Metabolism refers to the processes in your body that burn fuel and use energy. Your metabolic rate is the number of calories you burn during all these processes, so the “faster” your metabolism, the more calories you need to maintain your current weight.

Your base metabolism, or resting metabolic rate (RMR), is the minimum number of calories your body burns just to stay alive. Your brain, lungs, blood vessels and every cell in your body operate 24/7, so your RMR accounts for between 60 and 75 percent of the calories you burn each day.
Gender, age, body composition and body size affect RMR because muscle burns more calories than fat, even at rest. Our RMR goes down by 1 to 2 percent per decade, mostly due to loss of lean muscle.

In general, men and younger adults have more muscle mass than women and older adults. Someone who is overweight needs more calories to maintain weight because some of that extra weight is muscle.
A few health conditions can alter your RMR, including hypothyroidism and hyperthyroidism. So can some medications. RMR increases in infants, children and adolescents during growth periods and in women during pregnancy and breast-feeding. Your RMR is also higher when recovering from injury or serious illness.

Beyond RMR: Food and activity

Two factors increase your metabolism above resting. The first is food, because when you eat, your body has to do extra work to digest your food and eliminate waste. This burns about 7 to 10 percent of the calories you eat an average meal.

Physical activity is the other, bigger variable, accounting for up to 30 percent of your total daily calorie burn. Your metabolism temporarily speeds up when you are moving, because your muscles, lungs and heart have to work harder.

If you exercise intensely or for long periods, you also see a slight boost in your metabolism after exercise (the “afterburn”).