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Key Points: This study aimed to provide molecular insight into the differential effects of age and physical inactivity on the regulation of substrate metabolism during moderate-intensity exercise. Using the arteriovenous balance technique, we studied the effect of immobilization of one leg for 2 weeks on leg substrate utilization in young and older men during two-legged dynamic knee-extensor moderate-intensity exercise, as well as changes in key proteins in muscle metabolism before and after exercise. Age and immobilization did not affect relative carbohydrate and fat utilization during exercise, but the older men had higher uptake of exogenous fatty acids, whereas the young men relied more on endogenous fatty acids during exercise. Using a combined whole-leg and molecular approach, we provide evidence that both age and physical inactivity result in intramuscular lipid accumulation, but this occurs only in part through the same mechanisms., Abstract: Age and inactivity have been associated with intramuscular triglyceride (IMTG) accumulation. Here, we attempt to disentangle these factors by studying the effect of 2 weeks of unilateral leg immobilization on substrate utilization across the legs during moderate-intensity exercise in young (n = 17; 23 ± 1 years old) and older men (n = 15; 68 ± 1 years old), while the contralateral leg served as the control. After immobilization, the participants performed two-legged isolated knee-extensor exercise at 20 ± 1 W (∼50% maximal work capacity) for 45 min with catheters inserted in the brachial artery and both femoral veins. Biopsy samples obtained from vastus lateralis muscles of both legs before and after exercise were used for analysis of substrates, protein content and enzyme activities. During exercise, leg substrate utilization (respiratory quotient) did not differ between groups or legs. Leg fatty acid uptake was greater in older than in young men, and although young men demonstrated net leg glycerol release during exercise, older men showed net glycerol uptake. At baseline, IMTG, muscle pyruvate dehydrogenase complex activity and the protein content of adipose triglyceride lipase, acetyl-CoA carboxylase 2 and AMP-activated protein kinase (AMPK)γ3 were higher in young than in older men. Furthermore, adipose triglyceride lipase, plasma membrane-associated fatty acid binding protein and AMPKγ3 subunit protein contents were lower and IMTG was higher in the immobilized than the contralateral leg in young and older men. Thus, immobilization and age did not affect substrate choice (respiratory quotient) during moderate exercise, but the whole-leg and molecular differences in fatty acid mobilization could explain the age- and immobilization-induced IMTG accumulation., (© 2016 The Authors. The Journal of Physiology © 2016 The Physiological Society.)

We studied the effect of physical inactivity and subsequent retraining on cardiovascular risk factors in 17 young (Y; 23.4 ± 0.5 years) and 15 older adult (O; 68.1 ± 1.1 years) men who underwent 14 days of one leg immobilization followed by six weeks of training. Body weight remained unchanged. Daily physical activity decreased by 31 ± 9% (Y) and 37 ± 9% (O) (p < .001). Maximal oxygen uptake decreased with inactivity (Y) and always increased with training. Visceral fat mass decreased (p < .05) with training. Concentrations of lipids in blood were always highest in the older adults. FFA and glycerol increased with reduced activity (p < .05), but reverted with training. Training resulted in increases in HDL-C (p < .05) and a decrease in LDL-C and TC:HDL-C ratio (p < .05). A minor reduction in daily physical activity for two weeks increased blood lipids in both young and older men. Six weeks of training improved blood lipids along with loss of visceral fat.