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 orgams, muscles, and skin. The BMR decreases with age and increase with muscle mass.