The effect of age and unilateral leg immobilization for 2 weeks on substrate utilization during moderate-intensity exercise in human skeletal muscle.

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.

High-intensity interval training improves insulin sensitivity in older individuals.

AIM: Metabolic health may deteriorate with age as a result of altered body composition and decreased physical activity. Endurance exercise is known to counter these changes delaying or even preventing onset of metabolic diseases. High-intensity interval training (HIIT) is a time efficient alternative to regular endurance exercise, and the aim of this study was to investigate the metabolic benefit of HIIT in older subjects. METHODS: Twenty-two sedentary male (n = 11) and female (n = 11) subjects aged 63 ± 1 years performed HIIT training three times/week for 6 weeks on a bicycle ergometer. Each HIIT session consisted of five 1-minute intervals interspersed with 1½-minute rest. Prior to the first and after the last HIIT session whole-body insulin sensitivity, measured by a hyperinsulinaemic-euglycaemic clamp, plasma lipid levels, HbA1c, glycaemic parameters, body composition and maximal oxygen uptake were assessed. Muscle biopsies were obtained wherefrom content of glycogen and proteins involved in muscle glucose handling were determined. RESULTS: Insulin sensitivity (P = .011) and maximal oxygen uptake increased (P < .05) in both genders, while plasma cholesterol (P < .05), low-density lipoprotein (P < .05), visceral fat mass (P < .05) and per cent body fat (P < .05) decreased after 6 weeks of HIIT. HbA1c decreased only in males (P = .001). Muscle glycogen content increased in both genders (P = .001) and in line GLUT4 (P < .05), glycogen synthase (P = .001) and hexokinase II (P < .05) content all increased. CONCLUSION: Six weeks of HIIT significantly improves metabolic health in older males and females by reducing age-related risk factors for cardiometabolic disease.

Higher muscle content of perilipin 5 and endothelial lipase protein in trained than untrained middle-aged men.

A high VO(2)max in middle-age is related to high metabolic flexibility and lowered risk of metabolic diseases. However, the influence of a high VO(2)max induced by years of regular training in middle-age on protein expression related to muscle metabolism is not well studied. This study measures key proteins involved in mitochondrial oxidation, glucose and lipid metabolism in skeletal muscle of trained and untrained middle-aged men. 16 middle-aged men, matched for lean body mass, were recruited into an endurance trained (TR, n=8) or an untrained (CON, n=8) group based on their VO(2)max. A muscle biopsy was obtained from m. vastus lateralis and protein levels were analyzed by Western blotting. The TR had higher protein levels of mitochondrial complex III-V, endothelial lipase (EL) and perilipin 5 compared to the CON. Glycogen synthase (P=0.05), perilipin 3 (P=0.09) and ATGL (P=0.09) tended to be higher in TR than CON, but there was no difference in AKT I/II, HKII, GLUT4 and LPL protein expression. Lastly, there was a positive correlation between plasma HDL and EL (R(2)=0.53, P<0.01). In conclusion, a high VO(2)max in middle-aged men was as expected is reflected in higher muscle oxidative capacity, but also in higher endothelial lipase and perilipin 5 expression and a borderline higher glycogen synthase protein expression, which may contribute to a higher metabolic flexibility.