What Is Metabolism?

Metabolism (meh-TAB-uh-liz-um) is the chemical reactions in the body's cells that change food into energy. Our bodies need this energy to do everything from moving to thinking to growing.

Specific proteins in the body control the chemical reactions of metabolism. Thousands of metabolic reactions happen at the same time — all regulated by the body — to keep our cells healthy and working.

How Does Metabolism Work?

After we eat food, the digestive system uses enzymes to:

• break proteins down into amino acids
• turn fats into fatty acids
• turn carbohydrates into simple sugars (for example, glucose)

The body can use sugar, amino acids, and fatty acids as energy sources when needed. These compounds are absorbed into the blood, which carries them to the cells.

After they enter the cells, other enzymes act to speed up or regulate the chemical reactions involved with "metabolizing" these compounds. During these processes, the energy from these compounds can be released for use by the body or stored in body tissues, especially the liver, muscles, and body fat.

Metabolism is a balancing act involving two kinds of activities that go on at the same time:

• building up body tissues and energy stores (called anabolism)
• breaking down body tissues and energy stores to get more fuel for body functions (called catabolism)

Anabolism (uh-NAB-uh-liz-um), or constructive metabolism, is all about building and storing. It supports the growth of new cells, the maintenance of body tissues, and the storage of energy for future use. In anabolism, small molecules change into larger, more complex molecules of carbohydrates, protein, and fat.

Catabolism (kuh-TAB-uh-liz-um), or destructive metabolism, is the process that produces the energy needed for all activity in the cells. Cells break down large molecules (mostly carbs and fats) to release energy. This provides fuel for anabolism, heats the body, and enables the muscles to contract and the body to move.

As complex chemical units break down into more simple substances, the body releases the waste products through the skin, kidneys, lungs, and intestines.

What Controls Metabolism?

Several hormones of the endocrine system help control the rate and direction of metabolism. Thyroxine, a hormone made and released by the thyroid gland, plays a key role in determining how fast or slow the chemical reactions of metabolism go in a person's body.

Another gland, the pancreas, secretes hormones that help determine whether the body's main metabolic activity at any one time are anabolic (an-uh-BOL-ik) or catabolic (kat-uh-BOL-ik). For example, more anabolic activity usually happens after you eat a meal. That's because eating increases the blood's level of glucose — the body's most important fuel. The pancreas senses this increased glucose level and releases the hormone insulin, which signals cells to increase their anabolic activities.

Metabolism is a complicated chemical process. So it's not surprising that many people think of it in its simplest sense: as something that influences how easily our bodies gain or lose weight. That's where calories come in. A calorie is a unit that measures how much energy a particular food provides to the body. A chocolate bar has more calories than an apple, so it provides the body with more energy — and sometimes that can be too much of a good thing. Just as a car stores gas in the gas tank until it is needed to fuel the engine, the body stores calories — primarily as fat. If you overfill a car's gas tank, it spills over onto the pavement. Likewise, if a person eats too many calories, they "spill over" in the form of excess body fat.

The number of calories someone burns in a day is affected by how much that person exercises, the amount of fat and muscle in his or her body, and the person's basal metabolic rate (BMR). BMR is a measure of the rate at which a person's body "burns" energy, in the form of calories, while at rest.

The BMR can play a role in a person's tendency to gain weight. For example, someone with a low BMR (who therefore burns fewer calories while at rest or sleeping) will tend to gain more pounds of body fat over time than a similar-sized person with an average BMR who eats the same amount of food and gets the same amount of exercise.

BMR can be affected by a person's genes and by some health problems. It's also influenced by body composition — people with more muscle and less fat generally have higher BMRs. But people can change their BMR in certain ways. For example, a person who exercises more not only burns more calories, but becomes more physically fit, which increases his or her BMR.