Explain the variation of the basal metabolic rate with; i) sex ii) age

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.