It’s tough to define metabolic age without first defining basal metabolic rate (or BMR). “Your basal metabolic rate is how much caloric energy or energy from calories you need to keep your body functioning, including the organs, digestion, muscles, and everything else working over a period of rest,” explains functional medicine doctor Heather Moday, M.D. “If you were lying in bed all day long, it’s what your body would need or burn in order to function.” BMR is based on your gender, height, weight, lean muscle mass—and a range of other factors. The higher your BMR, the better your metabolic age.