Whilst a number of neuroendocrine afferent signals are implicated in body-weight homeostasis, the major efferent pathway is the sympathetic nervous system (SNS), which affects both energy expenditure and substrate utilization. Thyroid hormones and their interactions with the SNS may also have a role to play. Some of the variability in resting energy expenditure can be explained by differences in SNS activity, and ?2-blockade can reduce energy expenditure and diet-induced thermogenesis in Caucasians. Excess energy intake leads to SNS activation and increased diet-induced thermogenesis. A relationship has also been demonstrated between spontaneous physical activity and SNS activity. In many animal models the SNS activates brown adipose tissue thermogenesis, hence increasing diet-induced thermogenesis and dissipating excess energy as heat. This effect is mediated via ?23-adrenoceptors and activation of an uncoupling protein unique to brown adipose tissue. Homologous proteins have been identified in human tissues and may play a role in human energy expenditure. How the SNS is implicated in this process is unclear at present. ?23-Adrenoceptor polymorphism has been associated both with lower resting energy expenditure in some populations and with reduced autonomic nervous system activity. SNS effects on substrate cycling may also play a role. In the development of obesity the effects of the SNS in promoting lipolysis and fat oxidation are likely to be at least as important as its effects on thermogenesis. ?2-Blockade has relatively small effects on energy expenditure, but more pronounced effects on reducing lipid oxidation, so tending to favour fat storage and weight gain. Low lipid oxidation is a risk factor for weight gain, and there is some evidence that low basal sympathetic nerve activity in muscle is associated with this process. Overall, the relationship between SNS activity and obesity is complex, with evidence of low SNS activity occurring in some, but not all, studies.
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