How Much Protein Should Be In Your Diet?
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 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
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.
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 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 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 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.
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 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.
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 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 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.
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 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 is a nonessential amino acid that is essential for the metabolism of fats as well as fatty acids.
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 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 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 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.]