Understanding Metabolism
11:40 AM
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-tah-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.
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
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 (pronounced: uh-nah-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-tah-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 hormoneinsulin (pronounced: in-suh-lin),
which signals cells to increase their anabolic activities.
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 meh-luh-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.
Excerpts taken from “Metabolism” reviewed by Steven Dowshen, MD.
Read more at http://kidshealth.org/teen/your_body/body_basics/metabolism.html#
Best Teen Diets recommends healthy well balanced
eating that includes fruits, vegetables, whole grains, protein and low-fat or
fat-free dairy. We offer nutrition information for teens, parents, and
educators that emphasizes the importance of healthy eating for teens. For more
information visit www.bestteendiets.org
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