Vitamin C

Vitamin C—or ascorbic acid—is probably the best-known of all vitamins. It is water-soluble and can neither be made nor stored by the body. Thus, humans must obtain a steady supply of vitamin C from foods in their diet in order to prevent the devastating disease known as scurvy.

Vitamin C has essential roles in the human body, both as a coenzyme or cofactor that is required for the activity of various enzymes and also as a powerful antioxidant that helps protect cells from damage from free radicals and other active oxygen species that are both produced during normal metabolism and that enter the body from the environment. Vitamin C is required as a coenzyme in the production of collagen, which is a major component of the skin, blood vessels, cartilage, bones, teeth, tendons, and ligaments. Thus, vitamin C is necessary for the growth, maintenance, and repair of tissues throughout the body, as well as for wound healing and the formation of scar tissue. Vitamin C is a necessary coenzyme for the production of neurotransmitters that move signals between nerve cells, and it is necessary for metabolizing storage molecules into energy. As an antioxidant, vitamin C helps protect against free radicals that are responsible for aging and are involved in many diseases and conditions, including cancer, heart disease, and arthritis.

Vitamin C

SOURCE: Office of Dietary Supplements. National Institutes of Health. “Vitamin C: Fact Sheet for Health Professionals.” https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional (accessed April 19, 2018).

There are many claims for the benefits of large doses of vitamin C taken as dietary supplements, including prevention of everything from the common cold to cancer and heart disease; however, these claims remain unproven. High doses of vitamin C may be used to treat or prevent urinary tract infections, since high levels of vitamin C increase the acidity of urine, creating an inhospitable environment for bacteria in the urinary tract.

Vitamin C, the essential nutrient that prevents scurvy, was isolated in 1932. By 1934, the pharmaceutical company Hoffman-La Roche had produced a synthetic version of vitamin C. The symptoms of scurvy were described as early as 1550 BCE by the ancient Egyptians. In 1540, the French explorer Jacques Cartier learned from Native Americans to treat scurvy with a hot-water extract of pine needles. Early Spanish explorers planted orange trees in Florida and the Caribbean to provide a source of citrus fruit for their long voyages back to Europe. By the mid-eighteenth century, the Edinburgh physician James Lind had proved that oranges and lemons could prevent and cure scurvy, which was still decimating crews on the long voyages of discovery, and eventually British ships began stocking lemon juice. Nevertheless, between 1500 and 1800, as many as two million sailors died of scurvy, and the disease continued to afflict armies at war, miners of the California Gold Rush, polar explorers, and civilian populations during times of famine.

Although all animals require vitamin C, most can make their own. Only humans, along with apes, guinea pigs, and a few other animals, have lost the ability to make vitamin C. Not only are humans unable to produce vitamin C, it cannot be stored in the body. As a water-soluble vitamin, any amount that is not used immediately is excreted in the urine. Nor is vitamin C 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.

Antioxidants such as vitamin C react with and deactivate free radicals. Free radicals are formed during normal cellular metabolism and during exposure to ultraviolet light and toxins such as cigarette smoke. Free radicals cause damage by oxidizing DNA, as well as fats and proteins in cell membranes. It is the antioxidant properties of vitamin C that are the basis for many of the controversial health claims for the benefits of large vitamin C doses.

The Institute of Medicine (IOM) of the U.S. National Academy of Sciences has developed dietary reference intake (DRI) values for vitamins and minerals. The DRIs consist of three sets of numbers. The recommended dietary allowance (RDA) defines the average daily amount of a 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 AIs, RDAs, and ULs for vitamin C are in milligrams (mg) as follows:

These RDAs and ULs set by the IOM are controversial, since they are based on the amount of vitamin C necessary for preventing scurvy. Some researchers believe that doses many times higher— 400–3,000 mg per day for healthy adults—are needed to prevent certain chronic diseases. Furthermore, it is generally accepted that some people, in addition to pregnant and breastfeeding women, require higher doses of vitamin C. The recommended RDAs are 125 mg for men who smoke and 110 mg for women who smoke. People recovering from surgery, cancer patients, and burn victims may also require higher doses of vitamin C.

People need a continuous supply of vitamin C in their diet. This is not difficult for most Americans, since vitamin C is found in many foods. Citrus fruits and their juices, cantaloupe, kiwi fruit, mangoes, pineapples, raspberries, blueberries, cranberries, and watermelons are rich sources of vitamin C from fruit. Other good fruit sources include:

Among vegetables, Brussels sprouts; green peppers; leafy greens including spinach, cabbage, and turnip greens; and winter squash are rich in vitamin C, as are:

The best sources of vitamin C are fresh, raw fruits and vegetables. Vitamin C is unstable and is lost when food is exposed to air, temperature changes, or 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. Microwaving or steaming foods may preserve more of their vitamin C. Storing foods for long periods also reduces their vitamin C content.

