Protein
Protein is part of every cell; it is needed in thousands of chemical reactions. The
word protein was discovered by the Dutch chemist Gerardus Mulder in 1838,
and comes from the Greek word protos, meaning “of prime importance. Our
bodies constantly syntheses, break down, and use proteins, so we need protein
in our diet to replace what is being used. When we eat more protein than we
need, the excess is either used to make energy or stored as fat.
Plant foods such as dried beans and peas, grains, nuts, seeds, and vegetables
also provide protein, not just animal foods. Many protein-rich plant foods are
also rich in vitamins and minerals. These plant foods usually are low in fat and
calories.
When the diet lacks protein, the body breaks down body tissue such as muscle
and uses it as a protein source. This causes loss, or wasting, of muscle, organs,
and other tissues. Protein deficiency also increases susceptibility to infection,
and impairs digestion and absorption of nutrients.
Amino Acids: The Building Blocks of Protein
Amino acids are the basic building blocks of protein. Proteins are sequences of
amino acids. The body needs 20 different amino acids to choose from when
building these sequences. Nine of these amino acids are called essential amino
acids because your body cannot make them and must get them in the diet. Your
body can manufacture the remaining 11, called nonessential amino acids, when
enough nitrogen, carbon, hydrogen, and oxygen are available. Nonessential
amino acids do not need to be supplied in your diet.
Sometimes, certain nonessential amino acids can become essential. Tyrosine
and cysteine are both considered conditionally essential amino acids. Under
normal circumstances, your body makes tyrosine from the essential amino acid
phenylalanine, and cysteine from the essential amino acid methionine. When
your intake of phenylalanine and methionine is low, however, your body needs
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tyrosine and cysteine from your diet to free phenylalanine and methionine for
protein formation.
People with the disease phenylketonuria (PKU) must control their
consumption of phenylalanine. PKU is a genetic disorder that impairs
phenylalanine metabolism. People with PKU lack sufficient amounts of an
enzyme (phenylalanine hydroxylase) that converts phenylalanine to tyrosine, so
tyrosine becomes an essential amino acid.
Phenylalanine hydroxylase
Phenylalanine
tyrosine
1) Melanine.
2) Epinephrine and norepinephrine.
3) Thyroxine.
People with PKU must carefully monitor the amount of phenylalanine in their
diets so they have enough to support growth and maintenance of body tissue,
but not too much. Excess phenylalanine and byproducts of its abnormal
metabolism (called phenylketones) can build up in the body and contribute to
irreversible brain damage. Without treatment, the IQ of individuals with PKU
averages about 40; however, those who are treated starting at birth have IQs in
the normal range.
Other amino acids also can become essential under certain circumstances. The
amino acid glutamine is the main fuel for rapidly dividing cells and plays a key
role in transporting nitrogen between organs. Although normally considered
nonessential, glutamine can become essential after trauma or during periods of
critical illness that increase the body’s need for it. The amino acid arginine can
also become essential during conditions of illness or severe physiological stress.
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Wasting: The breakdown of body tissue such as muscle and organ for use as a
protein source when the diet lacks protein.
Essential amino acid: An amino acid the body cannot make at all or cannot
make enough of to meet physiological needs. Essential amino acids must be
supplied in the diet.
Nonessential amino acid: An amino acid the body can make if supplied with
adequate nitrogen. Nonessential amino acids do not need to be supplied in the
diet.
Conditionally essential amino acid: An amino acid that is normally made in
the body (nonessential) but becomes essential under certain circumstances, such
as during critical illness.
Structure of an amino acid
All amino acids have a similar structure. Attached to a carbon atom is a
hydrogen (H), an amino group (—NH2), an acid group (—COOH) and a side
group (R). The side group gives each amino acid its unique identity.
Forming a peptide bond
When two amino acids join together, the carboxyl group of one amino acid is matched with
the amino group of another. A condensation reaction forms a peptide bond and releases
water.
