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 has been determined for some vitamins. Intake above this level increases the risk of a harmful effect (toxicity).

Consuming too little or too much of certain vitamins can cause a nutritional disorder. People who eat a variety of foods are unlikely to develop a vitamin deficiency. However, people who follow restrictive diets may not consume enough of a particular vitamin. For example, strict vegetarians, who consume no animal products, may become deficient in vitamin B₁₂, which is available only in animal products. On the other hand, consuming large amounts (megadoses) of vitamin supplements, without medical supervision, may have harmful effects.

Vitamins are called essential micronutrients because they are required by the body in small amounts. Some vitamins--A, D, E, and K--are fat soluble. Other vitamins--B vitamins and vitamin C--are water soluble. B vitamins include vitamins B₁ (thiamin), B₂ (riboflavin), niacin, pantothenic acid, B₆ (pyridoxine), biotin, B₁₂ (cobalamin), and folic acid (folate).

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

Disorders that impair the intestine's absorption of fats can reduce the absorption of fat-soluble vitamins--A, D, E, and K--and increase the risk of a deficiency. Such disorders include Crohn's disease, cystic fibrosis, obstruction of the bile ducts, and malabsorption disorders.

See the table Vitamins.