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Metabolism Basics

Our bodies get the energy they need from food through metabolism, the chemical reactions in the body's cells that convert the fuel from food into the energy needed to do everything 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 (meh-TAB-uh-liz-um) 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, a living thing dies.

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 the molecule chlorophyll (which gives plants their green color) to build sugars from water and carbon dioxide in a process known as photosynthesis.

When people and animals eat the plants (or, if they're carnivores, when 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 (for example, 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.

In this way, 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:

The Endocrine System

Several of the hormones of the endocrine system are involved in controlling the rate and direction of metabolism. Thyroxine, a hormone produced and released by the thyroid 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 secretes 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 happens 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, 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.

What Factors Influence BMR?

To a certain extent, BMR is inherited. Sometimes health problems can affect BMR, but people can change their BMR in certain ways. For example, exercising more not only will cause someone 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.

Metabolic Disorders

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 involve either abnormal levels of enzymes or hormones, or problems with how those enzymes or hormones work.

When the metabolism of body chemicals is blocked or defective, it can cause a buildup of toxic substances in the body or a lack of substances needed for normal body function, either of which can lead to serious symptoms.

Some metabolic diseases are inherited. These are called inborn errors of metabolism. When babies are born, they're tested for many of these in a newborn screening test. Many inborn errors of metabolism can lead to serious complications or even death if they're not controlled with diet or medicine from an early age.

G6PD deficiency: Glucose-6-phosphate dehydrogenase (G6PD) is just one of the many enzymes that play a role in cell metabolism. G6PD is produced by red blood cells (RBCs) and helps the body metabolize carbohydrates. Without enough normal G6PD to help RBCs handle certain harmful substances, the cells can be damaged or destroyed, leading to hemolytic anemia. In a process called hemolysis, RBCs are destroyed prematurely, and the bone marrow (the soft, spongy part of the bone that produces new blood cells) may not be able to produce enough new red blood cells.

Kids with G6PD deficiency may be pale and tired and have a rapid heartbeat and breathing. They may also have an enlarged spleen or jaundice (yellowing of the skin and eyes). G6PD deficiency is usually treated by stopping medicines or treating the illness or infection causing the stress on the RBCs.

Galactosemia: Babies born with this inborn error of metabolism lack the enzyme that converts galactose (one of two sugars found in lactose) into glucose, a sugar the body is able to use. As a result, milk (including breast milk) and other dairy products must be eliminated from the diet. Otherwise, galactose can build up in the system and damage the body's cells and organs, leading to blindness, severe intellectual disability, growth deficiency, and even death.

Hyperthyroidism: This is when an overactive thyroid gland releases too much of the hormone thyroxine, which increases 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 medicines or through surgery or radiation treatments.

Hypothyroidism: This is when an absent or underactive thyroid (due to a developmental problem or thyroid disease) causes the release of too little of the hormone thyroxine, which lowers BMR.

If not treated, this condition can result in stunted growth and intellectual disability in infants and young children. Hypothyroidism slows body processes and causes fatigue (tiredness), slow heart rate, excessive weight gain, and constipation. Kids and teens with this condition can be treated with oral thyroid hormone.

Phenylketonuria: Also known as PKU, this is caused by a defect in the enzyme that breaks down the amino acid phenylalanine. This amino acid is necessary for normal growth in infants and children and for normal protein production. However, if too much of it builds up in the body, brain tissue is affected and intellectual disability happens.

Early diagnosis and dietary restriction of the amino acid can prevent or lessen the severity of these complications.

Type 1 diabetes: This is 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, it can cause kidney problems, pain due to nerve damage, blindness, and heart and blood vessel disease. Kids and teens with type 1 diabetes need to get regular insulin injections and to control their blood sugar levels to reduce the risk of developing complications.

Type 2 diabetes: This is when the body can't respond normally to insulin. Symptoms are similar to those of type 1 diabetes. Many kids who develop type 2 diabetes are overweight, which is thought to play a role in their decreased responsiveness to insulin. Some can be treated successfully with dietary changes, exercise, and oral medicine, 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 happen with type 1 diabetes.

Reviewed by: Steven Dowshen, MD
Date reviewed: June 2015

Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor.

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