kinetics of multiple indispensable amino acids in four adult female subjects studied both in fed and fasted state. The subjects ingested one dose of uniformly ¹³C-labeled algae (Spirulina platensis). The isotopic enrichments (measured with negative chemical ionization gas chromatography-mass spectrometry) of the branched chain amino acids, phenylalanine, lysine and threonine were followed for 24 h in both the plasma and in VLDL-apolipoprotein B-100 (apoB-100). Fasting lowered body protein degradation when measured with the branched chain amino acids, increased it when measured with phenylalanine and had no statistically significant effect when determined from the kinetics of lysine and threonine. These apparent differences challenge the adequacy of current models of whole-body protein turnover. The ratio of the peak labeling of amino acids in plasma and apoB-100 was used as an estimate of the isotopic dilution in the hepatic pool. In contrast to our earlier observations during intravenous tracer amino acid administration, in the present study fasting lowered the ratio of the peak isotopic enrichments of apoB-100 and plasma amino acids. This supports our contention that feeding increases the use of hepatic portal amino acids for hepatic secretory protein synthesis.

KEY WORDS: • humans • amino acid metabolism • stable isotopes

¹ This work is a publication of the USDA/ARS Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine and Texas Children's Hospital, Houston, TX. Funding has been provided from the USDA/ARS under Cooperative Agreement No. 5862-5-01003. The contents of this publication do not necessarily reflect the views or policies of the USDA, nor does mention of trade names, commercial products, or organizations imply endorsement by the U.S. 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 Current address: Department of Clinical Pharmacology, University of Bonn, Sigmund-Freud-Strasse 25, 53105 Bonn, Germany.

⁴ To whom correspondence should be addressed.

Manuscript received 11 August 1994. Revision accepted 18 April 1995.

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