The thermic effect of feeding in older men: the importance of the sympathetic nervous system.

We have previously published direct evidence that approximately one third of the thermic effect of feeding (TEF) in young healthy men can be accounted for by the meal-induced increment in sympathetic nervous system (SNS) activity. The elderly are known to have abnormal beta-adrenergic mechanisms and blunted responsiveness to sympathetic stimulation. Therefore, we postulated that the elderly might also have a blunted thermic response to a meal. In the present study, we evaluated the TEF in 25 young (age, 29.4 +/- 4.6 years) and 12 older (66.6 +/- 7.0 years), healthy weight-stable, untrained, nonsmoking men on no medications. Energy expenditure (ventilated hood system) and SNS activity (arterialized plasma catecholamine concentrations and norepinephrine [NE] kinetics) were measured before and following ingestion of an 800-kcal high-carbohydrate meal. At baseline, arterialized plasma NE concentration (P = .001) and appearance rate (P = .05) were 40% and 28% higher, respectively, in the elderly. Resting energy expenditure was related to fat-free mass (r = .54, P less than .01), and was 21% lower in the older men (P less than .01). Energy expenditure increased in both groups following the meal, but this TEF was 48% lower in the older men (P less than .001). This reduced TEF observed in the older subjects was associated with only a slight, nonsignificant, blunting of the SNS response to the meal. The TEF was related to the arterialized plasma NE appearance rate in the young, but not the older group. The TEF was unrelated to either glucose or insulin concentrations in either group.(ABSTRACT TRUNCATED AT 250 WORDS)