Abstract
The effect of aging on β-adrenergically mediated substrate utilization was investigated in nine young (25.2 ± 1.7 yr old) and eight older males (52.9 ± 2.1 yr old), matched for body weight and body composition. In a first experiment, the nonselectiveβ -agonist isoprenaline (ISO) was infused in increasing standardized doses, and during each infusion period energy expenditure and substrate utilization were determined by indirect calorimetry. In a second experiment, forearm skeletal muscle metabolism was studied during a standardized infusion dose of ISO (19 ng/kg fat-free mass·min). During β-adrenergic stimulation there was an increased carbohydrate oxidation (at an ISO infusion dose of 24 ng/kg fat-free mass·min, 31% vs. 21% of total energy expenditure; P < 0.05) and a decreased fat oxidation (51 vs. 62 of total energy expenditure; P < 0.05) in older compared to young subjects. Skeletal muscle lactate release significantly increased in the older subjects (from −175 ± 32 to −366 ± 66 nmol/100 mL forearm tissue·min), whereas there was no change in young subjects (from− 32 ± 21 to 23 ± 57 nmol/100 mL forearm tissue·min; interaction group × ISO, P < 0.01). Additionally, there was a tendency toward a blunted ISO-induced increase in nonesterified fatty acid uptake in the older subjects (interaction group × ISO, P = 0.062). Thus, middle-aged subjects have a blunted ability to oxidize fat during β-adrenergic stimulation compared to young subjects. This diminished fat oxidation may be an important etiological factor in the age-related increase in body fatness and obesity by favoring fat storage above oxidation.
Components of Total Energy Expenditure in Healthy Young Women Are Not Affected after 14 Days of Feeding with Medium-Versus Long-Chain Triglycerides