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 Recommended Daily Intakes for Vitamins 
and Sources, Functions, and Effects of Vitamins 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 Nutrient-Drug Interactions Nutrition can affect the body’s response to drugs; conversely, drugs can affect the body’s nutrition. Foods can enhance, delay, or decrease drug absorption. Foods impair absorption of many antibiotics... read more and table Potential Vitamin-Drug Interactions Potential Vitamin-Drug Interactions
)
Mild vitamin deficiency is common among frail and institutionalized elderly people who have protein-energy undernutrition Protein-Energy Undernutrition (PEU) Protein-energy undernutrition (PEU), previously called protein-energy malnutrition, is an energy deficit due to deficiency of all macronutrients. It commonly includes deficiencies of many micronutrients... read more .
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 > 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 References 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 , 2 References 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 ) or falls (3 References 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 , 4 References 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 , 5 References 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 , 6 References 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 ).
References
1. Myung SK, Ju W, Cho B, et al: Efficacy of vitamin and antioxidant supplements in prevention of cardiovascular disease: Systematic review and meta-analysis of randomized controlled trials. BMJ 346:f10, 2013. doi: 10.1136/bmj.f10
2. Sesso HD, Christen WG, Bubes V, et al: Multivitamins in the prevention of cardiovascular disease in men: The Physicians' Health Study II randomized controlled trial. JAMA 308 (17):1751–1756, 2012. doi: 10.1001/jama.2012.14805
3. Bischoff-Ferrari HA, Dawson-Hughes B, Orav EJ, et al: Monthly high-dose vitamin D treatment for the prevention of functional decline: A randomized clinical trial. JAMA Intern Med 176 (2):175–183, 2016. doi: 10.1001/jamainternmed.2015.7148
4. Cummings SR, Kiel DP, Black DM: Vitamin D supplementation and increased risk of falling: A cautionary tale of vitamin supplements retold. JAMA Intern Med 176 (2):171–172, 2016. doi: 10.1001/jamainternmed.2015.7568
5. Uusi-Rasi K, Patil R, Karinkanta S, Kannus P, et al: Exercise and vitamin D in fall prevention among older women: A randomized clinical trial. JAMA Intern Med 75 (5):703–711, 2015. doi: 10.1001/jamainternmed.2015.0225
6. LeBlanc ES, Chou R: Vitamin D and falls—Fitting new data with current guidelines. JAMA Intern Med 175 (5):712–713, 2015. doi: 10.1001/jamainternmed.2015.0248
