The basal metabolic rate decreases almost linearly with age. ... It is shown that the decrease in muscle mass relative to total body may be wholly responsible for the age-related decreases in basal metabolic rate. Energy consumption by physical activity also decreases with atrophic changes of skeletal muscle.
Explanation:
About 90% of daily energy expenditure is due to muscle mass, fat mass, and activity level. Women do tend to have slower metabolisms than men, but the difference is primarily a function of muscle mass and body size, not gender. We might expect someone with more muscle and less fat to be more metabolically healthy.
