Metabolism refers to the processes that the body needs to function. Basal Metabolic Rate is the amount of energy expressed in calories that a person needs to keep the body functioning at rest. Some of those processes are breathing, blood circulation, controlling body temperature, cell growth, brain and nerve function, and contraction of muscles. Basal metabolic rate (BMR) affects the rate that a person burns calories and ultimately whether you maintain, gain, or lose weight. Your basal metabolic rate accounts for about 60 to 75% of the calories you burn every day. It is influenced by several factors.
The more muscle you have, the higher your metabolic rate tends to be even at rest. Muscle burns 3-5 times more calories than fat does.
The metabolic rate is highest during the periods of rapid growth. As you get older, the amount of muscle decreases and metabolism naturally slows about 2-5% per decade after age 40 due to decrease in lean mass and a greater percentage of body fatness.