Basal metabolic rate, (BMR) The amount of
energy needed by an animal just to stay
alive. It is measured when the animal is
awake but resting, and includes the energy
required to keep the heart beating, sustain
breathing, repair tissues, and keep the brain
and nerves functioning. Measuring the
animal's consumption of oxygen gives an
accurate value for BMR, because oxygen is
needed to release energy from food. A cruder
measure of BMR estimates the amount of heat
given off, some heat being released when food
is used up. BMR varies from one species to
another, and from males to females. In
humans, it is highest in children and
declines with age. Disease, including mental
illness, can make it rise or fall. Hormones
from the thyroid gland control the BMR.