Henson L C, Heber D
J Clin Endocrinol Metab. 1983 Aug;57(2):316-9. doi: 10.1210/jcem-57-2-316.
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.
众所周知，禁食7天会导致尿尿素氮排泄显著减少。在本研究中，对9名肥胖受试者在禁食12小时和7天后，系统地研究了全身蛋白质分解率以及一些参与蛋白质合成代谢和分解代谢的激素循环水平的变化。通过L-[U-¹⁴C]赖氨酸的单次静脉注射和持续输注来测量全身蛋白质分解率，结果显示，禁食7天后（1.54±0.12克/千克·天）的全身蛋白质分解率低于禁食12小时后（1.96±0.10克/千克·天）。禁食7天后，血浆胰岛素水平下降，胰高血糖素水平升高，导致胰高血糖素与胰岛素的摩尔比增加。禁食7天后，血浆皮质醇、尿游离皮质醇排泄量、血浆反三碘甲状腺原氨酸（rT3）水平和支链氨基酸水平均升高。用于计算全身蛋白质分解率的血清赖氨酸水平未发生变化。我们得出以下结论：1）全身蛋白质分解减少显著导致了肥胖受试者禁食时氮排泄的减少；2）游离T3循环水平的降低可能导致禁食肥胖受试者蛋白质分解的这种适应性降低，因为所测量的其他激素要么没有变化，要么朝着分解代谢方向变化。
