The main components of the human body are proteins, fats, and carbohydrates. While fats and carbohydrates play important roles in fuel production and metabolism, the proteins perform a consortium of important physiological functions. The building blocks of proteins are amino acids (AAs), and the human body must have some 20 in the right amounts to function properly.
The name amino acid describes their composition with an amine group and an acidic group (carboxylic acid) in their structure. Depending on individual chemical structure, AAs can be classified as linear-chain or branched-chain. There are four elements that compose the AAs: carbon, oxygen, hydrogen, and nitrogen. When they combine to form proteins, amino acids join their amine groups with their acid groups to form bonds known as peptide bonds. The creation of peptide bonds by the body is not a simple, direct chemical reaction. Rather, it involves several pathways leading to the formation of the bond. Nevertheless, the end result is a chain of AAs lined up together to form the final structure known as a peptide or protein. If the chain has fewer than 50 AAs, the product is called a peptide; if it has more than 50, it is a protein.
Among the celebrated proteins in the human body is insulin. This important protein is composed of 51 AAs forming two chains held together with bridges containing sulfur known as the disulfide bonds. Insulin is a small protein, but it has a huge job. It regulates how the body utilizes glucose as a fuel for cells, especially muscle cells and brain cells. If the human body does not produce enough insulin, then Type 1 diabetes develops. If it cannot utilize the insulin produced by the pancreas, then Type 2 diabetes develops. Diabetic patients are commonly prescribed insulin to regain control of their glucose utilization. Insulin has normally been given by injection because the digestive system contains enzymes that will degrade insulin given orally. Research is ongoing globally to create an injectionless insulin for Type 1 diabetics (Madhumathi, 2011). Other routes of insulin administration being investigated include an aerosol via the lungs, nasally, or as an oral spray.
Enzymes are also made of amino acids. Enzymes perform a colossal number of activities within the body, including digestion, building, storage, and transportation. Other examples of biological materials that are made of AAs include chemicals involved in brain signaling known as neurotransmitters, the building components of the genetic material known as the nucleotides, and the protein essential for carrying oxygen in the blood known as hemoglobin.
An individual is capable of making the majority of the amino acids needed to sustain life. However, a few are not synthesized by the body and must be supplied by diet. They are labeled essential amino acids (EAAs) because they must be obtained from an outside source. The eight EAAs are isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
The other amino acids, known as nonessential, are alanine, arginine, asparagine, aspartic acid, cysteine, glutamic acid, glutamine, glycine, histidine, ornithine, proline, taurine, selenocysteine, serine, and tyrosine. Five other amino acids are sometimes listed as conditionally essential because they may be essential under special circumstances, such as in the case of infants, children, or those suffering from certain diseases. All amino acids can be easily obtained from a balanced diet, including vegetarian diets. The important point to remember is the word balanced, because some sources may be lacking or may contain only a small amount of the needed AAs. This is especially important with special diets such as vegan and vegetarian diets.
In addition, the amino acids arginine, cysteine, glutamine, leucine, proline, and tryptophan are known as functional AAs. The term functional signifies their role in regulating key metabolic processes important for immunity, reproduction, growth, and overall maintenance. Some food products are considered complete sources of amino acids (such as animal meats and soy products), and others may be incomplete (such as vegetables).
In 1994, the United States Congress passed the Dietary Supplement Health and Education Act. This act classified and distinguished products containing herbs/botanicals, vitamins, minerals, amino acids, enzymes, glandular, organ tissues, metabolites, or a combination as dietary supplements. The supplements became popular for a variety of uses, including fitness training, weight loss, and certain chronic diseases. Although many of these supplements continue to be used by the public as medications to help manage different diseases, the manufacturers cannot legally claim a medical or clinical effect for its content. The U.S. Food and Drug Administration (FDA) only allows a structure/function claim to be placed on the label. For example, a label for the supplement containing the amino acid arginine may state, “Amino acids are the building blocks of protein and help fuel skeletal muscles.”
The FDA also requires the manufacturer to place a statement on the same label to indicate the product is not intended for medical use: “The label claims have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease. Consult your healthcare professional before taking any dietary supplements.” It is extremely important for anyone thinking of using dietary supplements, including products containing amino acids, to consult first with his or her health care provider as many of these supplements can adversely interact with prescription or over-the-counter medications, which can lead to life-threatening conditions. Advertising on television and radio has led to a greater public demand for a number of supplements.
Research studies conducted on the use of amino acids in the treatment of diseases are constantly emerging in medical literature. In the case of arginine, several clinical effects have been suggested for its use. As an amino acid, arginine plays an important role in the cellular synthesis of many compounds, including nitric oxide, polyamines, creatine, and the amino acid ornithine. Being a substrate for nitric oxide synthases, the family of enzymes that produce nitric oxide, arginine consumption suggests a potential benefit from this amino acid in the management of asthma and high blood pressure. Due to the vasodilation potential of arginine use, many patients with erectile dysfunction have promoted this supplement for the purpose of overcoming this condition. Also, clinical studies of arginine oral administration have demonstrated a reduction in the inflammatory markers in the blood of angina pectoris patients, a beneficial effect for heart failure patients.
