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Vitamin C, also called ascorbic acid or antiscorbutic vitamin, is a water-soluble organic compound needed to prevent scurvy. Scurvy is marked by beeding gums and bone malformation in children. Humans cannot make or store vitamin C, so they must get a steady supply of it from foods in their diet.
Vitamin C is a powerful antioxidant that helps protect cells from damage. Vitamin C also is needed to make and repair collagen, move fat into cells where it can be converted into energy, and make neurotransmitters. There are also disputed claims that vitamin C, taken in large quantities as a dietary supplement, can prevent
(Illustration by GGS Information Services/Thomson Gale.)
cancer, heart disease, the common cold, cataracts, and many other diseases.
High dose vitamin C may be used to treat or prevent urinary tract infections. High levels of vitamin C increase the acidity of urine, creating an unhospitable environment for bacteria growing in the urinary tract.
Long before people knew what vitamin C was, they understood that eating certain foods, especially citrus fruit, would prevent a severe disease called scurvy. Vitamin C turned out to be the essential health-promoting compound in these foods. This vitamin was isolated in the early 1930s, and by 1934, a synthetic version of vitamin C was produced by the pharmaceutical company Hoffman-La Roche.
All animals need Vitamin C, but most animals can make their own. However, humans, along with apes, guinea pigs, and a few other animals, have lost that ability. In humans, this occurs because of a gene mutation thatcontrolsanenzyme neededtomake vitamin C. As a result, humans are completely dependent on getting enough of the vitamin from foods in their diet. In addition, vitamin C cannot be stored in the body. It is a water-soluble vitamin, and any amount that cannot be used immediately is excreted in urine. Vitamin C is not evenly distributed throughout the body. The adrenal glands, pituitary gland, thymus, retina, brain, spleen, lungs, liver, thyroid, testicles, lymph nodes, kidney, and pancreas all contain much higher levels of vitamin C than are found in circulating blood.
Vitamin C's role in health
Vitamin C functions as an antioxidant and as a coenzyme. Molecules called free radicals are formed during normal cell metabolism and with exposure to ultraviolet light or toxins such as cigarette smoke. Free radicals cause damage by reacting with fats and proteins in cell membranes and genetic material. This process is called oxidation. Antioxidants like vitamin C are compounds that attach themselves to free radicals so that it is impossible for the free radical to react with, or oxidize, other molecules. In this way, antioxidants protect cells from damage. The antioxidant properties of vitamin C are the basis for many of the controversial health claims made for it.
Vitamin C also functions as a coenzyme. Coen-zymes are small molecules that make it possible for metabolic activities to occur in cells. They are needed to break down food into its building-block molecules, build up new molecules from these building blocks, and convert nutrients into energy in cells. Vitamin C functions as a coenzyme in reactions that create collagen. Collagen is a protein that is found in cartilage, ligaments, tendons, bones, skin, and blood vessels. Vitamin C also is required to make the neurotransmitters dopamine, norepinephrine (noradrenaline), and epinephrine (adrenaline). Neurotransmitters are molecules that carry chemical messages from one nerve to another. Epinephrine is also made in the adrenal gland in response to stress. It prepares the body for a fight or flight response. Vitamin C may also be involved in cholesterol metabolism.
Normal vitamin C requirements
The United States Institute of Medicine (IOM) of the National Academy of Sciences has developed values called Dietary Reference Intakes (DRIs) for vitamins and minerals. The DRIs consist of three sets of numbers. The Recommended Dietary Allowance (RDA) defines the average daily amount of the nutrient needed to meet the health needs of 97–98% of the population. The Adequate Intake (AI) is an estimate set when there is not enough information to determine an RDA. The Tolerable Upper Intake Level (UL) is the average maximum amount that can be
taken daily without risking negative side effects. The DRIs are calculated for children, adult men, adult women, pregnant women, and breastfeeding women.
The IOM has not set RDAs for vitamin C in children under one year old because of incomplete scientific information. Instead, it has set AI levels for this age group. RDA2s and ULs for vitamin C are measured in milligrams (mg). The RDAs and ULs set by the IOM are highly controversial. They are set at a level based on preventing scurvy. Many researchers believe that doses hundreds of times higher are needed to prevent certain chronic diseases. They argue that large doses of vitamin C have minimal side effects and that RDAs and ULs should be much higher. These researchers suggest of anywhere from 400-3,000 mg per day for health adults.
The following lit gives the daily RDAs and IAs and ULs for vitamin C for healthy individuals as established by the IOM.
Vitamin C is the most commonly taken dietary supplement taken by Americans. As a single-ingredient supplement, it is available as tablets, capsules, and powder. It is found in multivitamin and antiox-idant supplements. It is also combined with minerals such as calcium (e.g. Ester-C) to make it less acidic and thus less irritating to the stomach in large doses. Vitamin C can be made synthetically or derived from corn or palm oil (ascorbyl palmate). There is little evidence that one form is more effective than another. Vitamin C is added to some skin creams, throat lozenges, energy drinks, and energy bars, and to some processed foods. In 2007, the two largest American soft drink manufacturers announced that they were going to produce carbonated drinks fortified with vitamins and minerals, including vitamin C.
