ESSENTIAL AMINO ACIDS
An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized de novo (from scratch) by the organism, and thus must be supplied in its diet. The nine amino acids humans cannot synthesize are phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine.
Histidine is an α-amino acid that is used in the biosynthesis of proteins. Initially thought essential only for infants, longer-term studies have shown it is essential for adults also.
Isoleucine is an α-amino acid that is used in the biosynthesis of proteins. It is essential in humans, meaning the body cannot synthesize it, and must be ingested in our diet.
Leucine is an α-amino acid used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH+3 form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO− form under biological conditions), and an isobutyl side chain, classifying it as a nonpolar (at physiological pH) amino acid.
Lysine is an α-amino acid that is used in the biosynthesis of proteins. Lysine is a base, as are arginine and histidine. The ε-amino group often participates in hydrogen bonding and as a general base in catalysis.
Methionine is an essential amino acid in humans. Together with cysteine, methionine is one of two sulfur-containing proteinogenic amino acids. Excluding the few exceptions where methionine may act as a redox sensor, methionine residues do not have a catalytic role.
Phenylalanine is an α-amino acid with the formula C9H11NO2. This essential amino acid is classified as neutral, and nonpolar because of the inert and hydrophobic nature of the benzyl side chain. The L-isomer is used to biochemically form proteins, coded for by DNA. The codons for L-phenylalanine are UUU and UUC. Phenylalanine is a precursor for tyrosine; the monoamine neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline); and the skin pigment melanin.
Threonine is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated −NH+3 form under biological conditions), an α-carboxylic acid group (which is in the deprotonated −COO− form under biological conditions), and an alcohol containing side chain, classifying it as a polar, uncharged (at physiological pH) amino acid. It is essential in humans, meaning the body cannot synthesize it, and must be ingested in our diet.
Tryptophan is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group, an α-carboxylic acid group, and a side chain indole, classifying it as a non-polar, aromatic amino acid. It is essential in humans, meaning the body cannot synthesize it and thus it must be obtained from the diet. Tryptophan is also a precursor to the neurotransmitter serotonin and the hormone melatonin.
Valine is an α-amino acid that is used in the biosynthesis of proteins. It is essential in humans, meaning the body cannot synthesize it and thus it must be obtained from the diet. Human dietary sources are any proteinaceous foods such as meats, dairy products, soy products, beans and legumes. Along with leucine and isoleucine, valine is a branched-chain amino acid.
Six other amino acids are considered conditionally essential in the human diet, meaning their synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress. These six are arginine, cysteine, glycine, glutamine, proline, and tyrosine. Five amino acids are dispensable in humans, meaning they can be synthesized in the body. These five are alanine, aspartic acid, asparagine, glutamic acid and serine.
Essential amino acids are useful components in a variety of metabolic processes. Even though, some roles can be highlighted as a function of an amino acid, it is important to be aware that they are part of complex pathways and biological systems. The function and use of an amino acid is dependent on other amino acids, mineral elements, carbohydrate and fatty acids and has indirect effects that are manifested in myriad metabolisms. Below are the functions of the essential amino acids in brief.
One of the most important characteristics of Histidine is that it can be converted into various substances, including histamine, glutamate and hemoglobin.
Necessary for the synthesis of neurotransmitter serotonin and the hormone melatonin. It helps relieve migraine and depression.
Essential for muscle development. Side effects of high levels of valine in the body include hallucinations.
One of the main functions of threonine is that it helps to maintain the proper protein balance in the body.
Necessary for the synthesis of hemoglobin, major constituent of red blood cells. It is one of three branched chain amino acids that help with muscle recovery.
Beneficial for muscle, skin, bone and tissue wound healing. It promotes growth hormone synthesis.
Component of muscle protein, and is needed in the synthesis of enzymes and hormones. It is also a precursor for L-carathine which is essential for healthy nervous system function.
Methionine is important in angiogenesis, the growth of new blood vessels, and supplementation may benefit those suffering from Parkinson's, drug withdrawal, schizophrenia, copper poisoning, asthma, allergies, alcoholism, or depression. It is also an antioxidant. It helps in breakdown of fats and aids in reducing muscle degeneration. It is also good for healthy skin and nail.
Beneficial for healthy nervous system. It boosts memory and learning. It may be useful against depression and suppressing appetite.
High protein foods are a good source of essential amino acids.
