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.
Humans need nine water-soluble vitamins. These vitamins dissolve in water and are not stored in the body for long periods. Most excess water-soluble vitamins are removed by the kidneys and leave the body in urine.
In general, B vitamins tend to be involved in reactions that convert nutrients to energy and reactions that synthesize new molecules. There are gaps in the numbering of the B-complex vitamins, because compounds originally named as vitamins, such as B4 (adenine), were renamed after further research showed that they did not meet the definition of a vitamin.
Water-soluble vitamins include:
- Vitamin B1 (thiamin): needed to convert carbohydrates to energy
- Vitamin B2 (riboflavin): helps breakdown proteins, fats, and carbohydrates and make other vitamins and minerals available to the body
- Vitamin B3 (niacin): helps the body process fats and proteins
- Vitamin B5 (pantothenic acid): helps regulate the chemical reactions that produce energy
- Vitamin B6 (pyridoxine): involved in the transmission of nerve impulses, formation and functioning of red blood cells, and creation of new cells
- Vitamin B12 (cobalamin): necessary for healthy red blood cells, creating new deoxyribose nucleic acid (DNA), and maintaining nerve cells
- Vitamin C (ascorbic acid): helps form cartilage and connective tissue; as an antioxidant, protects cells from free radical damage
- Vitamin H (biotin): joins with enzymes that regulate the breakdown of foods and their use in the body
- Folic acid (folate): helps make new cells; important in development of the fetal nervous system
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.
In general, the fat-soluble vitamins have antioxidant activity that helps protect cells from damage.
Fat-soluble vitamins include:
- Vitamin A (retinol): needed for vision, a healthy immune system, development of the fetus, tissue repair; as an antioxidant, protects cells from free radical damage
- Vitamin D (calciferol): involved in building bones, muscle contraction, and nerve impulse transmission
- Vitamin E (tocopherol): acts as an antioxidant to protect the body against damage caused by free radicals
- Vitamin K: needed for blood clotting.
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 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 nutraceuticals, have added vitamins, minerals, and herbs.
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:
- the elderly, especially those on restricted diets
- vegans, because they eat no animal products
- breastfed babies of vegan mothers
- people with lactose intolerance or those who do not eat dairy products
- people with alcoholism
- people who have had part of their stomachs or intestines surgically removed
- pregnant women or those who could become pregnant
- people with diseases that interfere with vitamin metabolism
- people taking drugs that interfere with vitamin metabolism
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.