Amino Acids Are Identified by Their Side Groups
The side group gives each amino acid its identity. It can vary from a simple
hydrogen atom, as in glycine, to a complex ring of carbon and hydrogen atoms,
as in phenylalanine. The side groups mean that amino acids differ in shape, size,
composition, electrical charge, and pH. When amino acids are linked to form a
protein, these characteristics work together to determine that protein’s specific
function.
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Protein Structure: Unique Three-Dimensional Shapes and
Functions
Proteins are very large molecules. Their chains of linked amino acids twist,
fold, or coil into unique shapes. The body combines amino acids in different
sequences to form a nearly infinite variety of proteins. For this reason, protein
molecules are more diverse than either carbohydrates or lipids.
Amino Acid Sequence
Amino acids link in specific sequences to form strands of protein (called
peptides) up to hundreds of amino acids long. One amino acid is joined to the
next by a peptide bond. To form a peptide bond, the carboxyl (—COOH) group
of one amino acid bonds to the amino (—NH2) group of another amino acid,
releasing water (H2O) in the process. A dipeptide is two amino acids joined by a
peptide bond, while a tripeptide is three amino acids joined by peptide bonds.
The term oligopeptide refers to a chain of 4 to 10 amino acids, while a
polypeptide contains more than 10 amino acids. Proteins in the body and in the
diet are long polypeptides, most with hundreds of linked amino acids.
Protein Shape
The three-dimensional shape of a protein determines its function and its
interaction with other molecules. For example, the unique folded and twisted
shape of hemoglobin, the iron-carrying protein in red blood cells. In the lungs,
hemoglobin binds oxygen and releases carbon dioxide. Hemoglobin delivers
oxygen to other tissues and picks up carbon dioxide for the return trip to the
lungs.
Some amino acids carry electrical charges and therefore are attracted to the
charged ends of water molecules (hydrophilic amino acids). Other amino acids
are electrically neutral and do not interact with water (hydrophobic amino
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acids). The amino acid cysteine, which has sulfur atoms in its side group,
sometimes will chemically bond to another cysteine in the chain, creating a
disulflde bridge, which helps stabilize the protein’s structure.
Dipeptide: Two amino acids joined by a peptide bond.
Tripeptide: Three amino acids joined by peptide bonds.
Oligopeptide: Four to 10 amino acids joined by peptide bonds.
Polypeptide: More than 10 amino acids joined by peptide bonds.
Hemoglobin: The oxygen-carrying protein in red blood cells that consists of
four heme groups and four globin polypeptide chains. The presence of
hemoglobin gives blood its red color.
Hydrophilic amino acids: Amino acids that are attracted to water (waterloving).
Hydrophobic amino acids: Amino acids that are water-fearing.
Disulfide bridge: A bond between the sulfur components of two sulfurcontaining amino acids that helps stabilize the structure of protein.
Protein Denaturation
Acidity, alkalinity, heat, alcohol, and oxidation can all disrupt the chemical
forces that stabilize a protein’s three-dimensional shape, causing it to unfold and
lose its shape (denature). Since a protein’s shape determines its function,
denatured proteins lose their ability to function properly.
Denaturation is the first step in breaking down protein for digestion. Stomach
acids denature protein, uncoiling the structure into a simple amino acid chain
that digestive enzymes can start breaking apart.
Denaturation: An alteration in the three-dimensional structure of a protein
resulting in an unfolded polypeptide chain that usually lacks biological activity.
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Protein digestion
The first step in using dietary protein is digesting its long polypeptide chains
into amino acids. Like the other energy-yielding nutrients, digestion requires
enzymes. Digestion of protein begins in the stomach.
Stomach
In the stomach, hydrochloric acid (HC) denatures a protein, unfolding it and
making the amino acid chain more accessible to the action of enzymes. Glands
in the stomach lining produce the proenzyme pepsinogen, an inactive precursor
of the enzyme pepsin. When pepsinogen comes in contact with HC, it is
converted to the active enzyme pepsin. Pepsin breaks down protein into
individual amino acids and peptides of various lengths.
