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How Much Protein Should I Eat?
Protein is required for the growth and development of
all animals including human beings. Body proteins serve many functions,
they include structural components of cells and tissues, enzyme
catalysts of biochemical reactions, hormone messengers, and components
of the immune system. The RDA for adult men and women is 0.8 g/kg of
body weight per day.
There is little evidence that muscular activity increases the need for
protein, except for the small amount required for the development of
muscles during physical conditioning. [Torun, B., Scrimshaw, N.S., and
Young, V.R., Effects of isometric exercises on body potassium and
dietary protein requirements of young men , American Journal of Clin
Nutr, 30, 1977]
Proteins are hydrolyzed by digestive enzymes to peptides and amino acids
which are absorbed and transported by the blood to various body tissues.
Tissue proteins are continually being broken down and resynthesized,
ingested amino acids being incorporated and those already present being
eliminated. Certain amino acids necessary for growth and maintenance
must be included in the diet, but others can be synthesized from
non-protein precursors.
Protein is formed from various combinations of aminoacids, of which
there are twenty two known. Plants can synthesize all of their amino
acids from carbon, hydrogen, nitrogen and oxygen, but humans lack the
ability to synthesize eight of them and must obtain them from their
diet. These are called essential amino acids and include isoleucine,
leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and
valine. Histidine is considered essential only for infants and children.
The nitrogen present in amino acids is eliminated in the urine in the
form of urea. Enzymes in the liver oxidize amino acids to ketoacids and
ammonia which is detoxified by conversion with carbon dioxide to urea.
Protein is found in many foods ranging from vegetables to animal
products. Many beans (eg. Soya) contain equal or more protein than beef.
All legumes, nuts and seeds have vast quantities of protein. By eating a
variety and combining these plant products your Recommended Dietary
Allowances (RDA's) for all essential amino acids will be met.
Contrary to common misbelief, all amino acids are obtainable from plant
sources. The plant foods that are high in protein also have vast
quantities of other essential vitamins and minerals.
Although protein is necessary for growth and the functioning of the
body, it is often over estimated how much protein a person requires. For
best performance 10% to 20% of your calories consumed should come from
protein. Protein from meat tends to behigh in fat, so meat consumption
should be reduced in favour of fish, grains, beans and other legumes.
In the average Western Diet the most common sources of protein are
animal products, (hamburgers, bacon, eggs and dairy products), and in
addition to supplying the consumer with an abundance of protein these
foods contain high quantities of saturated fats and cholesterol. Thus
the cause of most deaths, in the USA, is Coronary Heart disease.
An excess of protein can also cause the formation of toxic substances
and become a burden to our digestive system. There has been much
research done showing the negative and dangerous results of diets with
too high a protein content. It has been deemed prudent to maintain an
upper bound of no more than twice the RDA for protein.
A protein deficiency is unlikely to occur as an isolated condition. It
is usually only associated with severe malnutrition or kwashiorkor in
Third World populations. Where protein intake is exceptionally low,
there are physical signs - stunting, poor musculature, edema, thin and
fragile hair, and skin lesions.
Amino Acids
Amino acids are the molecular units that make up proteins. All proteins
are various compositions of twenty two specific naturally occurring
amino acids.
Table 1.1 Amino Acids
Non-essential
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Essential
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Alanine
Arginine
Asparagine
Aspartic Acid
Cysteine
Cystine
Glutamic Acid
Glutamine
Glycine
Ornithine
Proline
Serine
Tyrosine |
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine |
Alanine:
Alanine is synthesized in your muscle tissue from
branched chain amino acids. It helps regulate your blood
sugar levels and chronic deficiencies may lead to muscle
loss and poor glucose tolerance.
Arginine:
This amino acid is necessary for the normal functioning
of the pituitary gland. Together with ornithine,
phenylalanine, and other neuro chemicals, arginine is
required for the synthesis and release of the pituitary
gland's growth hormone.