The essential amino acid lysine has its own therapeutic claims as well. Lysine is found in meat products, beans (including lentils and soybeans), and wheat germ (although wheat itself is a poor source for lysine). This amino acid plays a significant role in the synthesis of carnitine (important for generating energy from fats) and collagen (important for healthy skin, bones, tendons, and cartilages). Clinical studies have shown lysine improves the absorption of calcium from the gastrointestinal tract and at the same time reduces the amount of calcium excreted in urine. Together these two effects may help maintain bone health and prevent osteoporosis. In clinical trials, lysine was shown to be effective in reducing the rate of infection with herpes simplex virus, in particular when the diet was high in lysine and low in arginine. Another possible application of lysine comes from limited observational data that suggest that a high daily dose of lysine (6 g) may help in relieving the painful symptoms of angina pectoris. However, no controlled clinical trials are available to support the pain relief action of lysine in angina pectoris patients.
Another amino acid with a positive effect on health is the essential amino acid tryptophan. It is abundant in meat products, especially in turkey. Tryptophan is widely used for sleep or insomnia and for anxiety and depression. Scientific and medical research on tryptophan supports the anecdotal folklore use of this amino acid, at least to a certain extent. Tryptophan gets converted in the body to serotonin, which is a neurotransmitter that affects mood. In turn, serotonin is converted to melatonin, the sleep hormone that is secreted from the pineal gland. There is an added benefit to tryptophan's mood and sleep normalizing effects: Any portion of tryptophan that gets absorbed from the intestine undergoes conversion to niacin by the liver, and niacin, or vitamin B3, is important in the control of cholesterol and triglycerides in the blood.
The best sources of the amino acid tyrosine are meat products, oat, and wheat. The human body is also capable of converting the amino acid phenylalanine to tyrosine. However, in the case of phenylketonuria, a genetic disease, patients are not able to convert phenylalanine to tyrosine due to a deficient amount of the converting enzyme in their liver. Patients who suffer from this disease are placed on natural protein restriction and given a protein substitute enriched with tyrosine without any added phenylalanine. For these patients, tyrosine is considered an essential amino acid. Tyrosine is also used to make several important neurotransmitters, including dopamine, epinephrine, and norepinephrine, which regulate mood. Low levels of these neurotransmitters cause sadness and irritability.
Exposure to cold temperatures produces a drop in cognitive, physical, and psychomotor functions; tyrosine supplementation was shown to reverse this reduction to normal levels (O'Brien, 2007). Researchers suggest tyrosine overcomes the effect of cold by suppressing the depletion of epinephrine and norepinephrine during exposure to environmental stress. While there is some support for using tyrosine in the management of attention deficit hyperactivity disorder, there are no clinical trials that show a strong enough connection at this time.
Another popular use of protein or amino acid supplements is by professional athletes to enhance their physical performance. For example, use of beta-alanine is growing among athletes because it is the precursor of carnosine, a dipeptide or two amino-acid peptides found in high concentration in the skeletal muscles. Studies have shown that athletes trained in different sports such as sailing, resistance training, heavy-load training programs, cycling, and others have all experienced, to some extent, a degree of enhanced endurance performance while taking mixtures of amino acids (Sharp, 2010). In general, athletes should rely more on a high-quality balanced protein diet to support their exercise training programs rather than supplements.
What does a well balanced protein diet look like? For protein consumption, this translates into 25 to 30 grams of high-quality protein for each meal, the equivalent to two average-size hamburger meat patties. These nutrients along with micronutrients of minerals and vitamins should come from as many natural unprocessed foods as possible.
When Americans try to lose weight, they often adopt different fad diets, some of which advocate the elimination of whole groups of nutrients. For example, the Atkins diet consists of low-carbohydrate and high-protein food choices. The diet became popular after Robert Atkins published Dr. Atkins’ Diet Revolution in 1972. While the Atkins diet can deliver what it promises regarding weight loss for some dieters, it has been plagued by numerous reports of potentially harmful cardiovascular effects. In addition, high-protein unbalanced diets can cause a significant loss in calcium, which promotes bone loss and the development of osteoporosis and can lower citric acid secretion in urine. This can lead to the development of kidney stones.
Amino acid supplements, while considered relatively safe beyond the possible development of osteoporosis and kidney stones, have been shown at higher doses to cause rapid heart rate, anxiety, and restlessness. Nonprotein amino acids from plant sources can result in toxicity if ingested by humans in the diet. Examples of AAs that are not harmful are canavanine from alfalfa and homoarginine from lentil. Neurotoxicity can come from ingesting some of the seeds of the Lathyrus species.
Beyond the naturally occurring amino acids, synthetic AAs also play a growing role in science and medicine. More importantly, biotechnology is now a major player in the production of therapeutic proteins, also known as recombinant proteins, which are produced by the recombinant DNA technology. Human insulin production and its use in the management of diabetes is an example of this progress. The insulin is produced from microorganisms specifically engineered to synthesize it. Following its production, the hormone is extracted, purified, and then packaged into sterile products. The obvious advantage of human insulin is that it is a human form of the hormone. Older forms of insulin were from animal sources. Other new proteins and peptides biotechnology products include Aranesp (Amgen, California), which is used as an erythropoiesis-stimulating protein, and Avonex (Biogen Idec, Massachusetts), which is a form of beta-inteferon employed in the treatment of multiple sclerosis.
See also Atkins, Robert C.; Calcium; Diabetes; Insomnia; Osteoporosis.
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