The Basal Metabolic Rate (BMR) is the amount of energy you need while resting in a temperate environment during the post-absorptive state, or when your digestive system is inactive. In such a state, your energy will be used only to maintain your vital organs, which include the heart, lungs, kidneys, the nervous system, intestines, liver, lungs, sex organs, muscles, and skin. The BMR decreases with age and increases with muscle mass.
The BMR is measured under very restrictive circumstances while awake. An accurate BMR measurement requires that a person’s sympathetic nervous system is inactive, which means the person must be completely rested. Basal metabolism is usually the largest component of a person’s total caloric needs. The daily calorie needs is the BMR value multiplied by a factor with a value between 1.2 and 1.9, depending on the activity level.
In most situations, the BMR is estimated with equations summarized from statistical data. The most commonly used one is the Mifflin – St Jeor equation: