Overview of vitamins

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Vitamins may be

Fat soluble (vitamins A, D, E, and K)
Water soluble (B vitamins and vitamin C)

The B vitamins include biotin, folate, niacin , pantothenic acid, riboflavin (B2), thiamin (B1), B6 (eg, pyridoxine), and B12 (cobalamins).
For dietary requirements, sources, functions, effects of deficiencies and toxicities, blood levels, and usual therapeutic dosages for vitamins, see table Recommended Daily Intakes for Vitamins and Sources, Functions, and Effects of Vitamins.
Dietary requirements for vitamins (and other nutrients) are expressed as daily recommended intake (DRI). There are 3 types of DRI:

Recommended daily allowance (RDA): RDAs are set to meet the needs of 97 to 98% of healthy people.
Adequate intake (AI): When data to calculate an RDA are insufficient, AIs are based on observed or experimentally determined estimates of nutrient intake by healthy people.
Tolerable upper intake level (UL): ULs are the largest amount of a nutrient that most adults can ingest daily without risk of adverse health effects.

In developed countries, vitamin deficiencies result mainly from the following:

Poverty
Food faddism
Drugs (see Nutrient-Drug Interactions and table Potential Vitamin-Drug Interactions)
Alcoholism
Prolonged and inadequately supplemented parenteral feeding
Malabsorption

Mild vitamin deficiency is common among frail and institutionalized elderly people who have protein-energy undernutrition.
In developing countries, vitamin deficiencies can result from lack of access to nutrients.
Deficiencies of water-soluble vitamins (except vitamin B12) may develop after weeks to months of undernutrition. Deficiencies of fat-soluble vitamins and of vitamin B12 take gt; 1 year to develop because the body stores them in relatively large amounts. Intakes of vitamins sufficient to prevent classic vitamin deficiencies (such as scurvy or beriberi) may not be adequate for optimum health. This area remains one of controversy and active research.
Vitamin dependency results from a genetic defect involving metabolism of a vitamin. In some cases, vitamin doses as high as 1000 times the DRI improve function of the altered metabolic pathway.
Vitamin toxicity (hypervitaminosis) usually results from taking megadoses of vitamin A, D, C, B6, or niacin .
Because many people eat irregularly, foods alone may provide suboptimal amounts of some vitamins. In these cases, the risk of certain cancers or other disorders may be increased. However, routine daily multivitamin supplements have not been proved to reduce cancer. Supplementation with vitamins does not appear to prevent cardiovascular disease (1, 2) or falls (3, 4, 5, 6).