The effect of diet or exercise on plasma norepinephrine kinetics in moderately obese young men

An increase in sympathetic nervous system (SNS) activity in the obese has been described by some but not all investigators. It is possible that an increase in SNS tone may play a role in the predisposition to atherosclerotic cardiovascular disease noted in the obese. The effect of dietary weight loss or exercise training on resting SNS activity in moderately obese subjects has not been extensively studied and the results of previous studies are conflicting. Therefore, we prospectively evaluated resting SNS activity in healthy moderately obese subjects randomized to either a three month dietary weight loss (n = 13) or endurance exercise training (n = 18). All subjects were weight stabilized on a constant composition diet for 10 days prior to study both before and after the interventions. Although both groups lost weight, weight loss was greater in the diet group (-13.6 +/- 6.7 vs -2.3 +/- 3.4 kg, P less than 0.001). The composition of weight loss was also different with 32 percent of total weight loss as fat free mass (FFM) in the dieters compared to no significant change in FFM in the exercisers. The caloric requirement for weight stabilization declined after the diet but increased following exercise training (-247 vs + 202 kcal/day, P less than 0.001). No significant changes in blood pressure occurred in either group, and neither group had a significant change in resting plasma norepinephrine concentration. Plasma epinephrine concentrations were also unchanged. However, SNS activity as reflected by arterialized plasma NE kinetics revealed that NE appearance rate declined by 17 percent after dietary weight loss (P less than 0.01), but was not significantly changed after exercise training. These results suggest that dietary weight loss is more effective than exercise training in reducing overall resting SNS activity in normotensive subjects. Since exercise training is known to reduce the SNS response to a given submaximal workload, a combination of diet plus exercise might be the most effective way to reduce overall SNS activity and its possible role in the premature atherosclerosis associated with obesity.