Abnormal basal metabolic rate

An abnormal basal metabolic rate is not necessarily indicative of disease; a number of physiological factors can alter the BMR by influencing cellular metabolic activity.[1] For instance, males are more likely than females to have a high BMR, and in women, the BMR may rise to abnormal levels during pregnancy or lactation.[2] An individual's BMR varies greatly with age: infants and children typically have a high BMR, required for growth, while the elderly have a low BMR.[1] Tall, thin people have a higher BMR than their shorter counterparts, even with the same weight, due to the greater surface area of their skin.[3] The metabolic rate also decreases during sleep and increases in exercise, and individuals who exercise regularly have a higher BMR than those who are sedentary.[1] Environmental temperature also has an effect: the BMR is increased in both heat and cold.[3]

A common pathological cause for a high BMR is fever, since a rise in body temperature increases the rate of cellular metabolic reactions.[1] It is estimated that for every degree Fahrenheit of rise in body temperature, the BMR increases by 7 percent.[3]

Thyroid disease also has a marked effect on BMR, since thyroid hormones regulate the rate of cellular metabolism.[3] Hyperthyroidism—in which there is an increase in the production of thyroid hormones—leads to a high BMR, while hypothyroidism—in which thyroid hormones are depleted—causes a low BMR.[1]

Prolonged periods of abnormal nutrition cause an adaptive change in BMR; this helps the body to maintain a stable body weight in response to the change in food supply. In prolonged malnutrition, the BMR declines, while in prolonged overnutrition, the BMR is increased.[1] Cancer sometimes causes an increase in BMR, perhaps because the cancer cells that form tumors have a high level of metabolic activity.[1]