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Every time you swallow a bite of sandwich or slurp a smoothie, your body works hard to process the nutrients you've eaten. Long after the dishes are cleared and the food is digested, the nutrients you've taken in become the building blocks and fuel needed by your body. Your body gets the energy it needs from food through a process called metabolism.

What Is Metabolism?

Metabolism (pronounced: muh-TA-buh-lih-zum) is a collection of chemical reactions that takes place in the body's cells. Metabolism converts the fuel in the food we eat into the energy needed to power everything we do, from moving to thinking to growing. Specific proteins in the body control the chemical reactions of metabolism, and each chemical reaction is coordinated with other body functions. In fact, thousands of metabolic reactions happen at the same time — all regulated by the body — to keep our cells healthy and working.

Metabolism is a constant process that begins when we're conceived and ends when we die. It is a vital process for all life forms — not just humans. If metabolism stops, living things die.

Here's an example of how the process of metabolism works in humans — and it begins with plants. First, a green plant takes in energy from sunlight. The plant uses this energy and a molecule called chlorophyll (which gives plants their green color) to build sugars from water and carbon dioxide. This process is called photosynthesis, and you probably learned about it in biology class.

When people and animals eat the plants (or, if they're carnivores, they eat animals that have eaten the plants), they take in this energy (in the form of sugar), along with other vital cell-building chemicals. The body's next step is to break the sugar down so that the energy released can be distributed to, and used as fuel by, the body's cells.

After food is eaten, molecules in the digestive system called enzymes break proteins down into amino acids, fats into fatty acids, and carbohydrates into simple sugars (e.g., glucose). In addition to sugar, both amino acids and fatty acids can be used as energy sources by the body when needed. These compounds are absorbed into the blood, which transports 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.

A Balancing Act

The process of metabolism is really a balancing act involving two kinds of activities that go on at the same time — the building up of body tissues and energy stores and the breaking down of body tissues and energy stores to generate more fuel for body functions:

  • Anabolism (pronounced: uh-NA-buh-lih-zum), 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 use in the future. During anabolism, small molecules are changed into larger, more complex molecules of carbohydrate, protein, and fat.
  • Catabolism (pronounced: kuh-TA-buh-lih-zum), or destructive metabolism, is the process that produces the energy required for all activity in the cells. In this process, cells break down large molecules (mostly carbohydrates and fats) to release energy. This energy release provides fuel for anabolism, heats the body, and enables the muscles to contract and the body to move. As complex chemical units are broken down into more simple substances, the waste products released in the process of catabolism are removed from the body through the skin, kidneys, lungs, and intestines.

Several of the hormones of the endocrine system are involved in controlling the rate and direction of metabolism. Thyroxine (pronounced: thigh-RAHK-sun), a hormone produced and released by the thyroid (pronounced: THIGH-royd) gland, plays a key role in determining how fast or slow the chemical reactions of metabolism proceed in a person's body.

Another gland, the pancreas (pronounced: PAN-kree-us) secretes (gives off) hormones that help determine whether the body's main metabolic activity at a particular time will be anabolic or catabolic. For example, after eating a meal, usually more anabolic activity occurs because eating increases the level of glucose — the body's most important fuel — in the blood. The pancreas senses this increased level of glucose and releases the hormone insulin (pronounced: IN-suh-lin), 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 fat on the body.

The number of calories a person 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. The basal metabolic rate, or 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, a person 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, compared with a similar-sized person with an average BMR who eats the same amount of food and gets the same amount of exercise.

What factors influence a person's BMR? To a certain extent, a person's basal metabolic rate is inherited — passed on through the genes the person gets from his or her parents. Sometimes health problems can affect a person's BMR (see below). But people can actually change their BMR in certain ways. For example, exercising more will not only cause a person to burn more calories directly from the extra activity itself, but becoming more physically fit will increase BMR as well. BMR is also influenced by body composition — people with more muscle and less fat generally have higher BMRs.

Things That Can Go Wrong With Metabolism

Most of the time your metabolism works effectively without you giving any thought to it. But sometimes a person's metabolism can cause major mayhem in the form of a metabolic disorder. In a broad sense, a metabolic disorder is any disease that is caused by an abnormal chemical reaction in the body's cells. Most disorders of metabolism involve either abnormal levels of enzymes or hormones or problems with the functioning of those enzymes or hormones. When the metabolism of body chemicals is blocked or defective, it can cause a buildup of toxic substances in the body or a deficiency of substances needed for normal body function, either of which can lead to serious symptoms.

Some metabolic diseases and conditions include:

Hyperthyroidism (pronounced: hi-per-THIGH-roy-dih-zum). Hyperthyroidism is caused by an overactive thyroid gland. The thyroid releases too much of the hormone thyroxine, which increases the person's basal metabolic rate (BMR). It causes symptoms such as weight loss, increased heart rate and blood pressure, protruding eyes, and a swelling in the neck from an enlarged thyroid (goiter). The disease may be controlled with medications or through surgery or radiation treatments.

Hypothyroidism (pronounced: hi-po-THIGH-roy-dih-zum). Hypothyroidism is caused by a nonexistent or underactive thyroid gland, and it results from a developmental problem or a destructive disease of the thyroid. The thyroid releases too little of the hormone thyroxine, so a person's basal metabolic rate (BMR) is low. Not getting treatment for hypothyroidism can lead to brain and growth problems in infants and children. Hypothyroidism slows body processes and causes fatigue, slow heart rate, excessive weight gain, and constipation. Teens with this condition can be treated with oral thyroid hormone to achieve normal levels in the body.

Inborn errors of metabolism. Some metabolic diseases are inherited. These conditions are called inborn errors of metabolism. When babies are born, they're tested for many of these metabolic diseases. Inborn errors of metabolism can sometimes lead to serious problems if they're not controlled with diet or medication from an early age. Examples of inborn errors of metabolism include galactosemia (babies born with this inborn error of metabolism do not have enough of the enzyme that breaks down the sugar in milk called galactose) and phenylketonuria (this problem is due to a defect in the enzyme that breaks down the amino acid phenylalanine, which is needed for normal growth and protein production). Teens may need to follow a certain diet or take medications to control metabolic problems they've had since birth.

Type 1 diabetes mellitus (pronounced: dye-uh-BEE-teez MEL-uh-tus). Type 1 diabetes occurs when the pancreas doesn't produce and secrete enough insulin. Symptoms of this disease include excessive thirst and urination, hunger, and weight loss. Over the long term, the disease can cause kidney problems, pain due to nerve damage, blindness, and heart and blood vessel disease. Teens with type 1 diabetes need to receive regular injections of insulin and control blood sugar levels to reduce the risk of developing problems from diabetes.

Type 2 diabetes. Type 2 diabetes happens when the body can't respond normally to insulin. The symptoms of this disorder are similar to those of type 1 diabetes. Many children and teens who develop type 2 diabetes are overweight, and this is thought to play a role in their decreased responsiveness to insulin. Some teens can be treated successfully with dietary changes, exercise, and oral medication, but insulin injections are necessary in other cases. Controlling blood sugar levels reduces the risk of developing the same kinds of long-term health problems that occur with type 1 diabetes.

Reviewed by: Steven Dowshen, MD
Date reviewed: February 2012

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