Vitamins

Vitamins are organic compounds found in plants and animals that are necessary in small quantities for life and health. Thirteen different vitamins have been identified as necessary for humans. The body can make small quantities of two of these vitamins, vitamins D and K. All other vitamins must be obtained either from food or from dietary supplements.

Each of the 13 vitamins has specific functions, and taken together vitamins play a role in almost every function in the body. They help convert food to energy and are involved in processes as diverse as blood clotting, vision, reproduction, and transmission of nerve impulses.

For centuries before vitamins were formally discovered, people knew that eating certain foods prevented certain diseases. For example, the ancient Egyptians knew that eating liver (later shown to be high in vitamin A) prevented night blindness. Sailors on long voyages often developed a serious disease called scurvy. James Lind, a Scottish surgeon who sailed with the British navy, conducted the first controlled experiment on vitamins in 1753. He supplemented the regular diet of four groups of sailors with four different foods. The group that received oranges and lemons as supplements did not develop scurvy, while the other three groups did. Although Lind did not know why citrus fruit was essential to health (it is high in vitamin C, and scurvy is caused by vitamin C deficiency), he recognized that it contained some substance that the sailors needed.

Essential vitamins

Humans need four fat-soluble vitamins. Unlike water-soluble vitamins, fat-soluble vitamins can be stored in the body. High levels of these vitamins can cause health problems. Below is a list of the water-soluble vitamins and a very brief description of their importance to health. In general, the fat-soluble vitamins have antioxidant activity that helps protect cells from damage. Fat-soluble vitamins include:

Before the twentieth century, all vitamins had to come from food. Often individuals on limited diets with little variety developed vitamin deficiency diseases. The period from the 1920s to the 1940s was a time of active research on vitamins. Out of this research came a food fortification program in the United States that continues today. Beginning in the late 1930s, the addition of vitamins to common foods such as flour, milk, and breakfast cereal substantially reduced vitamin deficiency diseases. Commercially manufactured vitamin supplements also began to appear, and taking a daily multivitamin supplement became popular. By 2007, more than 100 million Americans regularly took some form of vitamin supplement.

Vitamin supplements come as tablets, capsules, and elixirs (liquids). Supplements can contain a single vitamin, a group of related vitamins that work together in the body (e.g., B-complex vitamins), or a mixture of vitamins and minerals (e.g., vitamin D and calcium that work together to build bones). Vitamins are also added to foods that can then be labeled “fortified” or “enriched.” Many so-called functional foods, or nutra-ceuticals, have added vitamins, minerals, and herbs.

In the United States, the Food and Drug Administration (FDA) regulates dietary supplements under the 1994 Dietary Supplement Health and Education Act (DSHEA). Under DSHEA, supplements are subject to the same regulation as food, which is much less rigorous than the regulation of prescription or over-the-counter drugs. Vitamin manufacturers do not have to prove that their products are safe or effective before they can be sold to the public. By contrast, manufacturers of conventional prescription and over-the-counter drugs must prove both safety and effectiveness in humans before their product can be marketed.

In 2007, ConsumerLab, an independent testing company in New York, evaluated 21 brands of multivitamins. They found that only 10 of these multivitamins contained all the vitamins and minerals in the quantities listed on the label. In addition, some brands contained contaminants, including lead. To get the most out of vitamin supplements, consumers should:

Experts agree that vitamin supplements are not a substitute for nutrients from food. Most healthy people in developed countries who eat a varied diet high in fruits, vegetables, and whole grains get enough vitamins and do not need a vitamin supplement, although many take a daily multivitamin as “insurance.” However, some groups do tend to need either general supplementation with a multivitamin or supplementation with specific vitamins to prevent vitamin deficiency diseases. People in these groups should discuss their vitamin requirements with their healthcare provider. They include:

Although vitamins play an undeniable role in maintaining health, large doses of vitamins in healthy individuals can cause adverse effects. Almost all vitamin excess (hypervitaminosis) occurs because of supplementation; it is almost impossible to get too many vitamins from food. Although a great deal of advertising, especially on the Internet, suggests that megadoses of certain vitamins can improve athletic performance, prevent and treat chronic disease, delay aging, and increase longevity, there is little or no evidence from independent, well-controlled human clinical trials to support these claims. One exception is high dose niacin, which has been used to treat high blood cholesterol levels. Although niacin is very safe at normal doses, the levels needed to lower serum cholesterol, it has been associated with liver damage and, commonly, severe facial flushing. Otherwise, excess water-soluble vitamins are removed from the body in urine. Although large doses of water-soluble vitamins rarely cause health problems, they cannot be used by the body and are a waste of money. Fat-soluble vitamins that are stored in the body can build up to very high levels and cause serious health concerns.

Both too little and too much of any of the 13 human vitamins may cause health consequences. People interested in taking a multivitamin or other vitamin supplement should first talk to a healthcare provider.

The interactions among various vitamins, enzymes, coenzymes, drugs, and herbal supplements are complex and incompletely understood. People should consult with their healthcare provider before taking any dietary supplements, especially if they are taking prescription medications.

Vitamins acquired by eating fruits and vegetables promote health. No complications are expected from vitamins in food. Vitamin supplements may cause hypervitaminosis or interact with other supplements, prescription drugs, over-the-counter drugs, and herbal supplements in ways that cause undesirable side effects.

Parents should encourage their children to eat a healthy and varied diet high in fruits, vegetables, and whole grains to meet their vitamin needs.

Most vitamin poisonings and deaths occur in children under age 6 as the result of accidental intake of excessive vitamin supplements. Parents should treat vitamin supplements as they would any drug and store them out of the reach of children.

See also Folate ; Vitamin A ; Vitamin B6 ; Vitamin B12 ; Vitamin C ; Vitamin D ; Vitamin E ; Vitamin K ; Vitamin toxicity .