Small intestine
From the stomach, amino acids and polypeptides pass into the small intestine,
where most protein digestion takes place. In the small intestine, proteases
(protein-digesting enzymes) break down large peptides into smaller peptides.
Both the pancreas and the small intestine make digestive proenzymes. The
pancreas makes trypsinogen and chymotrypsinogen, which are secreted into the
small intestine in response to the presence of protein. Here, these proenzymes
are cleaved into their active forms, trypsin and chymotrypsin, respectively.
These activated proteases break polypeptides into smaller peptides. Pancreatic
enzymes completely digest only a small percentage of proteins into individual
amino acids; most of the proteins at this point are dipeptides, tripeptides, and
still larger polypeptides.
The final stages of protein digestion take place on the surface of the intestine’s
lining, and require enzymes secreted by the intestinal lining cells. Brush border
(microvilli) peptidases react with intestinal fluids that come in contact with the
cell surface and split the remaining larger polypeptides into tripeptides,
dipeptides and even some all the way into amino acids. These smaller units are
transported across the microvilli membranes into the cell. Inside the cell many
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other peptidases specifically attack the linkages between the amino acids. These
peptidases digest all the remaining dipeptides and tripeptides into individual
amino acids for absorption into the bloodstream.
Undigested protein
Any parts of proteins that are not digested and absorbed in the small intestine
continue on through the large intestine. People with celiac disease, for example,
cannot properly digest gluten—a protein found in wheat, barley, rye, and oats.
Unless treated with a gluten-free diet, people with celiac disease show poor
growth, weight loss, and other symptoms resulting from poor absorption of
protein and other nutrients.
Celiac disease: A disease that involves an inability to digest gluten, a protein
found in wheat, rye, oats, and barley. If untreated, it causes flattening of the villi
in the intestine, leading to severe malabsorption of nutrients. Symptoms include
diarrhea, fatty stools, and extreme fatigue.
Amino acid and peptide absorption
Absorption of some amino acids requires active transport while others are
absorbed via facilitated diffusion. Although there are several active transport
mechanisms, similar amino acids share the same active transport system. The
amino acids leucine, isoleucine, and valine, for example, all depend on the same
carrier molecule for absorption. Normally proteins in foods supply a mix of
many amino acids, so amino acids that share the same transport system are
absorbed fairly equally.
Most protein absorption takes place in the cells that line the duodenum and
jejunum. After they are absorbed, most amino acids are transported via the
portal vein to the liver and then released into general circulation. Some amino
acids remain in the intestinal cells and are used to synthesize intestinal enzymes
and new cells.
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Active transport: Amino acids are actively transported into the intestinal cell
by a sodium co-transport strategy. First energy is used to pump sodium out of
the cell. A special transport protein in the cell membrane allows the sodium to
reenter the cell when accompanied by an amino acid.
Note: All amino acids use facilitated diffusion to leave the cell and enter the
bloodstream.
Conclusion: Protein digestion begins in the stomach, where the enzyme pepsin
breaks proteins into smaller peptides. Digestion continues in the small intestine,
where proteases break polypeptides into smaller peptide units, which are then
absorbed into cells where additional enzymes (proteases) complete digestion to
amino acids.
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Functions of Body Proteins
1) Structural and Mechanical Functions
Structures such as bone, skin, and hair owe their physical properties to unique
proteins. Collagen is the most abundant protein in mammals, and gives skin and
bone their elastic strength. Hair and nails are made of keratin. So protein is
essential for building these structures; therefore, protein deficiencies during a
child’s development can be dangerous.
Motor proteins are proteins that turn energy into mechanical work. These
proteins are the final step in converting our food into physical work. Specialized
motor proteins are also involved in a variety of processes including cell division
and muscle contraction.
Collagen: The most abundant fibrous protein in the body, it is the major
constituent of connective tissue, forms the foundation for bones and teeth, and
helps maintain the structure of blood vessels and other tissues.
Keratin: A water-insoluble fibrous protein that is the primary constituent of
hair, nails, and the outer layer of the skin.
Motor proteins: Proteins that use energy and convert into some form of
mechanical work. Motor proteins are active in processes such as dividing cells
and contracting muscle.
2) Enzymes
Enzymes are proteins that catalyze chemical reactions. Every cell contains
thousands of types of enzymes, each with its own purpose. During digestion, for
example, enzymes help break down carbohydrates, proteins, and fats into
monosaccharides, amino acids, and fatty acids for absorption into the body.
Enzymes release energy from these nutrients to fuel thousands of body
processes. Enzymes also catalyze the reactions that build muscle and tissue.
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3) Hormones
Hormones are chemical messengers that are made in one part of the body but
act on cells in other parts of the body. Many are proteins with important
regulatory functions. Insulin, for example, is a protein hormone that plays a key
role in regulating the amount of glucose in the blood. It is released from the
pancreas in response to a rise in blood glucose levels and functions to lower
those levels.
Thyroid-stimulating protein (TSH) and leptin are two other protein hormones.
The pituitary gland produces TSH, which stimulates the thyroid gland to
produce the hormone thyroxine. Thyroxine, a modified form of the amino acid
tyrosine, increases the body’s metabolic rate.
4) Immune Function
Proteins play an important role in the immune system, which is responsible for
fighting infection and invasion by foreign substances. Antibodies are blood
proteins that attack and inactivate bacteria and viruses that cause infection.
When your diet does not contain enough protein, your body cannot make as
many protein antibodies as it needs. Your immune response is weakened and
your risk of infection and illness increases.
Antibody: A large blood protein produced by B lymphocytes in response to
exposure to a particular antigen (e.g., a protein on the surface of a virus or
bacterium). Each type of antibody specifically binds to and helps eliminate its
matching antigen from the body. Once formed, antibodies circulate in the blood
and help protect the body against subsequent infection.
5) Fluid Balance
Fluids in the body are intracellular (inside cells) or extracellular (outside cells).
There are two types of extracellular fluid—intercellular or interstitial, (between
cells) and intravascular (in the blood). These interior and exterior fluid levels
must stay in balance for body processes to work properly. Proteins in the blood
help to maintain appropriate fluid levels in the vascular system. The force of the
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heart’s beating pushes fluid and nutrients from the capillaries out into the fluid
surrounding the cells. But blood proteins like albumin and globulin are too large
to leave the capillary beds. These proteins remain in the capillaries, where they
attract fluid. This provides a balancing and partially counteracting force that
keeps fluid in the circulatory system.
If the diet does not have enough protein to maintain normal levels of blood
proteins, fluid will leak into the surrounding tissue and cause swelling, also
called edema. Children with protein malnutrition often suffer from severe
edema.
6) Acid-Base Balance
The body works hard to keep the pH of the blood near 7.4, or nearly neutral.
Only a few hours with a blood pH above 8.0 or below 6.8 will cause death.
Proteins help maintain stable pH levels in body fluids by behaving as buffers;
they pick up extra hydrogen ions when conditions are acidic, and they donate
hydrogen ions when conditions are alkaline. If proteins are not available to
buffer acidic or alkaline substances, the blood can become too acidic or too
alkaline, resulting in either acidosis or alkalosis. Both conditions can be serious;
either can cause proteins to denature, and this can lead to coma or death.
7) Transport Functions
Many substances pass in and out of cells via proteins that cross cell membranes
and act as channels and pumps. Channels allow substances to flow rapidly
through the membranes by passive diffusion and require no input of energy.
Pumps (active transporters), in contrast, must use energy to drive the transport
of substances across membranes. More than one-third of the energy your body
consumes at rest is used by sodium-potassium protein pumps that control cell
volume and nerve impulses and drive the active transport of sugars and amino
acids.
Proteins also act as carriers, transporting many important substances in the
bloodstream for delivery throughout the body. Lipoproteins, for example,
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carries lipid particles in the blood. The protein transferrin carries iron in the
blood. In the liver, iron is stored as part of ferritin.