Vitamin C deficiency produces a disease called scurvy. From the earliest times, scurvy was a problem for sailors on long voyages where there was no way to store fresh fruits and vegetables. In 1746, a doctor in the British navy proved that eating lemons and oranges could prevent scurvy among sailors. Early Spanish explorers planted orange trees in Florida and the Caribbean so that they would have a source of oranges to prevent scurvy on their long voyages back to Europe. Today scurvy occurs infrequently. As little as 10 mg per day of vitamin C can prevent the disease. People with alcoholism, elderly individuals on extremely restricted diets, and malnourished infants in developing countries are at higher risk for developing scurvy. Symptoms include fatigue, easy bruising, excessive bleeding, hair loss, sore gums, tooth loss, and joint pain. Left untreated, death can occur, usually through sudden cardiac attack. Smoking increases the body's need for vitamin C, but is not, by itself, a cause of scurvy.
Sources of vitamin C
People need a continuous supply of vitamin C from their diet because of the role it plays in many metabolic processes. Vitamin C is found in many foods. Good natural sources of vitamin C include citrus fruits and their juices, papaya, red bell peppers, broccoli, and tomatoes.
Vitamin C is unstable and is lost when food is exposed to air, temperature changes, and water. About one-quarter of the vitamin C content of vegetables is lost by brief boiling, steaming, or freezing and thawing. Canning fruits and vegetables reduces their vitamin C content by about one-third, as does longer cooking at higher temperatures. However, both the American Cancer Society and the American Heart Association recommend that people meet their vitamin C (and many other vitamin requirements) through a healthy diet that includes eating a minimum of 5 servings of fruits and vegetables daily.
The following list gives the approximate vitamin C content for some common foods:
Controversial health claims for vitamin C
Controversy about vitamin C centers on its usefulness in preventing or treating disease when taken in very large quantities as a dietary supplement. Most of these claims have not been substantiated by well-designed, well-controlled studies. Many are still being investigated in government-sponsored clinical trials. Individuals interested in participating in a clinical trial at no charge can find a list of open trials at http://www.clinicaltrials.gov.
COLDS. Nobel prize-winning chemist Linus Pauling popularized the idea that large doses (1,000 mg or more) of vitamin C daily, will prevent, shorten the duration, or reduce the severity of symptoms of the common cold. More than 30 trials have compared colds in people taking up to 2,000 mg of vitamin C daily and those taking a placebo (pill with no nutritional value). These studies found no difference in the number or severity of colds in the two groups, with one exception. Skiers, marathon runners, and soldiers training in Arctic conditions who took vitamin C supplements had 50% fewer colds than people who took no extra vitamin C. All the people who benefited from taking vitamin C supplements were putting their bodies under extreme stress. It appears that for elite athletes and others under physical stress, dietary supplements of vitamin C may be of value in preventing colds.
CARDIOVASCULAR HEALTH. Because vitamin C is involved in the production of collagen in blood vessels, researchers have examined the relationship between vitamin C intake and cardiovascular health. Some studies found no benefit to vitamin C supplementation, while others reported that a relatively low dose of vitamin C reduced the risk of death from strokes. Vitamin C does not reduce blood levels of cholesterol. The American Heart Association recommends that to improve cardiovascular health individuals should increase their intake of vitamin C (and other vitamins and mineral) by increasing the amount of fresh vegetables in their diet. Research continues in this area.
CATARACTS. Cataracts are the leading cause of vision impairment worldwide. They develop, usually in older individuals, because of changes in the proteins in the lens of the eye. Initial studies suggested that vitamin C could prevent these changes because of its antioxidant properties. A recent a 7-year follow-up study found vitamin C supplements to be of no benefit in preventing cataracts.
OTHER HEALTH CLAIMS. Claims have been made that vitamin C can treat or prevent lead poisoning, high blood pressure (hypertension), asthma, Alzheimer's disease, attention deficit hyperactivity disorder (ADHD), infertility, macular degeneration, premature birth, stomach ulcers, autism, and many other diseases and disorders. None of these health claims have been proved to the satisfaction of practitioners of conventional medicine.
People who smoke cigarettes need more vitamin C than those who do not. People with cancer also seem to need more vitamin C.
Large doses of vitamin C as a dietary supplement may cause indigestion or diarrhea that stops when the dose is reduced.
Vitamin C has few interactions with drugs or other vitamins. Large doses of vitamin C increase the amount of iron absorbed from food in the small intestine. In healthy people, this does not cause any problems and may be beneficial.
Large daily doses of vitamin C may interfere with the absorption of vitamin B12.
Vitamin C can be taken in enormous doses without any serious side effects. At very high doses, it causes diarrhea. Some researchers who believe that large doses of vitamin C prevent disease think that the appropriate daily dose is an amount just slightly less than the amount that causes diarrhea. This amount varies considerably form person to person.
Generally, parents should have few concerns about children getting either too much or too little Vitamin C. Vitamin C is safe for women to take during pregnancy and while breastfeeding. It passes into breast milk. Children under age one should not be given a dietary supplement containing vitamin C; their needs should be met through the foods the eat.
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