Thermic effect of glucose in man. Obligatory and facultative thermogenesis

Abstract

The contribution of the sympathetic nervous system to the thermic effect of intravenously infused glucose and insulin was studied in 10 healthy young men before and after β-adrenegic receptor blockade with propranolol during conditions of normoglycemia (90 mg/dl) at two levels of hyperinsulinemia (~90 μU/ml and ~620 μU/ml). During steady state conditions of glucose uptake (0.515 ± 0.046 and 0.754 ± 0.056 g/min), significant increases were observed in energy expenditure (0.10 ± 0.02 kcal/min, P < 0.001, and 0.21 ± 0.02 kcal/min, P < 0.01, respectively). Similarly, glucose oxidation increased from 0.100 ± 0.015 to 0.266 ± 0.022 g/min (P < 0.001) at ~90 μU/ml insulin and from 0.082 ± 0.013 to 0.295 ± 0.018 g/min (P < 0.001) at ~620 μU/ml insulin. Concomitantly, the rate of nonoxidative glucose disposal or 'glucose storage' was 0.249 ± 0.033 and 0.459 ± 0.048 g/min, respectively. At this time the thermic effect of infused glucose/insulin was 5.3 ± 0.9 and 7.5 ± 0.7%, and the energy cost of 'glucose storage' was 0.50 ± 0.16 kcal/g and 0.47 ± 0.04 kcal/g at the two different levels of glucose uptake. After β-adrenergic receptor blockade with propranolol, glucose uptake, oxidation, and 'storage' were unchanged in both studies, but significant decreases in energy expenditure were observed (1.41 ± 0.06-1.36 ± 0.05 kcal/min, P < 0.01 at ~90 μU/ml insulin, and 1.52 ± 0.07-1.43 ± 0.05 kcal/min, P < 0.005 at ~620 μU/ml insulin) causing significant falls in both the estimated thermic effect of infused glucose/insulin and the energy cost of 'glucose storage'. Regression analysis of the results from both studies indicated a mean energy cost for 'glucose storage' of 0.36 kcal/g (r = 0.74, P < 0.001), which fell significantly (P < 0.005) to 0.21 kcal/g (r = 0.49, P < 0.05) during β-adrenergic receptor blockade with propranolol. The latter is in close agreement with that calculated on theoretical grounds for the metabolic cost of glucose storage as glycogen, i.e., obligatory thermogenesis. It is concluded that β-adrenergically mediated sympathetic nervous activity is responsible for almost the entire rise in energy expenditure in excess of the obligatory requirements for processing and storing glucose during conditions of normoglycemia and hyperinsulinemia in healthy man, and that the energy cost of 'glucose storage' is not different at normal (~90 μU/ml) and supraphysiological (~620 μU/ml) plasma insulin concentrations.