Effects of nutrient intake on sympathoadrenal activity and thermogenic mechanisms.

Ingestion of carbohydrate results in a diphasic activation of the sympathoadrenal system. One component is an insulin-mediated activation of the sympathetic nervous system (SNS). This activation is partly a haemodynamic reflex, but it may cause a weak thermogenic effect via beta 1-adrenoceptors in white adipose tissue, the liver and the heart. The second thermogenic component of carbohydrate occurs later when the blood glucose concentration decreases towards baseline levels. This elicits an increased secretion of adrenaline from the adrenal medulla, and the circulating level exceeds the physiological threshold for thermogenic effect. The target is mainly skeletal muscle where thermogenesis is stimulated via beta 2-adrenoceptors. Also the basal metabolic rate and the thermogenic responses to cold and heat exposure, mental stress and exercise, have facultative components. Inhibition of facultative thermogenesis by beta-blockers such as propranolol, diminishes the daily energy expenditure and promotes weight gain and obesity. Although thermogenesis mediated by the sympathoadrenal system accounts for only a small part of the daily energy expenditure, it is sufficient to explain the positive energy balance and weight gain reported in patients receiving treatment with beta-adrenoceptor blocking agents.