The need for arginine is especially important for males,
since seminal fluids contain much of this amino acid.
The best natural sources are nuts, popcorn, brown rice,
oatmeal, raisins, sunflower and sesame seeds, and whole
wheat bread.
Arginine is necessary for adults because after the age
of thirty there is almost a complete cessation of its
secretion from the pituitary gland.
Any physical trauma increases your need for arginine.
Aspartic acid:
Aspartic acid helps in the expulsion of harmful ammonia
from the body. When ammonia enters the circulatory
system it acts as a highly toxic substance. By disposing
of ammonia, aspartic acid helps protect the central
nervous system.
Cysteine and Cystine:
Cystine is the stable form of the sulfur-containing
amino acid cysteine. The body readily converts one into
the other as needed, and the two forms can be considered
as a single amino acid in metabolism. When cystine is
metabolized it yields sulfuric acid which acts with
other substances to help detoxify the system.
Glutamic acid and Glutamine:
Glutamic acid is primarily used by the brain. It has the
ability to pick up excess ammonia, which inhibits brain
functioning, and convert it into glutamine. Since
glutamine produces an elevation of glutamic acid, a
shortage in the diet can result in a shortage of
glutamic acid in the brain.
Glutamine has also been shown to help in the control of
alcoholism, shorten the healing time of ulcers and
alleviate fatigue, depression, and impotence. It has
also been used successfully in the treatment of
schizophrenia and senility.
Glycine:
Glycine has been found to be helpful in the treatment of
low pituitary gland function and, because it supplies
the body with additional creatine, it has also been
found effective in the treatment of progressive muscular
dystrophy.
It is also used for the treatment of hypoglycemia.
Glycine stimulates the release of glucagon, which
mobilizes glycogen, which is then released into the
blood as glucose.
Histidine:
Histidine is an essential amino acid during infancy, and
its synthetic pathways in older children and adults are
poorly understood. The importance of the amino acid
histidine lies in the fact that the body uses it to
manufacture histamine, and histamine is responsible for
a wide range of physiological processes.
Isoleucine
It is essential in human nutrition. Isoleucine is found
in especially high amounts in meats, fish, cheese, most
seeds and nuts, eggs, chickens and lentils. In the human
body Isoleucine is concentrated in the muscle tissues.
Isoleucine is necessary for hemoglobin formation and in
stabilizing and regulating blood sugar and energy
levels. A deficiency of isoleucine can produce symptoms
similar to those of hypoglycemia
Leucine
Leucine is an essential amino acid, needed in the diet;
the human body cannot synthesize it from simpler
metabolites. Young adults need about 31 mg of this amino
acid per day per kilogram (14 mg per lb) of body weight.
Lysine:
This essential amino acid is vital in the makeup of
critical body proteins. It's needed for growth, tissue
repair, and the production of antibodies, hormones, and
enzymes.
It promotes better concentration and properly utilizes
fatty acids needed for energy production.
The best natural sources are fish, milk, lima beans,
cheese, eggs, and soy products.
Methionine:
Methionine helps in some cases of schizophrenia by
lowering the blood level of histamine, which can cause
the brain to relay wrong messages. It also helps remove
toxic wastes from your liver, assists in the
regeneration of liver and kidney tissue, infuences hair
follicle health, and can be an effective antistress
factor.
A deficiency of methionine can inhibit the body's
ability to process urine and result in edema and
susceptibility to infection. A methionine deficiency has
also been linked to cholesterol deposits,
atherosclerosis, and hair loss in laboratory animals.
Phenylalanine:
Phenylalanine is an essential amino acid that is a neuro-transmitter
(a chemical that transmits signals between the nerve
cells and the brain).
The best natural sources are soy products, cottage
cheese, dry skim milk, almonds, peanuts, lima beans,
pumpkin, and sesame seeds.