Various attempts have been made to express the "quality" or "value" of various kinds of protein. Measures include the biological value, net protein utilization, protein efficiency ratio, protein digestibility-corrected amino acid score and complete proteins concept. These concepts are important in the livestock industry, because the relative lack of one or more of the essential amino acids in animal feeds would have a limiting effect on growth and thus on feed conversion ratio. Thus, various feedstuffs may be fed in combination to increase net protein utilization, or a supplement of an individual amino acid (methionine, lysine, threonine, or tryptophan) can be added to the feed.
Although proteins from plant sources tend to have a relatively lower concentrations of protein by mass in comparison to protein from eggs or milk, they are nevertheless "complete" in that they contain at least trace amounts of all of the amino acids that are essential in human nutrition. Eating various plant foods in combination can provide a protein of higher biological value. Certain native combinations of foods, such as corn and beans, soybeans and rice, or red beans and rice, contain the essential amino acids necessary for humans in adequate amounts.
Additionally, certain types of algae and marine life have both chlorophyll as do plants, and also all the essential amino acids, as do animal proteins.
If one of the essential amino acids is less than needed for an individual the utilization of other amino acids will be hindered and thus protein synthesis will be less than what it usually is, even in the presence of adequate total nitrogen intake.
Protein deficiency has been shown to affect all of the body's organs and many of its systems, including the brain and brain function of infants and young children; the immune system, thus elevating risk of infection; gut mucosal function and permeability, which affects absorption and vulnerability to systemic disease; and kidney function. The physical signs of protein deficiency include edema, failure to thrive in infants and children, poor musculature, dull skin, and thin and fragile hair. Biochemical changes reflecting protein deficiency include low serum albumin and low serum transferrin.
The amino acids that are essential in the human diet were established in a series of experiments led by William Cumming Rose. The experiments involved elemental diets to healthy male graduate students. These diets consisted of cornstarch, sucrose, butterfat without protein, corn oil, inorganic salts, the known vitamins, a large brown "candy" made of liver extract flavored with peppermint oil (to supply any unknown vitamins), and mixtures of highly purified individual amino acids. The main outcome measure was nitrogen balance. Rose noted that the symptoms of nervousness, exhaustion, and dizziness were encountered to a greater or lesser extent whenever human subjects were deprived of an essential amino acid.
Essential amino acid deficiency should be distinguished from protein-energy malnutrition, which can manifest as marasmus or kwashiorkor. Kwashiorkor was once attributed to pure protein deficiency in individuals who were consuming enough calories ("sugar baby syndrome"). However, this theory has been challenged by the finding that there is no difference in the diets of children developing marasmus as opposed to kwashiorkor. Still, for instance in Dietary Reference Intakes (DRI), lack of one or more of the essential amino acids is described as protein-energy malnutrition.
When a high-protein diet is the source of excess amino acids, it makes the kidneys work hard to maintain balance.
Amino acids obtained through your diet are usually safe, but following a very high-protein diet for a long time might cause a problem. Side effects aren’t common when taking amino acid supplements, but several individual amino acids carry a higher risk. If you’re pregnant, take medications or you’ve been diagnosed with diabetes, kidney disease or liver disease, talk to your doctor to be sure supplements are safe for you.
The body has the ability to separate amino acids into their component parts. This breakdown naturally occurs when one part -- the amino group -- is needed to make new amino acids. Another part of the amino acid can be converted into glucose and used for energy.
The amino group contains nitrogen, which exists in the form of ammonia after it's cleaved from the original amino acid. This toxic ammonia must be removed from the body, so it binds with another amino acid, then goes to the liver. The liver turns it into urea and sends it to the kidneys for elimination in urine.
Impact on Kidneys
The kidneys help regulate the body's acid-base balance. Some amino acids are acidic, while others are basic, but dietary protein exerts an overall acidic effect. When a high-protein diet is the source of excess amino acids, it makes the kidneys work hard to maintain balance.
If you get too many amino acids, they're broken down and eliminated because the body doesn't store them for future use. Then levels of ammonia and urea increase, which forces the kidneys to filter more blood. For this reason, people with chronic kidney disease usually follow a low-protein diet to alleviate stress on the kidneys.
Estimating the daily requirement for the indispensable amino acids has proven to be difficult; these numbers have undergone considerable revision over the last 20 years. The following table lists the WHO and United States recommended daily amounts currently in use for essential amino acids in adult humans, together with their standard one-letter abbreviations. Food sources are identified based on the USDA National Nutrient Database Release.
The recommended daily intakes for children aged three years and older is 10% to 20% higher than adult levels and those for infants can be as much as 150% higher in the first year of life. Cysteine (or sulphur-containing amino acids), tyrosine (or aromatic amino acids), and arginine are always required by infants and growing children.