Basal Metabolic Rate (BMR) is the number of calories you burn if you do nothing but rest for 24 hours. This is the minimum amount of energy needed to keep your body functioning, such as breathing and maintaining a normal body temperature. BMR measurements are typically taken in a test facility. Resting Metabolic Rate (RMR) is similar, but measurements are taken under less restricted conditions than BMR.

As you age, you lose lean body mass, which slows the BMR and RMR. Fasting or depriving yourself of food also lowers these measures. This means that to maintain your current weight (or lose weight), you need to increase your cardiovascular exercise to create more lean muscle mass and raise your BMR and RMR.