Department of Medicine, Washington University, St Louis, MO, USA.
Am J Clin Nutr. 2011 Jul;94(1):172-81. doi: 10.3945/ajcn.110.010959. Epub 2011 May 4.
Acute deviations in protein intake before the quantification of protein kinetics in older humans may explain the controversy over the effects of older age on muscle protein synthesis and proteolysis rates.
We hypothesized that an acute decrease in protein intake from the habitual intake is associated with lower muscle protein synthesis and higher proteolysis rates, whereas an acute increase in protein intake from the habitual intake is associated with higher muscle protein synthesis and lower proteolysis rates.
In 112 community-dwelling healthy men aged 65-90 y, we quantified resting whole-body [1,2-(13)C(2)]leucine kinetics, muscle mixed protein fractional synthesis rates (FSRs), and muscle proteasome proteolytic enzyme activities after participants consumed for 3 d controlled research meals (0.9-1.1 g protein · kg(-1) · d(-1)) that contained more or less protein than that habitually consumed and that induced alterations in nitrogen balance.
Protein kinetic parameters were not significantly different between the groups, despite controlled research protein intakes that were lower (-0.2 to -0.3 g · kg(-1) · d(-1)) or higher (+0.2 g · kg(-1) · d(-1)) than habitual intakes and that induced negative (-22 to -25 mg · kg(-1) · d(-1)) or positive (22-25 mg · kg(-1) · d(-1)) nitrogen balance. Within these acutely altered protein intake and nitrogen balance boundaries, a reduction in protein intake from habitual intake and induction of negative nitrogen balance were not associated with higher proteolysis or lower muscle FSR, and an acute increase in protein intake from habitual intake and induction of positive nitrogen balance were not associated with lower proteolysis or higher muscle FSR. A higher quantitative insulin sensitivity check index was associated with lower whole-body proteolysis rates.
The practice of acutely controlling protein intake, even at intakes lower than habitual intakes that induce negative nitrogen balance, before quantifying human protein kinetics does not significantly reduce muscle protein synthesis or increase proteolysis. Factors other than protein intake explain lower muscle protein synthesis rates with advanced age. This trial is registered at clinicaltrials.gov as NCT00183040.
在对老年人的蛋白质动态进行量化之前，蛋白质摄入量的急性偏差可能解释了老年人对肌肉蛋白质合成和蛋白分解率的影响的争议。
我们假设，与习惯摄入量相比，蛋白质摄入量的急性减少与较低的肌肉蛋白质合成和较高的蛋白分解率有关，而蛋白质摄入量的急性增加与较高的肌肉蛋白质合成和较低的蛋白分解率有关。
在 112 名居住在社区的健康男性中，年龄在 65-90 岁之间，我们在参与者连续 3 天食用控制研究餐（0.9-1.1 g 蛋白质·kg(-1)·d(-1)）后，量化了静息全身[1,2-(13)C(2)]亮氨酸动力学、肌肉混合蛋白合成率（FSR）和肌肉蛋白酶体蛋白酶活性，这些研究餐的蛋白质含量高于或低于习惯摄入量，并导致氮平衡发生变化。
尽管控制研究的蛋白质摄入量低于（-0.2 至-0.3 g·kg(-1)·d(-1)）或高于（0.2 g·kg(-1)·d(-1)）习惯摄入量，并导致负氮平衡（-22 至-25 mg·kg(-1)·d(-1)）或正氮平衡（22-25 mg·kg(-1)·d(-1)），但蛋白质动力学参数在两组之间没有显著差异。在这些急性改变的蛋白质摄入量和氮平衡范围内，从习惯摄入量减少蛋白质摄入并导致负氮平衡与较高的蛋白分解或较低的肌肉 FSR 无关，而从习惯摄入量急性增加蛋白质摄入并导致正氮平衡与较低的蛋白分解或较高的肌肉 FSR 无关。较高的定量胰岛素敏感性检查指数与较低的全身蛋白分解率相关。
在量化人体蛋白质动力学之前，即使在摄入低于习惯摄入量并导致负氮平衡的情况下，急性控制蛋白质摄入也不会显著降低肌肉蛋白质合成或增加蛋白分解。除了蛋白质摄入之外，其他因素可以解释老年人肌肉蛋白质合成率较低的原因。本试验在 clinicaltrials.gov 注册为 NCT00183040。
