Metabolism


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 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 cholorophyll
(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.Enzymes

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:

• Anabolism, 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, 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.
  

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 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, 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 (or 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 someone'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.
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What Factors Influence BMR?

To a certain extent, BMR is inherited. Sometimes health problems can affect BMR, 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.
Metabolism Problems

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

G6PD deficiency. Glucose-6-phosphate dehydrogenase, or G6PD, is just one of the many enzymes that play a role in cell metabolism. G6PD is produced by red blood cells and helps the body metabolize carbohydrates. Without enough normal G6PD to help red blood cells handle certain harmful substances, red blood cells can be damaged or destroyed, leading to a condition known as hemolytic anemia. In a process called hemolysis, red blood cells are destroyed prematurely, and the bone marrow (the soft, spongy part of the bone that produces new blood cells) may not be able to keep up with the body's need to produce more 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 — a yellowing of the skin and eyes. G6PD deficiency is usually treated by discontinuing medications or treating the illness or infection causing the stress on the red blood cells.

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. This enzyme is produced in the liver. If the liver doesn't produce enough of this enzyme, galactose builds up in the blood and can cause serious health problems. Symptoms usually occur within the first days of life and include vomiting, swollen liver, and jaundice. If galactosemia is not diagnosed and treated quickly, it can cause liver, eye, kidney, and brain damage.

Hyperthyroidism. 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.
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More Metabolic Disorders
Hypothyroidism. Hypothyroidism is caused by an absent 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. In infants and young children who don't get treatment, this condition can result in stunted growth and mental retardation. Hypothyroidism slows body processes and causes fatigue, slow heart rate, excessive weight gain, and constipation. Kids and teens with this condition can be treated with oral thyroid hormone to achieve normal levels in the body.
Phenylketonuria. Also known as PKU, this condition occurs in infants due to 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 mental retardation occurs. Early diagnosis and dietary restriction of the amino acid can prevent or lessen the severity of these complications.

Type 1 diabetes mellitus. 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. Kids and 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 kids who develop type 2 diabetes are overweight, and this is thought to play a role in their decreased responsiveness to insulin. Some 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: May 2009
Back
What Factors Influence BMR?



To a certain extent, BMR is inherited. Sometimes health problems can
affect BMR, 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.

Metabolism Problems



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



G6PD deficiency. Glucose-6-phosphate dehydrogenase,
or G6PD, is just one of the many enzymes that play a role in cell
metabolism. G6PD is produced by red blood cells and helps the body
metabolize carbohydrates. Without enough normal G6PD to help red blood
cells handle certain harmful substances, red blood cells can be damaged
or destroyed, leading to a condition known as hemolytic anemia. In a
process called hemolysis, red blood cells are destroyed prematurely, and
the bone marrow (the soft, spongy part of the bone that produces new
blood cells) may not be able to keep up with the body's need to produce
more 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 — a yellowing of the skin and eyes. G6PD
deficiency is usually treated by discontinuing medications or treating
the illness or infection causing the stress on the red blood cells.

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. This enzyme is produced in the liver. If the
liver doesn't produce enough of this enzyme, galactose builds up in the
blood and can cause serious health problems. Symptoms usually occur
within the first days of life and include vomiting, swollen liver, and
jaundice. If galactosemia is not diagnosed and treated quickly, it can
cause liver, eye, kidney, and brain damage.

Hyperthyroidism. 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.More Metabolic Disorders



Hypothyroidism. Hypothyroidism is caused by an
absent 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. In infants and young children who don't get treatment,
this condition can result in stunted growth and mental retardation.
Hypothyroidism slows body processes and causes fatigue, slow heart rate,
excessive weight gain, and constipation. Kids and teens with this
condition can be treated with oral thyroid hormone to achieve normal
levels in the body.

Phenylketonuria. Also known as PKU, this condition
occurs in infants due to 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
mental retardation occurs. Early diagnosis and dietary restriction of
the amino acid can prevent or lessen the severity of these
complications.

Type 1 diabetes mellitus. 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.
Kids and 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 kids who develop type 2
diabetes are overweight, and this is thought to play a role in their
decreased responsiveness to insulin. Some 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.