8) Source of Energy and Glucose
Although our bodies preferentially burns carbohydrate and fat for energy, if
necessary it can use protein for energy or to make glucose. Thus, carbohydrate
and fat are protein-sparing: they spare amino acids from being burned for
energy and allow them to be used for protein synthesis.
If the diet does not provide enough energy to sustain vital functions, the body
will use its own protein from enzymes, muscle, and other tissues to make
energy and glucose for use by the brain, lungs, and heart (e.g. starvation).
To release energy from an amino acid, the body first removes the nitrogen
group—a process called deamination. To make glucose, the body uses the
remaining carbon, hydrogen, and oxygen compounds.
If the diet contains more protein than is needed for protein synthesis, most of
the excess is converted to glucose or stored as fat.
The amino acid pool and protein turnover
Cells throughout the body constantly and simultaneously synthesize and break
down protein. When cells break down protein, the protein’s amino acids return
to circulation. These available amino acids, found throughout body tissues and
fluids, are collectively referred to as the amino acid pool. Some of these amino
acids may be used for protein synthesis; others may have their amino group
removed and be used to produce energy or non-protein substances such as
glucose.
The constant recycling of proteins in the body is known as protein turnover.
Each day, more amino acids in your body are recycled than are supplied in your
diet. Of the approximately 300 grams of protein synthesized by the body each
day, 200 grams are made from recycled amino acids. This remarkable recycling
capacity is the reason we need little protein in our diet. In a healthful diet, only
10 to 15 percent of our daily calories must come from protein, whereas
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carbohydrate should supply about 55 to 60 percent and fat no more than 30
percent.
Synthesis of non-protein molecules
Amino acids have other roles, not just as components of proteins; they are
precursors of many molecules with important biological roles. Your body
makes non-protein molecules from amino acids and the nitrogen they contain.
The vitamin niacin, for example, is made from the amino acid tryptophan.
Precursors of DNA, RNA, and many coenzymes derive in part from amino
acids. Your body also uses amino acids to make other important compounds,
such as neurotransmitters, norepinephrine and epinephrine and histamine.
Nitrogen balance
The balance of nitrogen, and therefore protein, can be estimated in the body by
comparing nitrogen intake to the sum of all sources of nitrogen excretion (e.g.
urine, feces, skin). If nitrogen intake exceeds nitrogen excretion, the body is
said to be in positive nitrogen balance. Positive nitrogen balance means that the
body is adding protein, as in the case for growing children, pregnant women, or
people recovering from protein deficiency or illnesses. If nitrogen excretion
exceeds nitrogen intake, the body is in negative nitrogen balance. This means
that the body is losing protein. People who are starving or on extreme weightloss diets or who suffer from fever, severe illnesses, or infections are in a state
of negative nitrogen balance. If nitrogen intake equals nitrogen excretion,
nitrogen balance is zero and the body is in nitrogen equilibrium. Healthy adults
are in nitrogen equilibrium, which means their dietary protein intake is adequate
to maintain and repair tissue. They have no net gain or loss of body protein and
they simply excrete excess dietary protein.
Negative nitrogen balance: Nitrogen intake is less than the sum of all sources
of nitrogen excretion.
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Nitrogen balance: Nitrogen intake minus the sum of all sources of nitrogen
excretion.
Nitrogen equilibrium: Nitrogen intake equals the sum of all sources of
nitrogen excretion; nitrogen balance equals zero.
Conclusion: Cells throughout the body constantly synthesize and break down
protein. This process known as protein turnover. Nitrogen-containing end
products of protein metabolism are excreted in urine via the kidneys.
Comparison of nitrogen intake (from dietary protein) to nitrogen excretion gives
a measure of nitrogen balance and indicates protein status in the body.
