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What are Amino Acids and What Do They Do?
Amino acids are the chemical units or “building blocks,” as they are popularly called, that make up proteins. They also are the end products of protein digestion, or hydrolysis. Amino acids contain about 16 percent nitrogen. Chemically, this is what distinguishes them from the other two basic nutrients, sugars and fatty acids, which do not contain nitrogen.
To understand how vital amino acids are, you must understand how essential proteins are to life. It is protein that provides the structure for all living things. Every living organism, from the largest animal to the tiniest microbe, is composed of protein. And in its various forms, protein participates in the vital chemical processes that sustain life.
Proteins are a necessary part of every living cell in the body. Next to water, protein makes up the greatest portion of our body weight. In the human body, protein substances make up the muscles, ligaments, tendons, organs, glands, nails, hair, and many vital body fluids, and are essential for the growth of bones. The enzymes and hormones that catalyze and regulate all bodily processes are proteins. Proteins help to regulate the body’s water balance and maintain the proper internal pH. They assist in the exchange of nutrients between the intercellular fluids and the tissues, blood, and lymph. A deficiency of protein can upset the body’s fluid balance, causing edema. Proteins form the structural basis of chromosomes, through which genetic information is passed from parents to offspring. The genetic “code” contained in each cell’s DNA is actually information for how to make that cell’s proteins.
Proteins are chains of amino acids linked together by what are called peptide bonds. Each individual type of protein is composed of a specific group of amino acids in a specific chemical arrangement. It is the particular amino acids present and the way in which they are linked together in sequence that gives the proteins that make up the various tissues their unique functions and characters. Each protein in the body is tailored for a specific need; proteins are not interchangeable.
The proteins that make up the human body are not obtained directly from the diet. Rather, dietary protein is broken down into its constituent amino acids, which the body then uses to build the specific proteins it needs. Thus, it is the amino acids rather than protein that are the essential nutrients.
In addition to those that combine to form the body’s proteins, there are other amino acids that are important in metabolic functions. Some, such as citrulline, glutathione, ornithine, and taurine, can be similar to (or byproducts of) the protein-building amino acids. Some act as neurotransmitters or as precursors of neurotransmitters, the chemicals that carry information from one nerve cell to another. Certain amino acids are thus necessary for the brain to receive and send messages. Unlike many other substances, neurotransmitters are able to pass through the blood-brain barrier. This is a kind of defensive shield designed to protect the brain from toxins and foreign invaders that may be circulating in the bloodstream. The endothelial cells that make up the walls of the capillaries in the brain are much more tightly meshed together than are those of capillaries elsewhere in the body. This prevents many substances, especially water-based substances, from diffusing through the capillary walls into brain tissue. Because certain amino acids can pass through this barrier, they can be used by the brain to communicate with nerve cells elsewhere in the body.
Amino acids also enable vitamins and minerals to perform their jobs properly. Even if vitamins and minerals are absorbed and assimilated by the body, they cannot be effective unless the necessary amino acids are present. For example, low levels of the amino acid tyrosine may lead to iron deficiency. Deficiency and /or impaired metabolism of the amino acids methionine and taurine has been linked to allergies and autoimmune disorders. Many elderly people suffer from depression or neurological problems that may be associated with deficiencies of the amino acids tyrosine, tryptophan, phenylalanine, and histidine, and also of the branched-chain amino acids—valine, isoleucine, and leucine. These are amino acids that can be used to provide energy directly to muscle tissue. High doses of branched-chain amino acids have been used in hospitals to treat people suffering from trauma and infection. Some people are born with an inability to metabolize the branched-chain amino acids. This potentially life-threatening condition, branched-chain ketoaciduria (often referred to as maple syrup urine disease because keto acids released into the urine cause it to smell like maple syrup) can result in neurological damage and necessitates a special diet, including a synthetic infant formula that does not contain leucine, isoleucine, or valine.
