Gender differences in resting metabolic rate and noradrenaline kinetics in older individuals.

The physiological factors mediating gender differences in resting metabolic rate (RMR) in older individuals are presently unclear. We examined the contribution of sympathetic nervous system activity to gender differences in resting metabolic rate in older men and women and its relation to body fat distribution. We performed measurements of noradrenaline (NA) kinetics from infusions of [3H]-NA, RMR, body fat distribution, body composition, peak Vo2 and dietary intake in 29 older men (69 +/- 6 years) and 26 older women (65 +/- 5 years). Older men weighed more (P < 0.01) and had a greater fat-free mass (P < 0.01) and a larger waist circumference (P < 0.01) than older women. Older men had a higher RMR (P < 0.05) than older women, which persisted after controlling for differences in fat-free mass and fat mass. Older men also showed a greater NA appearance rate (P < 0.01) at rest than older women. The higher NA appearance rate in older men was partly related to their greater waist circumference (r = 0.50, P < 0.01). We explored the sympathetic contribution to gender differences in RMR by statistically controlling for differences in body composition and NA appearance rate. After this procedure, we found no gender differences in adjusted RMR between older men (4.3 +/- 0.5 kJ min(-1)) and older women (4.3 +/- 0.4 kJ min(-1)). Our results suggest that: (a) older men have a higher RMR than older women independent of differences in body composition; (b) the higher RMR in older men may be partly due to higher levels of sympathetic nervous system activity; (c) the higher sympathetic nervous system activity in older men is partly related to their greater waist circumference, a proxy measure of central body fatness.