Fukagawa N K, Yu Y M, Young V R
University of Vermont, the Department of Medicine and the Clinical Research Center, Burlington 05405-0068, USA.
Am J Clin Nutr. 1998 Aug;68(2):380-8. doi: 10.1093/ajcn/68.2.380.
Earlier nitrogen balance studies led to the conclusion that requirements for methionine in older individuals are much higher than those in younger adults. Hence, we examined the kinetics of whole-body methionine, cysteine, and leucine metabolism postabsorptively using a continuous intravenous infusion of L-[C2H3, 1-(13)C]methionine, L-[2H3]leucine, and [3,3-2H2]cysteine in 12 elderly men (n = 5) and women (n = 7) given as a 3-h infusion after a 12-h fast (study 1) and in 8 elderly men (n = 4) and women (n = 4) as an 8-h infusion according to a 3-h fasted, 5-h fed protocol (study 2) for 6 d. Before tracer infusion, each of 3 L-amino acid diets supplying the following nominal, but known, amounts (mg x kg(-1) x d(-1)) of methionine and cysteine, respectively, were used in study 2: diet 1: 13 and 0; diet 3: 6.5 and 5.2; and diet 5: 6.5 and 21. Studies 1 and 2 gave values for plasma methionine flux that agreed with the leucine flux data, which, in turn, also appeared to be comparable with findings in healthy younger adults. In study 2, methionine oxidation rates were the same across all diets in the fasted state and the same with diets 1 and 3 in the fed state but lower with diet 5, suggesting a modest sparing effect of dietary cystine on methionine oxidation. Estimated daily methionine balance was at equilibrium for diet 1 and negative (significantly different from zero, P<0.05) with diets 3 and 5. The results were evaluated against our previous findings in younger adults.
早期的氮平衡研究得出结论，老年人对蛋氨酸的需求量远高于年轻人。因此，我们通过连续静脉输注L-[C₂H₃, 1-(¹³)C]蛋氨酸、L-[²H₃]亮氨酸和[3,3-²H₂]半胱氨酸，在12名老年男性（n = 5）和女性（n = 7）中研究了吸收后全身蛋氨酸、半胱氨酸和亮氨酸的代谢动力学。这些受试者在禁食12小时后接受3小时的输注（研究1），以及在8名老年男性（n = 4）和女性（n = 4）中，按照禁食3小时、进食5小时的方案进行8小时的输注，持续6天（研究2）。在进行示踪剂输注之前，研究2使用了三种L-氨基酸饮食，分别提供以下名义上但已知量（mg·kg⁻¹·d⁻¹）的蛋氨酸和半胱氨酸：饮食1：13和0；饮食3：6.5和5.2；饮食5：6.5和21。研究1和2得出的血浆蛋氨酸通量值与亮氨酸通量数据一致，而亮氨酸通量数据反过来似乎也与健康年轻人的研究结果相当。在研究2中，禁食状态下所有饮食的蛋氨酸氧化率相同，进食状态下饮食1和3的蛋氨酸氧化率相同，但饮食5的蛋氨酸氧化率较低，这表明饮食中的胱氨酸对蛋氨酸氧化有适度的节约作用。饮食1的每日估计蛋氨酸平衡处于平衡状态，饮食3和5的蛋氨酸平衡为负（与零有显著差异，P<0.05）。我们将这些结果与我们之前在年轻人中的研究结果进行了评估。
