Merck Manual

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Overview of Vitamins

By

Larry E. Johnson

, MD, PhD, University of Arkansas for Medical Sciences

Last full review/revision Aug 2019| Content last modified Aug 2019
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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 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:

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).

Table
icon

Recommended Daily Intakes for Vitamins

Age

Folate (mcg)

Niacin (mg NE*)

Riboflavin (mg)

Thiamin (mg)

Vitamin A (mcg)

Vitamin B6 (mg)

Vitamin B12 (mcg)

Vitamin  C (mg)

Vitamin  D (units) †

Vitamin  E (mg)

Vitamin  K (mcg)

Infants

0–6 months

65

2

0.3

0.2

400

0.1

0.4

40

400

4

2.0

7–12 months

80

4

0.4

0.3

500

0.3

0.5

50

400

5

2.5

Children

1–3 years

150

6

0.5

0.5

300

0.5

0.9

15

600

6

30

4–8 years

200

8

0.6

0.6

400

0.6

1.2

25

600

7

55

Males

9–13 years

300

12

0.9

0.9

600

1.0

1.8

45

600

11

60

14–18 years

400

16

1.3

1.2

900

1.3

2.4

75

600

15

75

19–70 years

400

16

1.3

1.2

900

1.3

2.4

90

600

15

120

> 70 years

400

16

1.3

1.2

900

1.7

2.4

90

800 ‡

15

120

Females

9–13 years

300

12

0.9

0.9

600

1.0

1.8

45

600

11

60

14–18 years

400

14

1.0

1.0

700

1.2

2.4

65

600

15

75

19–70 years

400

14

1.1

1.1

700

1.3

2.4

75

600

15

90

> 70 years

400

14

1.1

1.1

700

1.5

2.4

75

800 ‡

15

90

Pregnant women

19–50 years

600

18

1.4

1.4

770

1.9

2.6

85

600

15

90

Breastfeeding women

19–50 years

500

17

1.6

1.4

1300

2.0

2.8

120

600

19

90

Upper limit (UL) §

1000

35

ND

ND

3000

100

ND

2000

4000

1000

ND

NOTE: Recommended dietary allowances (RDAs) are shown in regular type. RDAs are set to meet the needs of 97 to 98% of healthy people.

Adequate intakes (AIs) are shown in bold type. When data to calculate the RDA for a nutrient are insufficient, AIs are based on observed or experimentally determined estimates of nutrient intake by healthy people.

* 1 niacin equivalent (NE) equals 1 mg niacin or 60 mg dietary tryptophan.

† 200 units of vitamin D equals 5 mcg cholecalciferol.

‡ 800 units of vitamin D is recommended for people 70 years.

§ UL is the largest amount of a nutrient that most adults can ingest daily without risk of adverse effects. The more the UL is exceeded, the greater the risk of adverse effects.

ND = not determinable because of lack of data (sources of intake should be limited to foods); RAE = retinol activity equivalents (1 mcg RAE of preformed vitamin A = 3.33 units).

Adapted from Dietary Reference Intakes, Food and Nutrition Board, Institute of Medicine. Washington, DC: National Academy Press.

Table
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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

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

Table
icon

Potential Vitamin-Drug Interactions

Nutrient

Drug

Antibiotics, antiseizure drugs

Alcohol, 5-fluorouracil, metformin, methotrexate, oral contraceptives, antiseizure drugs (eg, phenobarbital, phenytoin, primidone), sulfasalazine, triamterene, trimethoprim

Alcohol, isoniazid

Alcohol, barbiturates, phenothiazines, thiazide diuretics, tricyclic antidepressants

Alcohol; oral contraceptives; thiamin antagonists in coffee, tea, raw fish, and red cabbage

Cholestyramine, mineral oil

Alcohol, antiseizure drugs, corticosteroids, cycloserine, hydralazine, isoniazid, levodopa, oral contraceptives, penicillamine

Antacids, metformin, nitrous oxide (repeated exposure)

Corticosteroids

Antipsychotics, corticosteroids, mineral oil, antiseizure drugs, rifampin

Mineral oil, warfarin

Antibiotics, antiseizure drugs, mineral oil, rifampin, warfarin

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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 6. LeBlanc ES, Chou R: Vitamin D and falls—Fitting new data with current guidelines. JAMA Intern Med 175 (5):712–713, 2015.

Drugs Mentioned In This Article

Drug Name Select Trade
No US brand name
CUPRIMINE
AZULFIDINE
OTREXUP
CARAC
DYRENIUM
SEROMYCIN
GLUCOPHAGE
MYSOLINE
DILANTIN
LANIAZID
COUMADIN
Levodopa
RIFADIN, RIMACTANE
NIACOR, NIASPAN
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