Work and energy: Basal metabolic rate and energy expenditure

A person’s Basal Metabolic Rate (BMR) is the daily amount of energy their body requires if at rest in a temperate environment. This energy is used to support the functioning of the human body, and in particular is responsible for maintaining homeostasis. Some processes that factor into the BMR of a person are things like breathing, blood circulation, nerve function, brain activity, and support of vital organs. The energy used or “burned” in these processes generally ends up in the form of heat expended by the body.

An average person’s BMR accounts for around 70% of their total daily energy expenditure, while physical activity accounts for 20% and digestion accounts for the remaining 10%. The average BMR in the United States is 6250 Joules/day for women and 6950 Joules/day for men. This information is tabulated in Table 1.

Table 1. The BMR and daily energy expenditures of American adult males and females.

A nutritionist is interested in seeing the effects of increasing physical activity on the BMR. She has one of her patients undergo an exercise involving throwing a baseball of mass .14kg into the air and catching it. After twenty minutes of this activity daily for a week, the nutritionist discovered that the patient’s BMR rose and they were beginning to lose weight. She concluded that more physical activity causes an increase the BMR, resulting in further energy expenditure during the day.