24-hour energy expenditure and sympathetic activity in postobese women consuming a high-carbohydrate diet

It has been suggested that the energy expenditure (EE) of formerly obese subjects (postobese) is highly susceptible to the dietary carbohydrate content and that a high dietary carbohydrate-to-fat ratio may increase their 24-h EE. We studied eight obese women before and after weight normalization (postobese state) and compared them with eight matched controls. Twenty-four hour EE, substrate oxidations, and 24-h heart rate were measured in respiratory chambers on a fixed physical program, while the postobese and controls were in macronutrient balance on a high-carbohydrate diet. Mean 24-h EE decreased from the obese to the postobese state (P less than 0.01), but it remained higher in the postobese women than in the controls (8,292 +/- 153 vs. 7,646 +/- 190 kJ/day, P = 0.01). The higher EE in the postobese group was entirely covered by a 22% higher carbohydrate oxidation (P = 0.006). The mean 24-h heart rate, as measured by telemetry, was also higher in the postobese group than in the control group (74 vs. 66 beats/min, P less than 0.03). Plasma norepinephrine (NE) concentrations were greater by 50% in the postobese as compared with the controls (P = 0.004), and differences in plasma NE concentrations entirely accounted for the group difference in 24-h EE and heart rate between postobese and controls, as analyzed by stepwise regression analysis. We conclude that postobese women on a high-carbohydrate-low-fat diet have an enhanced sympathetic nervous system activity, which is responsible for the higher 24-h EE and heart rate. These findings may have implications for understanding the pathophysiology and for the treatment of obesity.