Overview of Vitamins
Vitamins are a vital part of a healthy diet. The recommended dietary allowance (RDA)—the amount most healthy people need each day to remain healthy—has been determined for most vitamins. A safe upper limit (tolerable upper intake level) has been determined for some vitamins. Intake above this limit increases the risk of a harmful effect (toxicity).
Consuming too little of a vitamin can cause a nutritional disorder. However, people who eat a variety of foods are unlikely to develop most vitamin deficiencies. Deficiency of vitamin D is an exception. Vitamin D deficiency is common among certain groups of people (such as older people) even if they eat a variety of foods. For other vitamins, a deficiency can develop if people follow a restrictive diet that does not contain enough of a particular vitamin. For example, vegans, who consume no animal products, may become deficient in vitamin B12, which is available in animal products. Deficiency of biotin or pantothenic acid almost never occurs.
Consuming large amounts (megadoses) of certain vitamins (usually as supplements) without medical supervision may also have harmful effects.
Vitamins are called essential micronutrients because the body requires them but only in small amounts.
The body does not store most vitamins. Deficiencies of these vitamins usually develop in weeks to months. Therefore, people must consume them regularly. Vitamins A, B12, and D are stored in significant amounts, mainly in the liver. Vitamins A and D are also stored in fat cells. Deficiencies of these vitamins take more than a year to develop.
Because many people eat irregularly or do not eat a variety of foods, they may not get enough of some vitamins from foods alone. If they do not get enough, the risk of certain cancers or other disorders may be increased. People may then take a multivitamin. However, for most people, taking multivitamins does not appear to reduce risk of developing cancer or heart or blood vessel (cardiovascular) disorders.
Some vitamins—A, D, E, and K—are fat soluble. Other vitamins—B vitamins and vitamin C—are water soluble. B vitamins include biotin, folate (folic acid), niacin, pantothenic acid, riboflavin (vitamin B2), thiamin (vitamin B1), and vitamins B6 (pyridoxine) and B12 (cobalamins).
Disorders that impair the intestine’s absorption of food (called malabsorption disorders) can cause vitamin deficiencies. Some disorders impair the absorption of fats. These disorders can reduce the absorption of fat-soluble vitamins—A, D, E, and K—and increase the risk of a deficiency. Such disorders include chronic diarrhea, Crohn disease, cystic fibrosis, certain pancreatic disorders, and blockage of the bile ducts.
Some types of weight-loss (bariatric) surgery can also interfere with absorption of vitamins.
Liver disorders and alcoholism can interfere with the processing (metabolism) or storage of vitamins.
In a few people, hereditary disorders impair the way the body handles vitamins and thus cause a deficiency.
If people must be fed intravenously for a long time or if the formula used lacks the needed nutrients, people may develop a vitamin (or mineral) deficiency.
Drugs can also contribute to deficiency of a vitamin. They may interfere with absorption, metabolism, or storage of a vitamin.