Calories are like money. If you earn (eat) just a little more than you spend (burn), your savings (fat) will gradually increase over time.

The "energy-balance equation" below summarizes what happens to energy (calories) that we eat. Neither your genetics nor your food environment can change this simple mathematical truth.

Your body stores extra energy mostly as fat. However, extra energy can also be stored as muscle if you are doing muscle-building exercises. If you eat fewer calories than you burn each day, then some of the stored energy (fat or a combination of fat and muscle) will be burned to make up the difference.1 The accumulation or loss of stored energy may be slow, but can result in a substantial change in weight after years or decades. Even a small change in the amount you eat and in your level of physical activity can make a big difference over time.

Your body burns calories through a process called "metabolism." Metabolism is required to sustain physical activity, to digest food, to maintain body temperature, and for organ function and tissue repair. The number of calories your body burns each day for organ function and tissue repair is called your "resting metabolic rate" (RMR) or your "basal metabolic rate" (BMR), and is often referred to in casual speech as your "resting metabolism" or simply "metabolism." This is the number of calories your body would burn if you did nothing but sleep all day. Your RMR is primarily determined by the mass of your organs (heart, liver, brain, etc.) and muscles. Fat also increases your RMR, but to a lesser degree. If you are an adult, your RMR is about 12 to 14 times your fat-free weight in pounds, no matter what your sex, size, or genetics. For example, a 160 pound individual with 25% body fat would have an RMR of about 1440 to 1680 Calories/day (Calculated as 120 pounds of fat-free weight multiplied by 12 to 14 Calories). Thus, this person would burn about 1500 Calories (about 3 cheeseburgers worth) each day without even getting out of bed.

Women tend to have slower resting metabolisms than do men, mostly because of their typically smaller frames and less muscle mass. People who are naturally larger framed and muscular have relatively fast resting metabolisms. Thus, the most important genes affecting your resting metabolism are the ones that determine your frame size and muscle mass. Having a slower resting metabolism than someone else is not a problem unless you try to eat as much as they do. Your body simply doesn't need as many calories to function.

The RMR of a sedentary (non-exercising) adult typically decreases 2 to 5 percent per decade, primarily due to reduction in physical activity and loss of muscle and organ mass. You can reverse this trend and increase your RMR at any age with muscle-building exercises. 2

Beyond calories burned by resting metabolism, a sedentary adult will burn about 20% more calories each day just moving around, digesting food, and maintaining body temperature. A moderately active adult (for example, one who plays an active sport or exercises 3-5 days/week) will burn a total of about 50% more calories. 3