Americans take more vitamin C than any other dietary supplement. As a single-ingredient supplement, vitamin C is available as tablets, capsules, and powders. It is also found in multivitamin and antioxidant supplements. It may be combined with minerals such as calcium to make it less acidic and thus less irritating to the stomach in large doses. Vitamin C can be produced synthetically or derived from corn or palm oil (ascorbyl palmate). There is little evidence that one form is more effective than another. In addition, some cereals and other processed foods and beverages are fortified with vitamin C. Vitamin C is added to some skin creams, throat lozenges, energy drinks, and energy bars. Nevertheless, both the American Cancer Society and the American Heart Association recommend that people meet their vitamin C (and most other vitamin) requirements through a healthy diet that includes a minimum of five servings of fruits and vegetables daily.

As little as 10 mg per day of vitamin C is enough to prevent scurvy, and the disease is rare throughout most of the world. However, poverty can put anyone at risk for scurvy. It is epidemic in some refugee camps and remains a problem among the malnourished and populations that subsist primarily on grains.

In the United States, scurvy primarily affects alcoholics and older, malnourished adults, especially men suffering from “widower scurvy.” Babies between the ages of 6 and 18 months are sometimes affected if they are fed a diet that is deficient in fruits and vegetables or fortified foods. Nevertheless, some researchers believe that a significant sector of the American population may suffer from some degree of vitamin C deficiency. Smoking increases the body's need for vitamin C, but is not, by itself, a cause of scurvy.

Signs and symptoms of vitamin C deficiency include fatigue; easy bruising; excessive bleeding, especially from the nose and gums; tooth loss; hair loss; and swollen, painful joints. Scurvy can also cause anemia; susceptibility to infection; poor wound healing; dry and splitting hair; rough, scaly, and dry skin; and possible weight gain due to slowed metabolism. Scurvy causes malformed bones in children. If left untreated, scurvy is fatal, usually through sudden cardiac arrest.

During the 1970s, the Nobel prize-winning chemist Linus Pauling popularized the idea that large doses (1,000 mg or more) of daily vitamin C could prevent, shorten the duration of, or reduce the severity of symptoms of the common cold. However, a multitude of studies have found little effect of large doses of vitamin C on the frequency or severity of colds. A few small studies have suggested that the amount of vitamin C in a typical multivitamin may ease cold symptoms, but there is little evidence that megadoses either prevent colds or ease symptoms in most people.

Most other claims for the health benefits of large vitamin C doses have also failed to be substantiated in well-designed, well-controlled studies. Studies of the effects of vitamin C supplements on heart disease, cancer, and eye diseases such as cataracts and macular degeneration have not been definitive. Although many studies have demonstrated that diets low in fats and high in fresh fruits and vegetables reduce the risk of heart disease and various cancers, there is little evidence that these benefits are due in large part to the antioxidant effects of vitamin C. Likewise, studies have not shown that supplemental vitamin C reduces blood levels of cholesterol or improves cardiovascular health. Other health claims for vitamin C— including prevention or treatment of lead poisoning, high blood pressure (hypertension), asthma, Alzheimer's disease, attention deficit hyperactivity disorder (ADHD), infertility, stomach ulcers, autism, premature birth, and a multitude of other conditions—remain unproven.

Although Vitamin C can usually be taken in enormous doses without any serious side effects, large amounts of vitamin C can cause indigestion or diarrhea, which disappears when the dose is reduced.

Vitamin C rarely interacts with drugs or other supplements. Large doses increase the amount of iron absorbed from food by the small intestine, although this is not normally harmful. Large daily doses of vitamin C may interfere with vitamin B₁₂ absorption.

Even very large doses of vitamin C rarely cause complications other than possible diarrhea.

Parents generally do not need to be concerned about their children getting either too much or too little vitamin C. Vitamin C is safe for women during pregnancy and breastfeeding and passes into breast milk. Children under age one should get all of their vitamin C from breast milk, fortified formula, or foods. They should not be given supplemental vitamin C.

See also ADHD diet ; AIDS/HIV diet and nutrition ; Alcohol consumption ; Antioxidants ; Artificial preservatives ; Dietary supplements ; Iron ; Oral health and nutrition ; Osteoporosis diet ; Vitamins .