Byerley L O, Heber D
Clinical Research Center, Harbor-University of California-Los Angeles Medical Center, Torrance 90509-2910, USA.
J Clin Endocrinol Metab. 1996 Mar;81(3):968-76. doi: 10.1210/jcem.81.3.8772559.
The adaptation to fasting reduces muscle protein breakdown by switching from a carbohydrate to fat fuel economy in normal man. With the discovery of T3 and the observation that its formation from T4 was reduced significantly during starvation, it was proposed that T3 mediated many of these changes. To examine this possibility, otherwise healthy, obese subjects were fasted for 10 days and supplemented with T3 the last 3 days of the fast to bring circulating T3 levels within normal prefasting (weight maintenance) levels. The effects of the same dose of T3 for 3 days were tested during the last 3 days of a 10-day weight maintenance diet for comparison. Both metabolic rate and CO2 production decreased as expected with fasting and did not increase after T3 supplementation. Hepatic glucose appearance rates fell with fasting and increased significantly during T4 supplementation, but not to prefasting levels. Urinary urea nitrogen excretion decreased significantly with fasting and decreased further with T3 supplementation. Lysine appearance did not change during fasting or T3 supplementation, but leucine appearance decreased with T3 supplementation during fasting. These observations suggest that the fall in serum T3 during fasting may not mediate the observed decreases in protein breakdown that occur during fasting and prolonged starvation, but may instead initiate the fall in hepatic glucose appearance.
在正常人中，对禁食的适应通过从碳水化合物供能转换为脂肪供能来减少肌肉蛋白质分解。随着三碘甲状腺原氨酸（T3）的发现以及观察到在饥饿期间其从甲状腺素（T4）的生成显著减少，有人提出T3介导了许多这些变化。为了检验这种可能性，选取原本健康的肥胖受试者禁食10天，并在禁食的最后3天补充T3，以使循环T3水平恢复到禁食前（维持体重）的正常水平。为作比较，在为期10天的体重维持饮食的最后3天测试相同剂量T3持续3天的效果。如预期的那样，随着禁食，代谢率和二氧化碳生成均下降，补充T3后并未增加。肝葡萄糖生成率随着禁食而下降，在补充T4期间显著增加，但未恢复到禁食前水平。尿尿素氮排泄随着禁食显著减少，补充T3后进一步减少。赖氨酸生成在禁食或补充T3期间没有变化，但在禁食期间补充T3时亮氨酸生成减少。这些观察结果表明，禁食期间血清T3的下降可能并未介导禁食和长期饥饿期间所观察到的蛋白质分解减少，反而可能引发肝葡萄糖生成的下降。
