Understanding Metabolism

While it’s true that genetics play a part, your metabolism is not predetermined — lifestyle affects it, too. Understanding what metabolism is and how yours works is the first step in doing what you can to maximize its potential.

Calories in vs. Calories Out

When people talk about metabolism, they are generally referring to metabolic rate — the rate at which the body expends energy, or burns calories. Your metabolic rate for a 24-hour period is often called your total energy expenditure (TEE), and is expressed as calories.

Your body burns calories through physical activity—taking a walk or climbing a flight of stairs. But it also burns them by performing the most basic physiological functions. For example, breathing, maintaining body temperature and pumping blood around the body are all processes that require energy 24 hours a day. In fact, about 60 to 75 percent of your TEE consists of this type of passive calorie burn, known as resting metabolic rate (RMR).

Many factors influence whether someone has a high or low resting metabolic rate, and thus burns more or less calories during the day without even trying. These include:

Genetics. Although there are things you can do to increase or decrease your RMR, some people may be genetically predisposed to faster or slower metabolism.

Lean Body Mass Lean. Muscle tissue requires more oxygen than fat tissue does, which means your body has to spend more energy to maintain it. That’s why the more lean muscle (and thus, less body fat) you have, the higher your RMR will be.

Total Body Weight and Size. A larger, heavier body burns more calories at rest than a small one. So as you lose weight, your RMR will decrease.

As you get older, your RMR slows down and your body burns fewer calories at rest. This is due to fluctuating hormone levels, and, for most people, decreased muscle mass and less physical activity.

Calorie Intake Eating. significantly fewer calories than your body needs, like in the case of an extreme diet, can send your body into survival mode in which calories are conserved rather than burned, lowering your RMR.

Meal Frequency. About five to 10 percent of the calories you eat are used to fuel the digestion process. So, the more often you eat, the more calories you’ll burn digesting. (If eating more often also means eating more food throughout the day, you’ll wind up with a higher total calorie intake, too.)

External Factors. Lifestyle and environmental factors that can affect RMR include extreme hot or cold temperatures, certain medications, herbs and supplements, smoking, stress, the health of your immune system and the amount of sleep you get.

How Many Calories Does Your Body Burn?

These basic calculations cannot take into account all of the factors that affect your metabolism, but it is still possible to estimate your metabolic rate based on your body weight and fitness level. Some simple math may open your eyes to new ways of thinking about your weight loss or maintenance strategies.

To estimate your RMR, multiply your body weight (in pounds) by 10. (This number varies by physical fitness level; you could multiply by eight if you are 50 or more pounds overweight, and by 12 if you are an elite athlete, for example.)

So, if a 150-pound woman wanted to know her resting metabolic rate, she would use this calculation:

150 x 10 = 1,500 RMR

To find your TEE, or the number of calories you can expect to burn in a day, add 30 percent to that number—an estimate for what someone will burn based on even the most sedentary lifestyle. So, assuming that that same 150-pound woman doesn’t exercise, her TEE could be calculated as follows:

1500 + (1500 x 0.3) = 1,950 TEE

Now that you have these numbers, you can think more strategically about your physical activity, diet and weight. There are a variety of ways to boost your metabolism. Just remember that your metabolic rate is uniquely yours: What you may need to do to reach and maintain a healthy weight may be quite different than what someone else does.