Effect of diet on energy expenditure and plasma norepinephrine in lean and obese Pima Indians

To assess whether thermogenesis or sympathetic nervous system (SNS) function might differ between lean and obese human subjects, studies of thermic and sympathetic responses to standard stimuli were undertaken in Pima Indians, an ethnic group with a high prevalence of obesity. Plasma levels of norepinephrine (NE) and energy expenditure at rest and in response to feeding, exercise, and graded infusions of NE were compared in five lean and five obese Indians during a period of weight maintenance (WM), after 3 weeks of overfeeding (OF) and, in the obese, also after 6 weeks of underfeeding (UF). Basal energy expenditure, when adjusted for fat free mass, was equivalent during WM and increased 3% with OF (P < 0.01) in both groups. Thermic responses to exercise or a test meal did not differ in lean and obese and did not change with OF, while thermic responses to NE infusion fell during OF to a greater degree in obese than lean (P < 0.05). A similar pattern (decreased effect in obese with OF) was also noted in the glycemic response to infused NE (P < 0.05). Although not quantitatively different in lean and obese, the plasma NE concentration appeared to vary more in response to feeding or dietary alteration in the obese than lean, a finding that may reflect lower plasma clearance of NE in the obese. These studies, therefore, raise the possibility that overfeeding in obese Pima Indians may limit the contribution of sympathetically mediated thermogenesis to energy expenditure, though the implications of this for body weight regulation are speculative.