Protein and nitrogen excretion
Cells break down and recycle amino acids. Amino acid breakdown yields amino
groups (-NH2). -NH2 molecule is unstable and quickly converts to ammonia (NH3). Ammonia is toxic to cells, so it is entered the bloodstream, as a waste
product, and is carried to the liver. In the liver, and amino group and an
ammonia group react with CO2 through a series of reactions (urea cycle) to
generate urea and water. The nitrogen-rich urea is transported from the liver to
the kidneys, where it is filtered from the blood and sent to the bladder for
excretion in urine. Small amounts of other nitrogen-containing compounds,
such as ammonia, uric acid and creatinine, are also excreted in urine. Some
nitrogen is also lost through skin, GI cells, mucus, hair and nail cutting, but in a
very small amounts.
NH2
Urea:
C=O
NH2
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Proteins in the diet
Meat eggs, milk, legumes, grains, and vegetables are all sources of protein.
Fruits contain minimal amounts and, along with fats, are not considered protein
sources.
Adults
For adults, the RDA for protein intake is 0.8 grams per kilogram of body
weight. In clinical situations that require precise assessments, ideal body weight
(rather than actual body weight) is typically used to determine protein needs.
Other life stages
Infants 0 to 6 months of age require 2.2 grams of protein per kilogram body
weight, the highest protein need relative to body weight of any time of life. Both
pregnancy and lactation (production of breast milk) increase a woman’s need
for protein.
Conclusion: Infants, who are growing rapidly, have the highest protein needs
relative to body weight. The Recommended Dietary Allowance (RDA) for
protein declines from 2.2 grams per kilogram for infants 0 to 6 months old to
0.8 grams per kilogram for adults. Pregnancy and lactation can alter protein
requirements.
Protein quality
Although both animal and plant foods contain protein, the quality of protein in
these foods differs. Foods that supply all the essential amino acids are called
complete or high quality protein. A high-quality protein (1) provides all the
essential amino acids in the amounts the body needs, (2) provides enough other
amino acids to serve as nitrogen sources for synthesis of nonessential amino
acids, and (3) is easy to digest. Foods that lack adequate amounts of one or
more essential amino acids are called incomplete, or low-quality protein.
Complete protein: A protein that supplies all of the essential amino acids in the
proportions the body needs.
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Incomplete protein: A protein that lacks one or more essential amino acids in
the proportions needed by the body. Also called low-quality proteins.
Complete proteins
Animal foods generally provide complete protein; that is, they provide all the
essential amino acids in approximately the right proportions.
Red meats, poultry, fish, eggs, milk, and milk products (all animal foods)
contain complete protein. The protein isolated from soybeans also provides a
complete, high-quality protein equal to that of animal protein.
Complementary proteins
With the exception of soy protein, the protein in plant foods is incomplete; that
is, it lacks one or more essential amino acids and does not match the body’s
amino acid needs as closely as animal foods do. Although the protein in one
plant food may lack certain amino acids, the protein in another plant food may
be a complementary protein that completes the amino acid pattern. So the
protein of one plant food can provide the essential amino acid(s) that the other
plant food is missing.
For example, grain products such as pasta are low in the essential amino acid
lysine but high in the essential amino acids methionine and cysteine. Legumes
such as kidney beans are low in methionine and cysteine but high in lysine. In a
dish that combines these foods, such as a pasta-kidney bean salad, the protein
from pasta complements the protein from kidney beans, so together they
provide a complete protein. Generally, when you combine grains with legumes,
or legumes with nuts or seeds, you will get complete, high-quality protein.
Small amounts of animal foods can also complement the protein in plant
foods. Protein complementation is important only for people who consume little
or no animal proteins. For these people, a wide variety of plant protein sources
are the key to obtaining adequate amounts of all the essential amino acids.
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Protein-energy malnutrition
A deficiency of protein, energy, or both in the diet is called proteinenergy malnutrition, or PEM. Protein and energy intake are difficult
to separate because diets adequate in energy usually are adequate in
protein, and diets inadequate in energy inhibit the body’s use of
dietary protein for protein synthesis.
protein-energy malnutrition (PEM): A condition resulting from
long-term inadequate intakes of energy and protein that can lead to
wasting of body tissues and increased susceptibility to infection.
