Overview of Vitamins

ByLarry E. Johnson, MD, PhD, University of Arkansas for Medical Sciences
Reviewed/Revised Nov 2022

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).

Did You Know...

  • Consuming very large doses of certain vitamins can be harmful.

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 B12bariatric surgery, are on hemodialysis, or have alcohol use disorder) may benefit from a daily multivitamin.

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 are fat soluble. Other vitamins are water soluble. The difference between fat and water soluble affects nutrition in several ways.

Fat-soluble vitamins

Fat-soluble vitamins dissolve in fats (lipids) and include

  • Vitamin K

Fat-soluble vitamins are stored in the liver and in fatty tissues. If too much of the fat-soluble vitamins A or D is consumed, they can accumulate and may have harmful effects.

Because fats in foods help the body absorb fat-soluble vitamins, a low-fat diet may result in a deficiency. Some disorders, called malabsorption disorders

Cooking does not destroy fat-soluble vitamins.

Water-soluble vitamins

Water-soluble vitamins dissolve in water and include

  • B vitamins

  • Vitamin C


Water-soluble vitamins are eliminated in urine and tend to be eliminated from the body more quickly than fat-soluble vitamins. Water-soluble vitamins are more likely to be destroyed when food is stored and prepared. The following can help prevent the loss of these vitamins:

  • Refrigerating fresh produce

  • Storing milk and grains out of strong light

  • Using the cooking water from vegetables to prepare soups


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 alcohol use disorder 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.

Drugs Mentioned In This Article
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