Merck Manual

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Overview of Vitamins


Larry E. Johnson

, MD, PhD, University of Arkansas for Medical Sciences

Reviewed/Revised Nov 2022
Topic Resources

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 tables and .

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 countries with low rates of food insecurity, vitamin deficiencies result mainly from the following:

In countries with high rates of food insecurity, 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 Overview of Undernutrition Undernutrition is a form of malnutrition. (Malnutrition also includes overnutrition.) Undernutrition can result from inadequate ingestion of nutrients, malabsorption, impaired metabolism, loss... read more . Deficiencies of fat-soluble vitamins and of vitamin B12 take > 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, the latest 2022 report from the US Preventive Services Task Force finds that, after a thorough review of scientific research, there is insufficient evidence that routine daily multivitamin supplements or individual vitamin and mineral supplementation can reduce incidence of cancer or prevent cardiovascular disease (1 Reference 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)... read more ) in community-dwelling, nonpregnant adults.



Drugs Mentioned In This Article

Drug Name Select Trade
Cyto B7, YumVs, YumVs ZERO
No brand name available
No brand name available
B-Natal, Neuro-K-500
A Mulsin, Aquasol A, Dofsol-A
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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