There are approximately twenty-eight commonly known amino acids that are combined in various ways to create the hundreds of different types of proteins present in all living things. In the human body, the liver produces about 80 percent of the amino acids needed. The remaining
20 percent must be obtained from the diet. These are called the essential amino acids. The essential amino acids that must enter the body through diet are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The nonessential amino acids, which can be manufactured in the body from other amino acids obtained from dietary sources, include alanine, arginine, asparagine, aspartic acid, citrulline, cysteine, cystine, gamma-aminobutyric acid, glutamic acid, glutamine, glycine, ornithine, proline, serine, taurine, and tyrosine. The fact that they are termed “nonessential” does not mean that they are not necessary, only that they need not be obtained through the diet because the body can manufacture them as needed. And nonessential amino acids can become “essential” under certain conditions. For instance, the nonessential amino acids cysteine and tyrosine are made from the essential amino acids methionine and phenylalanine. If methionine and phenylalanine are not available in sufficient quantities, cysteine and tyrosine then become essential in the diet.
The processes of assembling amino acids to make proteins, and of breaking down proteins into individual amino acids for the body’s use, are continuous ones. When we need more enzyme proteins, the body produces more enzyme proteins; when we need more cells, the body produces more proteins for cells. These different types of proteins are produced as the need arises. Should the body become depleted of its reserves of any of the essential amino acids, it would not be able to produce the proteins that require those amino acids. An inadequate supply of even one essential amino acid can hinder the synthesis, and reduce body levels, of necessary proteins. This can result in negative nitrogen balance, an unhealthy condition in which the body excretes more nitrogen than it assimilates. Further, all of the essential amino acids must be present simultaneously in the diet in order for the other amino acids to be utilized—otherwise, the body remains in negative nitrogen balance. A lack of vital proteins in the body can cause problems ranging from indigestion to depression to stunted growth.
How could such a situation occur? More easily than you might think. Many factors can contribute to deficiencies of essential amino acids, even if you eat a well-balanced diet that contains enough protein. Impaired absorption, infection, trauma, stress, drug use, age, and imbalances of other nutrients can all affect the availability of essential amino acids in the body. Insufficient intake of vitamins and minerals, especially vitamin C, can interfere with the absorption of amino acids in the lower part of the small intestines. Vitamin B6 is needed also, for the transport of amino acids in the body.
If your diet is not properly balanced—that is, if it fails to supply adequate amounts of the essential amino acids—sooner or later, this will become apparent as some type of physical disorder. This does not mean, however, that eating a diet containing enormous amounts of protein is the answer. In fact, it is unhealthy. Excess protein puts undue stress on the kidneys and the liver, which are faced with processing the waste products of protein metabolism. Nearly half of the amino acids in dietary protein are transformed into glucose by the liver and utilized to provide needed energy to the cells. This process results in a waste product, ammonia. Ammonia is toxic to the body, so the body protects itself by having the liver turn the ammonia into a much less toxic compound, urea, which is then carried through the bloodstream, filtered out by the kidneys, and excreted.
As long as protein intake is not too great and the liver is working properly, ammonia is neutralized almost as soon as it is produced, so it does no harm. However, if there is too much ammonia for the liver to cope with—as a result of too much protein consumption, poor digestion, and /or a defect in liver function—toxic levels may accumulate. Strenuous exercise also tends to promote the accumulation of excess ammonia. This may put a person at risk for serious health problems, including encephalopathy (brain disease) or hepatic coma. Abnormally high levels of urea can also cause problems, including inflamed kidneys and back pain. Therefore, it is not the quantity but the quality of protein in the diet that is important (see DIET AND NUTRITION in Part One).
It is possible to take supplements containing amino acids, both essential and nonessential. For certain disorders, taking supplements of specific amino acids can be very beneficial. When you take a specific amino acid or amino acid combination, it supports the metabolic pathway involved in your particular illness. Vegetarians, especially vegans, would be wise to take a formula containing all of the essential amino acids to ensure that their protein requirements are met.