Tessari P, Barazzoni R, Zanetti M, Kiwanuka E, Tiengo A
Dipartimento di Medina e Sperimentale, Cattedra Malattie del Recambio, Policlinico, Padova, Italy.
Baillieres Clin Endocrinol Metab. 1996 Oct;10(4):511-32. doi: 10.1016/s0950-351x(96)80681-1.
Substrates are powerful modulators of amino acid and protein turnover in vivo (Table 4). Intravenous infusions of amino acids exert a protein-anabolic effect, because they directly inhibit endogenous protein degradation and stimulate protein synthesis at the whole-body level. A stimulation of protein synthesis has been observed also at the forearm level. These changes resulted in an improvement of body and tissue protein balance, which is the ultimate goal of any nutritional intervention aimed at preserving body protein stores. In humans acute intravenous infusions of carbohydrates do not appear to affect either protein degradation or leucine oxidation. However, animal studies support the view that glucose availability spares essential amino acids at least in the fetus. The effects of hypercaloric refeeding with high-carbohydrate diets may, however, result in increased protein turnover. Lipids, in the form of long-chain fatty acids, inhibit endogenous protein breakdown and may suppress leucine oxidation in the whole body. They do not affect protein synthesis. In contrast, medium-chain fatty acids apparently increased leucine oxidation, and therefore increased net protein catabolism. Ketone bodies may be anabolic provided that fatty acid concentrations are not concurrently decreased.
底物是体内氨基酸和蛋白质周转的强大调节剂（表4）。静脉输注氨基酸具有蛋白质合成代谢作用，因为它们直接抑制内源性蛋白质降解并在全身水平刺激蛋白质合成。在小臂水平也观察到了蛋白质合成的刺激作用。这些变化导致身体和组织蛋白质平衡得到改善，这是任何旨在保存身体蛋白质储备的营养干预的最终目标。在人类中，急性静脉输注碳水化合物似乎不会影响蛋白质降解或亮氨酸氧化。然而，动物研究支持这样的观点，即葡萄糖可用性至少在胎儿中节省必需氨基酸。然而，高热量高碳水化合物饮食再喂养的影响可能会导致蛋白质周转增加。长链脂肪酸形式的脂质抑制内源性蛋白质分解，并可能抑制全身亮氨酸氧化。它们不影响蛋白质合成。相比之下，中链脂肪酸明显增加亮氨酸氧化，因此增加净蛋白质分解代谢。只要脂肪酸浓度不同时降低，酮体可能具有合成代谢作用。
