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Overview of Vitamins

by Larry E. Johnson, MD, PhD

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 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 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.



Good Sources

Main Functions

Recommended Dietary Allowance

Safe Upper Limit


Liver, kidneys, egg yolks, milk, fish, dried yeast, cauliflower, nuts, and legumes

Required for the processing (metabolism) of carbohydrates and fatty acids

30 micrograms (but no RDA has been established)

Folate (folic acid)

Fresh green leafy vegetables, asparagus, broccoli, fruits (especially citrus), liver, other organ meats, dried yeast, and enriched breads, pastas, and cereals

(Note: Extensive cooking destroys 50–95% of the folate in food.)

Required for the formation of red blood cells, for DNA and RNA synthesis, and for normal development of the nervous system in a fetus

400 micrograms

600 micrograms for pregnant women

500 micrograms for breastfeeding women

1,000 micrograms

Niacin (nicotinic acid or nicotinamide )

Dried yeast, liver, meat, fish, legumes, and whole-grain or enriched cereal products

Required for the metabolism of carbohydrates, fats, and many other substances

14 milligrams for women

16 milligrams for men

35 milligrams

Pantothenic acid

Liver, beef, egg yolks, yeast, potatoes, broccoli, and whole grains

Required for the metabolism of carbohydrates and fats

5 milligrams (but no RDA has been established)

Riboflavin (vitamin B2)

Milk, cheese, liver, meat, fish, eggs, and enriched cereals

Required for the metabolism of carbohydrates and amino acids and for healthy mucous membranes, such as those lining the mouth

1.1 milligrams for women

1.3 milligrams for men

1.4 milligrams for pregnant women

1.6 milligrams for breastfeeding women

Thiamin (vitamin B1)

Dried yeast, whole grains, meat (especially pork and liver), enriched cereals, nuts, legumes, and potatoes

Required for the metabolism of carbohydrates and for normal nerve and heart function

1.1 milligrams for women

1.2 milligrams for men

1.4 milligrams for pregnant or breastfeeding women

Vitamin A (retinol)

As vitamin A: Fish liver oils, liver, egg yolks, butter, cream, and fortified milk

As carotenoids (converted to vitamin A in the body), such as beta-carotene: Dark green and yellow-orange vegetables and yellow-orange fruits

Required to form light-sensitive nerve cells (photoreceptors) in the retina, helping maintain night vision

Helps maintain the health of the skin, cornea, and lining of the lungs, intestine, and urinary tract

Helps protect against infections

700 micrograms for women

900 micrograms for men

770 micrograms for pregnant women

1,300 micrograms for breastfeeding women

3,000 micrograms

Vitamin B6

Dried yeast, liver, other organ meats, whole-grain cereals, fish, and legumes

Required for the metabolism of amino acids and fatty acids, for normal nerve function, for the formation of red blood cells, and for healthy skin

1.3 milligrams

1.5 milligrams for women older than 70

1.7 milligrams for men older than 70

1.9 milligrams for pregnant women

2.0 milligrams for breastfeeding women

100 milligrams

Vitamin B12 (cobalamins)

Meats (especially beef, pork, liver, and other organ meats), eggs, fortified cereals, milk, clams, oysters, salmon, and tuna

Required for the formation and maturation of red blood cells, for nerve function, and for DNA synthesis

2.4 micrograms

2.6 micrograms for pregnant women

2.8 micrograms for breastfeeding women

Vitamin C (ascorbic acid)

Citrus fruits, tomatoes, potatoes, broccoli, strawberries, and sweet peppers

Required for the formation, growth, and repair of bone, skin, and connective tissue; for healing of wounds and burns; and for normal function of blood vessels

Acts as an antioxidant, protecting cells against damage by free radicals

Helps the body absorb iron

75 milligrams for women

90 milligrams for men

85 milligrams for pregnant women

120 milligrams for breastfeeding women

35 milligrams more for smokers

2,000 milligrams

Vitamin D

Formed in the skin when the skin is exposed to direct sunlight

Fortified milk, fatty fish, fish liver oils, and egg yolks

Promotes the absorption of calcium and phosphorus from the intestine

Required for bone formation, growth, and repair

Strengthens the immune system and reduces the risk of autoimmune disorders

600 IU for people aged 1‒70

800 IU for people older than 70

4,000 IU

Vitamin E

Vegetable oil, margarine, nuts, and wheat germ

Acts as an antioxidant, protecting cells against damage by free radicals

15 milligrams (22 IU of natural or 33 IU of synthetic)

19 milligrams for breastfeeding women

1,000 milligrams

Vitamin K

Green leafy vegetables (such as collards, spinach, and kale) and soybean and canola oils

Helps in the formation of blood clotting factors and thus is necessary for normal blood clotting

Required for healthy bones and other tissues

90 micrograms for women

120 micrograms for men

DNA = deoxyribonucleic acid; IU = international unit; RNA = ribonucleic acid.

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).

The body does not store most vitamins. Therefore, people must consume them regularly. Vitamins A, B12, and D are stored in significant amounts, mainly in the liver.

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.

Some Drugs That Cause Vitamin Deficiency






Vitamin B6


Vitamin B12

Antibiotics, such as isoniazid, tetracycline, and trimethoprim-sulfamethoxazole

B vitamins


Vitamin K

Anticoagulants, such as warfarin

Vitamin E

Vitamin K

Anticonvulsants, such as phenytoin and phenobarbital



Vitamin B6

Vitamin D

Vitamin K

Antipsychotic drugs


Vitamin D

Barbiturates such as phenobarbital



Vitamin D

Chemotherapy drugs, such as methotrexate



Vitamin A

Vitamin D

Vitamin E

Vitamin K


Vitamin C

Vitamin D


Vitamin B6


Vitamin B6


Vitamin B6

Mineral oil (long-term use)

Vitamin A

Vitamin D

Vitamin E

Vitamin K



Vitamin B12

Nitrous oxide (repeated exposure)

Vitamin B12

Oral contraceptives



Vitamin B6


Vitamin B6





Vitamin D


Vitamin D

Vitamin K



Thiazide diuretics




Tricyclic antidepressants, such as amitriptyline and imipramine


Drugs can also contribute to deficiency of a vitamin. They may interfere with absorption, metabolism, or storage of a vitamin.

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.

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Drugs Mentioned In This Article

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