Your body burns calories to supply the energy that you need to live, work, and play. If you eat more calories than you need for your daily activities, your body stores the extra calories as fat. As long as you continue eating more calories than you need, your calorie reserves (fat) continue to grow. The equation below summarizes what happens to the calories in the food you eat. Neither your genetics nor your food environment can change this simple mathematical truth:
Calories stored = calories eaten – calories burned. Think of calories as money. If you earn (eat) just a little more than you spend (burn), your savings (fat) will gradually grow over time. When you eat fewer calories than you need for your daily activities, your body dips into the savings and some of your fat is burned to supply the needed energy. When this happens, you begin to lose weight. Even a small change in your eating or exercise habits, if continued long enough, can make a big difference in your weight over time.
Metabolism Made Easy
Your body burns calories through a process called metabolism. Metabolism supplies energy for physical activity, to digest food, to keep your body warm, and for organ function and repair. The number of calories your body burns each day just for organ function and repair is called your resting metabolic rate (RMR). Your RMR is often referred to in casual speech as your resting metabolism or simply metabolism. This is the number of calories your body would burn in a day if you did nothing but sleep. Your RMR is mostly determined by how much muscle tissue you have, and how big your organs (heart, liver, brain, etc.) are. The greater the mass of your muscles and organs, the faster your RMR. Body fat also increases your RMR, but to a lesser degree. Although there are various ways to calculate your RMR, the following simple equation gives a good rough estimate: RMR = fat-free weight x 10 + 500
For example, if you weigh 160 pounds, and 25 percent of that weight is from fat, then your fat-free weight is 120 pounds. You would calculate your RMR like this: RMR = 120 x 10 + 500 = 1,700 calories Thus, you would burn about 1,700 calories (about three cheeseburgers) each day without even getting out of bed. If you weigh yourself in kilograms instead of pounds, replace the 10 with 22 in the above equations.
To calculate your own RMR, you would, of course, need to know your percentage of body fat. Skinfold measurements provide a reasonably accurate way of doing this. The easiest way to take skinfold measurements is with body fat calipers, which can be purchased online and usually come with detailed instructions for calculating percentage of body fat. My point here, however, is not to advocate calorie counting as a tool for natural weight loss. It is to help you understand the relationship between metabolism and weight loss. Your resting metabolism mostly reflects the amount of organ and muscle tissue you have, and is about the same as the metabolism of the next man or woman who has the same amount of muscle and organ tissue. If that next man or woman seems to have an easier time keeping the weight off than you do, a faster metabolism is probably not the reason.
What Makes Your Metabolism Unique?
Women generally have slower RMRs than men, mostly because of their typically smaller frames and less muscle mass. People who are naturally larger framed and muscular have relatively fast RMRs. Thus, the most important genes affecting your metabolism are the ones that influence your frame size and muscle mass. Having a slower RMR 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 non-exercising adult typically decreases 2 to 5 percent every decade, mostly due to lack of physical activity and loss of muscle and organ mass. This trend is reversible. You can boost your metabolism at any age with muscle-building exercises. Losing weight can also decrease your RMR, primarily because when you lose weight, you usually lose some muscle as well as fat, and the lost muscle and fat are no longer using calories. Proper muscle building exercises during a period of gradual weight loss can help preserve your muscle mass and keep your metabolism up.
Your daily activities also affect your metabolism. In addition to the calories burned by resting metabolism, a nonexercising adult will burn about 20 percent more calories each day moving around, digesting food, and maintaining body temperature. A moderately active adult (for example, one who plays an active sport or exercises three to five days a week) will burn a total of about 50 percent more calories.