About 60 to 70 percent of the energy used by your body during the day is to maintain the body's systems, such as digestion of food, beating of the heart, breathing, maintaining body temperature, and many other life-sustaining functions. If you consume more calories than needed for basal metabolism, you will need to burn these calories off with some form of physical activity.
Several factors determine the basal metabolic rate.
This is due to the fact that during infancy and childhood there are many growth spurts requiring high amounts of calories. After you reach 30, your BMR slows down by a rate of one to two percent a year. It becomes increasingly harder to lose and keep weight off as you grow older. Another significant factor is the influence of your body composition. Muscle tissue is highly active even at rest.