Final answer:
The basal metabolic rate (BMR) decreases as you age due to a decrease in lean muscle mass, slower metabolic processes, and changes in body composition.
Step-by-step explanation:
The basal metabolic rate (BMR) decreases as you age due to several factors. One of the main reasons is the decrease in lean muscle mass, which occurs naturally as we get older. Muscle is more metabolically active than fat, so a decrease in muscle mass leads to a decrease in BMR.
Additionally, as we age, our body's metabolic processes tend to slow down, including digestion and energy production. This slower metabolism results in a lower BMR.
Lastly, changes in body composition, including an increase in body fat percentage, can also contribute to a lower BMR as we age.