Your Metabolism: What it is and How it Works

You may hear the word metabolism all the time—especially when you're talking about your weight. But just what does it really mean?

Metabolism is the set of body processes and chemicals that converts calories from the foods you eat into the energy you need to live. The metabolic process consists of the breakdown of food, transport of nutrients, and the elimination of waste. It involves your digestive system, kidneys, liver, pancreas, and blood.

The Role of Calories

To understand metabolism, it helps to understand what a calorie is and what a calorie isn’t. A calorie is not a "thing" in your food; it's a unit of heat energy, and the way we measure the amount of energy we get from foods.

"Our bodies use this energy for basic needs, like keeping our hearts pumping and lungs breathing, all movement and physical activity, and for the digestion and absorption of food," explains Toby Smithson, a registered dietician based in Hilton Head, SC, spokesperson for the Academy of Nutrition and Dietetics and author of Diabetes Meal Planning and Nutrition for Dummies.

How Much Energy Do You Need?

The minimum amount of energy you need to stay alive is known as your basal, or resting metabolism. That energy is represented by your basal metabolic rate (BMR), or the rate at which you burn the calories needed to maintain basic body processes such as heartbeat, nerve impulses, and blood circulation.

Basal metabolic rate varies from person to person; online BMR calculators use factors like age, sex, height, and weight to estimate how many calories you need to maintain these basic body processes. According to the MyFitnessPal BMR calculator, a 40-year-old, 5'11" man would burn about 1,749 calories over the course of 24 hours doing nothing but resting; a 5'4" woman the same age would burn 1,245 calories.

But you also need energy for the physical activities you do throughout the day, from getting out of bed in the morning to washing dishes or running on a treadmill. Plus you need energy to digest food, absorb nutrients, and convert food to energy. When you add these activities to your basal metabolic rate, you get your overall or total metabolic rate.

Your overall metabolic rate can be estimated by multiplying your BMR by a number ranging from 1.2 (if you're sedentary) to 1.9 (if you engage in intense vigorous activity on a daily basis). So the 40-year old, 5'4" woman whose basal metabolic rate is 1,245 calories would have a total metabolic rate of somewhere between about 1,494 calories (if she is sedentary) and 2,365 (if she's extremely active).

Eating more calories than you need results in weight gain as your body stores excess energy as fat; eating less results in weight loss as your body uses it’s own fat or muscle tissue as fuel. This energy balance is at the foundation of weight control.

However, this isn't the whole story. Energy balance is not a one-size-fits-all equation, according to Smithson. In addition to the factors listed above, genetics, diet, and health status also play a role in weight gain, loss, and maintenance.

Boosting Your Metabolism

Is your metabolism "fast" or "slow"? Metabolism speed refers to basic metabolic rate, and generally, it's faster in people who have more muscle tissue, because muscle burns calories more efficiently than fat. Men, growing children, and physically active people usually have the leanest body mass and the fastest metabolisms.

Unfortunately, we lose lean body mass as we age, and that slows down metabolism, as does weight gain from excess fat. Is there a way to fight the inevitable metabolic slowdown? Yes: Exercise is the best way to boost your metabolic rate, which can "help push your weight in a different direction," Smithson says. This is why she encourages her clients who want to lose weight to increase their physical activity.

Different kinds of exercise offers different benefits: Strength-building exercises, such as push-ups, pull-ups, planks, and free weights not only make you stronger, they also help maximize muscle, Smithson points out. This increases your metabolism and helps your body becomes more efficient at burning calories. Meanwhile, aerobic or endurance activities, such as dance classes, biking, running, and even fast walking, generally burn the most calories and improve your overall health and fitness.

"Intense physical activity speeds up your metabolism while you exercise," Smithson explains. "The bonus is that it also extends an increased metabolism for several hours after you stop exercising."

So don't fret too much about your "slow" metabolism—you have tools to up its speed, and reap the benefits.

Toby Smithson, MS, RDN, LD, CDE reviewed this article.


Smithson, Toby MS, RDN, LD, CDE. Email to author June 26, 2016.

Whitney, Ellie and Rolfes, Sharon Rady. Understanding Nutrition. California: Wadsworth, Cenage Learning, 2013.

"BMR Calculator." Page accessed June 28, 2016.

"How to Measure Your Metabolic Rate." Page accessed June 28, 2016.