Basal metabolic rate (BMR) is the energy the body uses at complete rest to run essential functions such as breathing, circulation and cell repair. It typically accounts for the largest share of daily energy expenditure, ahead of activity and digestion.
BMR varies with muscle mass, age, sex, hormones and genetics, and it adapts downward during aggressive dieting, which is one reason rapid weight loss often rebounds. Preserving muscle through adequate protein and resistance activity helps protect BMR during weight loss.