Effect of aging on β-adrenergically mediated thermogenesis in men.

The age-dependent alterations in β-adrenergically mediated thermogenesis were investigated in 11 young (mean ± SE age: 21.9 ± 0.5 yr) and 9 older (52.9 ± 2.1 yr) men during intravenous infusion of the nonselective β-agonist isoprenaline (Iso). The older men had higher basal plasma norepinephrine (327.7 ± 35.8 vs. 159.0 ± 18.2 pg/ml, P < 0.001) and epinephrine (75.1 ± 18.1 vs. 29.1 ± 5.3 pg/ml, P < 0.05) concentrations than the young. The β-adrenergically mediated thermogenesis was diminished in the older men, as reflected by the significantly higher plasma Iso concentration needed to increase resting energy expenditure by 15% (236.1 ± 51.0 vs. 107.6 ± 11.4 pg/ml, P < 0.05). Additionally, both dose (39.4 ± 6.6 vs. 19.1 ± 1.5 ng ⋅ kg fat-free mass-1 ⋅ min-1, P < 0.01) and plasma concentration (332.2 ± 59.1 vs. 119.3 ± 14.0 pg/ml, P < 0.01) of Iso needed to increase resting heart rate by 25 beats/min were higher in older than in younger subjects, suggesting that the age-related decline in β-adrenergic sensitivity is a generalized defect not related to a specific tissue or response. In conclusion, aging is associated with a diminished β-adrenergically mediated thermogenesis. This blunted thermogenic response may contribute to a positive energy balance and thus promote increased fat storage and obesity.