GENDER DIFFERENCES IN RESTING METABOLIC-RATE AND NORADRENALINE KINETICS IN OLDER INDIVIDUALS

The physiological factors mediating gender differences in resting meta bolic rate (RMR) in older individuals are presently unclear. We examin ed the contribution of sympathetic nervous system activity to gender d ifferences in resting metabolic rate in older men and women and its re lation to body fat distribution. We performed measurements of noradren aline (NA) kinetics from infusions of [H-3]-NA, RMR, body fat distribu tion, body composition, peak VO2 and dietary intake in 29 older men (6 9 +/- 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 large r waist circumference (P < 0.01) than older women. Older men had a hig her 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 gre ater waist circumference (r = 0.50, P < 0.01). We explored the sympath etic contribution to gender differences in RMR by statistically contro lling for differences in body composition and NA appearance rate. Afte r this procedure, we found no gender differences in adjusted RMR betwe en 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 th an older women independent of differences in body composition; (b) the higher RMR in older men may be partly due to higher levels of sympath etic nervous system activity; (c) the higher sympathetic nervous syste m activity in older men is partly related to their greater waist circu mference, a proxy measure of central body fatness.