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Vitamins may be fat soluble (vitamins A, D, E, and K) or 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 1: Vitamin Deficiency, Dependency, and Toxicity: Recommended Daily Intakes for Vitamins* and Table 2: Vitamin Deficiency, Dependency, and Toxicity: Sources, Functions, and Effects of Vitamins 

Table 1
Recommended Daily Intakes for Vitamins* This table is presented as a PDF and requires the free Adobe PDF reader. Get Adobe Reader

Table 2
Sources, Functions, and Effects of Vitamins  This table is presented as a PDF and requires the free Adobe PDF reader. Get Adobe Reader

Table 3
Potential Vitamin-Drug Interactions Nutrient Drug Biotin Antibiotics, anticonvulsants Folate Alcohol, 5- fluorouracil Some Trade Names ADRUCIL Click for Drug Monograph , metformin Some Trade Names GLUCOPHAGE Click for Drug Monograph , methotrexate Some Trade Names RHEUMATREX Click for Drug Monograph , oral contraceptives, phenobarbital Some Trade Names LUMINAL Click for Drug Monograph , phenytoin Some Trade Names DILANTIN Click for Drug Monograph , primidone Some Trade Names MYSOLINE Click for Drug Monograph , sulfasalazine Some Trade Names AZULFIDINE Click for Drug Monograph , triamterene Some Trade Names DYRENIUM Click for Drug Monograph , trimethoprim Some Trade Names PROLOPRIM TRIMPEX Click for Drug Monograph Niacin Alcohol, isoniazid Some Trade Names INH NYDRAZID Click for Drug Monograph Riboflavin Alcohol, barbiturates, phenothiazines, thiazide diuretics, tricyclic antidepressants Thiamin Alcohol; oral contraceptives; thiamin antagonists in coffee, tea, raw fish, and red cabbage Vitamin A Cholestyramine Some Trade Names QUESTRAN Click for Drug Monograph , mineral oil Vitamin B6 Alcohol, anticonvulsants, corticosteroids, cycloserine Some Trade Names SEROMYCIN Click for Drug Monograph , hydralazine Some Trade Names APRESOLINE Click for Drug Monograph , isoniazid Some Trade Names INH NYDRAZID Click for Drug Monograph , levodopa, oral contraceptives, penicillamine Some Trade Names CUPRIMINE Click for Drug Monograph Vitamin B12 Vitamin C Antacids, metformin Some Trade Names GLUCOPHAGE Click for Drug Monograph , nitrous oxide (repeated exposure) Vitamin D Corticosteroids Antipsychotics, barbiturates, cholestyramine Some Trade Names QUESTRAN Click for Drug Monograph , corticosteroids, mineral oil, phenytoin Some Trade Names DILANTIN Click for Drug Monograph , primidone Some Trade Names MYSOLINE Click for Drug Monograph , rifampin Some Trade Names RIFADIN RIMACTANE Click for Drug Monograph Vitamin E Mineral oil, warfarin Some Trade Names COUMADIN Click for Drug Monograph Vitamin K Antibiotics, anticonvulsants, mineral oil, rifampin Some Trade Names RIFADIN RIMACTANE Click for Drug Monograph , warfarin Some Trade Names COUMADIN Click for Drug Monograph and other anticoagulants

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 that of a nutrient most adults can ingest daily without risk of adverse health effects.

In developed countries, vitamin deficiencies result mainly from poverty, food faddism, drugs (see Nutrition: General Considerations: Nutrient-Drug Interactions and Table 3: Vitamin Deficiency, Dependency, and Toxicity: Potential Vitamin-Drug Interactions), alcoholism, or prolonged and inadequately supplemented parenteral feeding. Mild vitamin deficiency is common among frail and institutionalized elderly people who have protein-energy malnutrition. In developing countries, 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 > 1 yr to develop because the body stores them in relatively large amounts. Intakes of vitamins sufficient to prevent classic vitamin deficiencies (like scurvy or beriberi) may not be adequate for optimum health. This remains an area 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. Because of this risk, routine daily multivitamin supplements are sometimes recommended.

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

Content last modified April 2007