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Whole Body Protein Breakdown Rates and Hormonal Adaptation in Fasted Obese Subjects^*

LINDSEY C. HENSON and DAVID HEBER

General Clinical Research Center, Department of Medicine, Harbor-UCLA Medical Center Torrance, California 90509
University of California School of Medicine Los Angeles, California 90024

Address requests for reprints to: Lindsey C. Henson, M.D., Clinical Research Center, Harbor-UCLA Medical Center, 1000 West Carson Street, Torrance, California 90509.

Fasting is known to result in marked decreases in urinary urea nitrogen excretion over a 7-day period. In the present studies, changes in whole body protein breakdown rates and in the circulating levels of a number of hormones involved in protein anabolism and catabolism were systematically studied in nine obese subjects after 12 h and after 7 days of fasting. Whole body protein breakdown rates, measured with a primed continuous infusion of L-[U-¹⁴C]lysine, were decreased after 7 days of fasting (1.54 ± 0.12 g/kg·day) compared to those after 12 h of fasting (1.96 ± 0.10 g/kg·day). Plasma insulin decreased and glucagon increased after 7 days of fasting, resulting in an increased glucagon to insulin molar ratio. Plasma cortisol, urinary free cortisol excretion plasma rT₃ levels, and branched chain amino acid levels increased after 7 days of fasting. Serum lysine levels, used for the calculations of whole body protein breakdown rates, were not changed. We conclude: 1) decreased whole body protein breakdown contributes significantly to the decreased nitrogen excretion observed with fasting in obese subjects; and 2) a decrease in circulating levels of free T₃ may lead to this adaptive decrease in protein breakdown in fasted obese subjects, since the other hormones measured either did not change or changed in a catabolic direction.

^* This work was supported by funds from the NIH (General Clinical Research Center Grant RR-00425-14).

Recipient of Research Career Development Award K04-HD-00442-01.

Received September 23, 1982.

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