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.
Metabolism Basics
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.
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.
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.
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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.
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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.
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.
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 include:
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.
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: July 2007
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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