The human body is a powerhouse of energy, with every cell performing numerous functions that require energy. This energy production and consumption is measured by the metabolic rate, which quantifies the total heat generated by all the body's chemical reactions and mechanical work. This measurement helps to determine the rate of kilocalorie (kcal) consumption needed to fuel all ongoing activities.

The Basal Metabolic Rate (BMR) measures the energy expended at rest.

Several factors influence the BMR, including age, gender, body temperature, stress, and the hormone thyroxine. Younger individuals have a higher BMR due to their growth requirements, while older people experience a decline in BMR. Men typically have a higher metabolic rate than women due to their greater muscle mass.

Furthermore, BMR fluctuates with body temperature, rising during a fever. Stress, both physical and emotional, can also spike BMR by activating the sympathetic nervous system. Hormones like epinephrine and norepinephrine stimulate fat catabolism, raising BMR.

Lastly, thyroxine, a hormone from the thyroid gland, significantly influences BMR. Often referred to as the "metabolic hormone," thyroxine accelerates oxygen consumption and heat production. Therefore, the more thyroxine present, the higher the BMR.