DL-Phenylalanine (DLPA):
This form of the essential amino acid phenylalanine is a
mixture of equal parts of D (synthetic) and L (natural)
phenylalanine. By producing and activating endorphins it
intensifies and prolongs the body's own natural
pain-killing response to injury and disease.
Certain enzyme systems continually destroy endorphins,
but DL-phenylalanine effectively inhibits these enzymes,
allowing the pain-killing endorphins to work.
People who suffer from chronic pain have lower levels of
endorphin activity in their blood and cerebro-spinal
fluid. Since DLPA can restore normal endorphin levels,
it can assist the body in reducing pain naturally. It is
effective as a natural pain-killer for conditions such
as osteoarthritis, rheumatoid arthritis, lower back
pain, migraines, leg and muscle cramps, postoperative
pain, and neuralgia.
Proline
Proline contains no primary but a secondary amino group
and is therefore actually an alpha-imino acid, but it is
nevertheless referred to as an amino acid. It is the one
exception to the general structure of amino acids.
This is a nonessential amino acid that is synthesized by
the body from the amino acids glutamine or ornithine and
is involved in the body's production of glutamic acid.
In foods, it is found readily in dairy products and
eggs, with some found in meats or wheat germ.
Serine
Serine is a nonessential amino acid that is essential
for the metabolism of fats as well as fatty acids.
Threonine:
Excessive use of threonine can cause the formation of
too much urea and consequently ammonia toxicity in your
body. To be used effectively, threonine requires vitamin
B6, magnesium, and niacin. Both serine and glycine can
be synthesized from this amino acid.
Tryptophan:
Tryptophan is an essential amino acid that's used by the
brain along with vitamin B6, niacin, and magnesium to
produce serotin, a neurotransmitter that carries
messages between the brain and one of the body's
biochemical mechanisms of sleep. It acts as an
antidepressant reducing anxiety and tension.
The best natural sources are cheese, milk, fish turkey,
bananas, dried dates, and peanuts.
Tyrosine:
Tyrosine is a neuro-transmitter and is important because
of its role in stimulating and modifying brain activity.
For instance, in order for phenylalanine to be effective
as a mood elevator and appetite depressant, it must
first be converted into tyrosine.
Clinical studies have shown that tyrosine
supplementation has helped control medication-resistant
depression and anxiety, as well as enable patients
taking amphetamines (as mood elevators or diet drugs) to
reduce their dosages to minimal levels in a matter of
weeks.
Valine
Valine is a glycogenic amino that promtes mental vigor,
muscle coordination and emotional calm. It is essential
for the prevention of nervous and digestive disorders.
Valine has a stimulating effect on the body and is used
by body builders for muscle growth, tissue repair, and
energy.
The Origin of Amino Acids, Proteins and Life
In recent years there has been considerable speculation,
accompanied by some experimentation, regarding the
events which may have occured about 4 billion years ago
that led to the appearance of the first living cell on
earth. The subject is sometimes called "chemical
evolution," the chemical precursor of Darwinian
evolution.
It is now generally agreed that the primitive earth's
atmosphere was hydrogen-dominated, or reducing. The
elements carbon, nitrogen, and oxygen were present in
their reduced forms (methane, ammonia, water). In 1953
S. L. Miller, working in Professor H. C. Urey's
laboratory, subjected a mixture of methane, ammonia,
water vapour, and hydrogen to an electric discharge and
demonstrated that the amino acids glycine, alanine, and
aspartic acid were present in the complex mixture of
reaction products.
In subsequent experiments, other forms of high energy
radiation such as ultraviolet light and high
temperatures have been studied. Other gases such as
carbon dioxide, formaldehyde, and hydrogen cyanide have
been tried and most common amino acids have been
detected among the reaction products. Thus, the
possibility (not necessarily the actuality) of amino
acid synthesis from the earth's primordial atmosphere
has been established experimentally. [Hart. H., and
Schuetz. R.D., Organic Chemistry , 4th Edition, P.442,
1972.]
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