Basal and resting metabolic rates

The rate at which the body uses energy is termed the metabolic rate. Estimates of energy expenditure at rest and during exercise are often based on measurement of whole-body oxygen consumption(VO₂) and its caloric equivalent. At rest, an average person consumes about 0.3L of O₂/min. This equals 18L of O₂/h or 432L of O₂/day.

Knowing an individual’s VO₂ allows us to calculate that person’s caloric expenditure. Recall that at rest, the body usually burns a mixture of carbohydrate and fat. An RER value of 0.80is fairly common for most resting individuals on a mixed diet. The caloric equivalence of an RER value of 0.80 is fairly common for most resting individuals on a mixed diet. The caloric equivalence of an RER value of 0.80 is 4.80 kcal per liter of O₂ consumed. Using these common values, we can calculate this individual’s caloric expenditure as follows:

kcal/day = liters of O₂ consumed per day x kcal used per liter of O₂ =

              = 432L O₂/day x 4.80 kcal/L O₂ = 2,074 kcal/day.

This value closely agrees with the average resting energy expenditure expected for a 70kg(154lb) man. Of course, it does not include the extra energy needed for normal daily activity.

One standardized measure of energy expenditure at rest is the basal metabolic rate(BMR). The BMR is the rate of energy expenditure for an individual at rest in a supine position, measured immediately after at least 8h of sleep and at least 12h of fasting. This value reflects the minimum amount of energy required to carry on essential physiological functions.

Because muscle has high metabolic activity, the BMR is directly related to an individual’s fat-free mass and is generally reported in kilocalories per kilogram of fat-free mass per minute(kcal x kg FFM^-1 x min^-1). The more fat-free mass, the more total calories expended in a day. Because women tend to have a lower fat-free mass and a greater fat mass then men, women tend to have lower BMRs than men of similar weight.

Body surface area also affects BMR. The higher the surface area, the more heat loss occurs from the skin, which raises the BMR because more energy is needed to maintain body temperature. For this reason, the BMR is also often reported in kilocalories per square meter of body surface area per hour(kcal x m^-2 x h^-1). Because we are discussing daily energy expenditure, we’ve opted for a simpler unit: kcal/day.

Many other factors affect BMR, including these:

• Age: BMR gradually decreases with increasing age, generally because of a decrease in fat-free mass.
• Body temperature: BMR increases with increasing temperature.
• Psychological stress: stress increases activity of the sympathetic nervous system, which increases the BMR.
• Hormones: for example, thyroxine from the thyroid gland and epinephrine from the adrenal medulla both increase the BMR.

Instead of BMR, most researchers now use the term resting metabolic rate(RMR), because most measurements follow the same conditions required for measuring BMR but do not require the individual to sleep over in a hospital or research laboratory. Basal metabolic rate and RMR values are essentially identical, and they range from 1,200 to 2,400 kcal/day. But the average total metabolic rate of an individual engaged in normal daily activity ranges from 1,800 to 3,000 kcal.