The two famous forms of severe PEM are kwashiorkor and
Marasmus:
Kwashiorkor
The term kwashiorkor is a Ghanian word that describes the “evil spirit which
infects the first child when the second child is born.” In many cultures, babies
are breast-fed until the next baby comes along. When it does, the first baby is
weaned from nutritious breast milk and placed on a watered-down version of
the family’s diet. In areas of poverty this diet is often low in protein, or the
protein is not absorbed easily.
The symptom of kwashiorkor that sets it apart from marasmus is edema, or
swelling of body tissue, usually in the feet and legs. Lack of blood proteins
reduces the force that keeps fluid in the bloodstream, and instead, fluid leaks out
into the tissues. The belly (stomach) can also become bloated from both edema
and accumulation of fat in the liver, since no proteins are available to transport
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the fat. Other features of kwashiorkor include stunted weight and height,
increased susceptibility to infection, dry and flaky skin, and sometimes skin
sores, dry hair, and changes in skin color.
Kwashiorkor usually develops in children between 18 and 24 months of age,
about the time weaning occurs. Its onset can be rapid and is often triggered by
an infection or illness that increases the child’s protein needs. In hospital
settings, kwashiorkor can develop in situations where protein needs are
extremely high (trauma, infection, burns) but dietary intake is poor.
Kwashiorkor often caused by serious acute infection, especially measles, or
abrupt weaning from the breast onto a starchy diet.
Signs of kwashiorkor:
1) Edema.
2) Growth failure and a degree of wasting.
3) Hepatomegaly.
4) Changes to the hair and skin (hair sparse and thin).
5) Poor appetite and mental changes.
6) Anemia.
7) Moon face.
8) Loose stool.
Marasmus
Marasmus is derived from the Greek word marasmos, which means “withering
or to waste away.” Marasmus is more common than kwashiorkor. It develops
more slowly than kwashiorkor and results from chronic PEM. Protein, energy,
and nutrient intake are all inadequate, depleting body fat reserves and severely
wasting muscle tissue, including vital organs like the heart. Growth slows or
stops, and children are both short and very thin for their age. Because muscle
and fat are used up, a child with marasmus often looks like a frail elderly
person.
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Marasmus occurs most often in infants and children 6 to 18 months of age who
are fed diluted or improperly mixed formulas. Because this is a time of rapid
brain growth, marasmus can permanently stunt brain development and lead to
learning disabilities. Marasmus also occurs in adults during cancer and
starvation, including the self-imposed starvation of the eating disorder known as
anorexia nervosa.
Signs of marasmus:
1) Marked muscle wasting.
2) Loss of subcutaneous fat.
3) Growth failure.
4) “Old man’s” face.
5) Loose stool in some children.
- Growth failure: Inadequate gain in weight/or height compared with a wellnourished individual.
- Stunting: Shortness.
- Wasting: Thinness.
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Excess dietary protein
In industrialized countries, an excess of protein and energy is more common
than a deficiency. High protein intake can also cause health problems.
Kidney function
Since the kidneys must excrete the products of protein breakdown, high protein
intake can strain kidney function and is especially harmful for people with
kidney disease or diabetes.
To prevent dehydration, it is important to drink plenty of fluids to dilute the
byproducts of protein breakdown for excretion.
Mineral losses
High protein intake can cause the body to excrete more calcium, contributing to
bone mineral losses and increasing the risk of osteoporosis.
Obesity
High-protein foods are also often high in fat. A diet high in fat and protein may
provide too much energy, contributing to obesity.
Heart disease
Research has linked high intake of animal protein to high blood cholesterol
levels and increased risk of heart disease. Foods high in animal protein,
however, are also high in saturated fat and cholesterol.
Cancer
Some studies suggest a link between a diet high in animal protein foods and an
increased risk for certain types of cancers. Cancer of the colon, breast, pancreas,
and prostate have been linked to high protein and fat intake. As with obesity and
heart disease, however, the independent effects of protein and fat are difficult to
separate.
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