Merck Manual

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Overview of Vitamins


Larry E. Johnson

, MD, PhD, University of Arkansas for Medical Sciences

Reviewed/Revised Nov 2022
Topic Resources

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.

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 Vitamin D Deficiency Vitamin D deficiency is most commonly caused by a lack of exposure to sunlight. Some disorders can also cause the deficiency. The most common cause is lack of exposure to sunlight, usually when... read more 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 Vitamin B12 Deficiency Vitamin B12 deficiency can occur in vegans who do not take supplements or as a result of an absorption disorder. Anemia develops, causing paleness, weakness, fatigue, and, if severe, shortness... read more , which is available in animal products. Deficiency of the B vitamins biotin or pantothenic acid almost never occurs. People at high risk for vitamin deficiencies (for example, people who had bariatric surgery Metabolic and Bariatric Surgery Metabolic and bariatric (weight-loss) surgery alters the stomach, intestine, or both to produce weight loss in people have obesity or overweight and have metabolic disorders related to obesity... read more , are on hemodialysis Dialysis Dialysis is an artificial process for removing waste products and excess fluids from the body, a process that is needed when the kidneys are not functioning properly. There are a number of reasons... read more Dialysis , or have alcohol use disorder Alcohol Use Alcohol (ethanol) is a depressant (it slows down brain and nervous system functioning). Consuming large amounts rapidly or regularly can cause health problems, including organ damage, coma,... read more ) 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 A

  • Vitamin D

  • Vitamin E

  • 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 Overview of Malabsorption Malabsorption syndrome refers to a number of disorders in which nutrients from food are not absorbed properly in the small intestine. Certain disorders, infections, and surgical procedures can... read more , interfere with absorption of fats and thus of fat-soluble vitamins. Some drugs, such as mineral oil, have the same effect. Fat-soluble vitamins dissolve in mineral oil, which the body does not absorb. So when people take mineral oil (for example, as a laxative), it carries these vitamins unabsorbed out of the body.

Cooking does not destroy fat-soluble vitamins.

Water-soluble vitamins

Water-soluble vitamins dissolve in water and include

  • B vitamins

  • Vitamin C

B vitamins include biotin, folate (folic acid), niacin, pantothenic acid, riboflavin (vitamin B2), thiamin (vitamin B1), vitamin B6 (pyridoxine), and vitamin B12 (cobalamins).

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


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

Generic Name Select Brand Names
Calcidol, Calciferol, D3 Vitamin, DECARA, Deltalin, Dialyvite Vitamin D, Dialyvite Vitamin D3, Drisdol, D-Vita, Enfamil D-Vi-Sol, Ergo D, Fiber with Vitamin D3 Gummies Gluten-Free, Happy Sunshine Vitamin D3, MAXIMUM D3, PureMark Naturals Vitamin D, Replesta, Replesta Children's, Super Happy SUNSHINE Vitamin D3, Thera-D 2000, Thera-D 4000, Thera-D Rapid Repletion, THERA-D SPORT, UpSpring Baby Vitamin D, UpSpring Baby Vitamin D3, YumVs, YumVs Kids ZERO, YumVs ZERO
Cyto B7, YumVs, YumVs ZERO
No brand name available
A Mulsin, Aquasol A, Dofsol-A
Alph-E-Mixed , AQUA-E, Aquasol E , Aquavite-E
Fleet, Kondremul, Liqui-Doss, Muri-Lube
Folacin , Folicet, Q-TABS
B-Natal, Neuro-K-500
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