beta-adrenergically stimulated fat oxidation is diminished in middle-aged compared to young subjects

The effect of aging on P-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 nonselective P-agonist isoprenaline (ISO) was infused in i ncreasing standardized doses, and during each infusion period energy expend iture 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 P -adrenergic stimulation there was an increased carbohydrate oxidation (at a n ISO infusion dose of 24 ng/kg fat-free mass min, 31% vs. 21% of total ene rgy expenditure; P < 0.05) and a decreased fat oxidation (51 us. 62 of tota l energy expenditure; P < 0.05) in older compared to young subjects. Skelet al muscle lactate release significantly increased in the older subjects (fr om -175 +/- 32 to -366 +/- 66 nmol/100 mL forearm tissue min), whereas ther e was no change in young subjects (from -32 +/- 21 to 23 +/- 57 nmol/100 mt forearm tissue min; interaction group x ISO, P < 0.01). Additionally, ther e was a tendency toward a blunted ISO-induced increase in nonesterified fat ty acid uptake in the older subjects (interaction group x ISO, P = 0.062). Thus, middle-aged subjects have a blunted ability to oxidize fat during bet a-adrenergic stimulation compared to young subjects. This diminished fat ox idation may be an important etiological factor in the age-related increase in body fatness and obesity by favoring fat storage above oxidation.