Introduction: Vitamins: Merck Manual Home Edition

THE MERCK MANUAL MEDICAL LIBRARY: The Merck Manual of Medical Information--Home Edition

Index

Sections

A	B	C	D	E	F	G	H	I
J	K	L	M	N	O	P	Q	R
S	T	U	V	W	X	Y	Z

In This Topic

	Introduction

Section		Disorders of Nutrition and Metabolism

Subject		Vitamins

Topics		Introduction· Folate· Niacin· Riboflavin· Thiamin· Vitamin A· Vitamin B₁₂· Vitamin B₆· Vitamin C· Vitamin D· Vitamin E· Vitamin K

Introduction

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. It is common among certain groups of people (such as older people) even if they eat a variety of foods. For other vitamins, 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. 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.

Vitamins
Vitamin	Good Sources	Main Functions	Recommended Dietary Allowance	Safe Upper Limit
Biotin	Liver, kidneys, egg yolks, milk, fish, dried yeast, cauliflower, nuts, and legumes	Required for the 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 1,200 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 50 1.7 milligrams for men older than 50 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	200 IU for people aged 50 and younger 400 IU for people aged 51 to 70 600 IU for people older than 70	2,000 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	—
IU = international unit; DNA = deoxyribonucleic acid; 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). Deficiency of biotin or pantothenic acid almost never occurs.

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.

Vitamins: Fat Versus Water Soluble

Vitamins are classified as fat soluble:

Vitamin A
Vitamin D
Vitamin E
Vitamin K

or water soluble:

B vitamins
Vitamin C

This difference affects nutrition in several ways.

Fat-soluble vitamins: These vitamins dissolve in fats (lipids). They are stored in the liver and in fatty tissues. If too much of the fat-soluble vitamins A or D are 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 interfere with absorption of fats and thus of fat-soluble vitamins. Examples are chronic diarrhea, Crohn's disease, cystic fibrosis, pancreatitis, and blockage of the bile ducts. 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, it carries these vitamins unabsorbed out of the body.

Cooking does not destroy fat-soluble vitamins.

Water-soluble vitamins: These vitamins dissolve in water. They 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. Refrigerating fresh produce, storing milk and grains out of strong light, and using the cooking water from vegetables to prepare soups can help prevent the loss of the vitamins.

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's disease, cystic fibrosis, pancreatitis, and blockage of the bile ducts.

Did You Know...

Consuming very large doses of vitamins can be harmful.

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.

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

Some Drugs That Cause Vitamin Deficiency
Drug	Vitamin
Alcohol	Folate Thiamin Vitamin B₆
Antacids	Vitamin B₁₂
Antibiotics, such as isoniazidSome Trade Names INH NYDRAZID , tetracyclineSome Trade Names ACHROMYCIN V TETRACYN SUMYCIN , and trimethoprim-sulfamethoxazoleSome Trade Names BACTRIM SEPTRA	B vitamins Folate Vitamin K
Anticoagulants, such as warfarinSome Trade Names COUMADIN	Vitamin E Vitamin K
Anticonvulsants, such as phenytoinSome Trade Names DILANTIN and phenobarbitalSome Trade Names LUMINAL	Biotin Folate Vitamin B₆ Vitamin D Vitamin K
Antipsychotic drugs	Riboflavin Vitamin D
Barbiturates such as phenobarbitalSome Trade Names LUMINAL	Folate Riboflavin Vitamin D
Chemotherapy drugs, such as methotrexateSome Trade Names RHEUMATREX	Folate
CholestyramineSome Trade Names QUESTRAN	Many vitamins
Corticosteroids	Vitamin C Vitamin D
CycloserineSome Trade Names SEROMYCIN	Vitamin B₆
HydralazineSome Trade Names APRESOLINE	Vitamin B₆
LevodopaSome Trade Names DOPAR LARODOPA	Vitamin B₆
Mineral oil (long-term use)	Vitamin A Vitamin D Vitamin E Vitamin K
MetforminSome Trade Names GLUCOPHAGE	Folate Vitamin B₁₂
Nitrous oxide (repeated exposure)	Vitamin B₁₂
Oral contraceptives	Folate Thiamin Vitamin B₆
PenicillamineSome Trade Names CUPRIMINE	Vitamin B₆
Phenothiazines	Riboflavin
PrimidoneSome Trade Names MYSOLINE	Folate Vitamin D
RifampinSome Trade Names RIFADIN RIMACTANE	Vitamin D Vitamin K
SulfasalazineSome Trade Names AZULFIDINE	Folate
Thiazide diuretics	Riboflavin
TriamtereneSome Trade Names DYRENIUM	Folate
Tricyclic antidepressants, such as amitriptylineSome Trade Names ELAVIL ENDEP and imipramineSome Trade Names TOFRANIL	Riboflavin

Last full review/revision August 2007 by Larry E. Johnson, MD, PhD

Next: Folate

Audio

Figures

Photographs

Pronunciations

Sidebar

Tables

Videos