Overview of Vitamins
Vitamins may be
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:
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Recommended daily allowance (RDA): RDAs are set to meet the needs of 97 to 98% of healthy people.
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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.
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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:
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Poverty
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Food faddism
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Drugs (see Nutrient-Drug Interactions and table Potential Vitamin-Drug Interactions)
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Prolonged and inadequately supplemented parenteral feeding
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 > 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).
Recommended Daily Intakes for Vitamins
Sources, Functions, and Effects of Vitamins
|
Nutrient |
Principal Sources |
Functions |
Effects of Deficiency and Toxicity |
|
Folate (folic acid) |
Raw green leafy vegetables, fruits, organ meats (eg, liver), enriched cereals and breads |
Maturation of red blood cells Synthesis of purines, pyrimidines, and methionine Development of fetal nervous system |
Deficiency: Megaloblastic anemia, neural tube birth defects, confusion |
|
Niacin (nicotinic acid, nicotinamide) |
Liver, red meat, fish, poultry, legumes, whole-grain or enriched cereals and breads |
Oxidation-reduction reactions Carbohydrate and cell metabolism |
Deficiency: Pellagra (dermatitis, glossitis, gastrointestinal and central nervous system dysfunction) Toxicity: Flushing |
|
Riboflavin (vitamin B2) |
Milk, cheese, liver, meat, eggs, enriched cereal products |
Many aspects of carbohydrate and protein metabolism Integrity of mucous membranes |
Deficiency: Cheilosis, angular stomatitis, corneal vascularization |
|
Thiamin (vitamin B1) |
Whole grains, meat (especially pork and liver), enriched cereal products, nuts, legumes, potatoes |
Carbohydrate, fat, amino acid, glucose, and alcohol metabolism Central and peripheral nerve cell function Myocardial function |
Deficiency: Beriberi (peripheral neuropathy, heart failure), Wernicke-Korsakoff syndrome |
|
Vitamin A (retinol) |
As preformed vitamin: fish liver oils, liver, egg yolks, butter, vitamin A–fortified dairy products As provitamin carotenoids: dark green and yellow vegetables, carrots, yellow and orange fruits |
Formation of rhodopsin (a photoreceptor pigment in the retina) Integrity of epithelia Lysosome stability Glycoprotein synthesis |
Deficiency: Night blindness, perifollicular hyperkeratosis, xerophthalmia, keratomalacia, increased morbidity and mortality in young children Toxicity: Headache, peeling of skin, hepatosplenomegaly, bone thickening, intracranial hypertension, papilledema, hypercalcemia |
|
Vitamin B6 group (pyridoxine, pyridoxal, pyridoxamine) |
Organ meats (eg, liver), whole-grain cereals, fish, legumes |
Many aspects of nitrogen metabolism (eg, transaminations, porphyrin and heme synthesis, tryptophan conversion to niacin) Nucleic acid biosynthesis Fatty acid, lipid, and amino acid metabolism |
Deficiency: Seizures, anemia, neuropathies, seborrheic dermatitis Toxicity: Peripheral neuropathy |
|
Vitamin B12 (cobalamins) |
Meats (especially beef, pork, and organ meats [eg, liver]), poultry, eggs, fortified cereals, milk and milk products, clams, oysters, mackerel, salmon |
Maturation of red blood cells, neural function, DNA synthesis, myelin synthesis and repair |
Deficiency: Megaloblastic anemia, neurologic deficits (confusion, paresthesias, ataxia) |
|
Vitamin C (ascorbic acid) |
Citrus fruits, tomatoes, potatoes, broccoli, strawberries, sweet peppers |
Collagen formation Bone and blood vessel health Carnitine, hormone, and amino acid formation Wound healing |
Deficiency: Scurvy (hemorrhages, loose teeth, gingivitis, bone defects) |
|
Vitamin D ( cholecalciferol, ergocalciferol) |
Direct ultraviolet B irradiation of the skin (main source), fortified dairy products (main dietary source), fish liver oils, fatty fish, liver |
Calcium and phosphate absorption Mineralization and repair of bone Tubular reabsorption of calcium Insulin and thyroid function, improvement of immune function, reduced risk of autoimmune disease |
Deficiency: Rickets (sometimes with tetany), osteomalacia Toxicity: Hypercalcemia, anorexia, renal failure, metastatic calcifications |
|
Vitamin E group (alpha-tocopherol, other tocopherols) |
Vegetable oils, nuts |
Intracellular antioxidant Scavenger of free radicals in biologic membranes |
Deficiency: Red blood cell hemolysis, neurologic deficits Toxicity: Tendency to bleed |
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Vitamin K group (phylloquinone, menaquinones) |
Green leafy vegetables (especially collards, spinach, and salad greens), soy beans, vegetable oils Bacteria in the gastrointestinal tract after neonatal period |
Formation of prothrombin, other coagulation factors, and bone proteins |
Deficiency: Bleeding due to deficiency of prothrombin and other factors, osteopenia |
Potential Vitamin-Drug Interactions
References
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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.
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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.
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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.
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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.
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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.
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6. LeBlanc ES, Chou R: Vitamin D and falls—Fitting new data with current guidelines. JAMA Intern Med 175 (5):712–713, 2015.
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Drugs Mentioned In This Article
| Drug Name | Select Trade |
|---|---|
cholecalciferol |
No US brand name |
penicillamine |
CUPRIMINE |
sulfasalazine |
AZULFIDINE |
methotrexate |
OTREXUP |
fluorouracil |
CARAC |
triamterene |
DYRENIUM |
cycloserine |
SEROMYCIN |
metformin |
GLUCOPHAGE |
primidone |
MYSOLINE |
phenytoin |
DILANTIN |
isoniazid |
LANIAZID |
warfarin |
COUMADIN |
levodopa |
Levodopa |
rifampin |
RIFADIN, RIMACTANE |
niacin |
NIACOR, NIASPAN |
