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Vitamins

Last updated: November 3, 2023

Summarytoggle arrow icon

Vitamins are a group of chemically diverse organic compounds that an organism requires for normal metabolism. Apart from a few exceptions (e.g., vitamin D), the human body cannot synthesize vitamins on its own in sufficient amounts and must, therefore, ensure a steady supply through the diet. Vitamins are micronutrients that do not provide energy (like macronutrients) but instead have very specific biochemical roles. They can be coenzymes in various reactions (B vitamins, vitamins A and K) and/or antioxidants that protect the cell and its membrane from free radicals (vitamins C and E). They can also enable cell signaling (vitamin A) and gene transcription (vitamins A and E) or function as hormones (e.g., vitamin D). Vitamins are classified into fat-soluble vitamins, which the body can store, and water-soluble vitamins, which, with the exception of vitamins B9 (folate) and B12 (cobalamin), the body cannot store over significant periods of time and, therefore, require continuous intake. A balanced diet typically supplies the body with all vitamins it requires. Deficiencies occur mainly due to malnutrition, malabsorption disorders, or restrictive diets (e.g., vitamin B12 deficiency in a vegan diet).

Overview of vitaminstoggle arrow icon

Fat-soluble vs. water-soluble vitamins
Fat-soluble vitamins Water-soluble vitamins
Vitamins
Sources
  • Diet (except vitamin D which is predominantly synthesized in the body)
  • Intestinal flora: small amounts of vitamin K are synthesized by intestinal bacteria
  • Mainly diet
  • Intestinal flora: small amounts of vitamin B7, B9, and B12 are synthesized by intestinal bacteria
Absorption
  • Absorption depends on intestinal and pancreatic function
  • Require lipids for absorption
  • Absorption in the intestine via specific luminal transporters
Storage
  • Not stored in the body, except vitamins B9 and B12, which are stored in the liver
    • B9 stores last for approx. 3–4 months.
    • B12 stores last for approx. 3–4 years
Functions
Vitamin deficiency
Vitamin toxicity
  • Oversupplementation → accumulation in adipose tissue → toxic excess
  • Vitamin toxicity occurs more often than with water-soluble vitamins (see respective sections below)
  • Accumulation and subsequent toxicity are very rare (kidneys detect and remove excess in urine).
  • No toxicity has been described for vitamins B1, B2, B5, B7, B9, and B12.

Accumulation and toxicity occur almost exclusively with fat-soluble vitamins.

Fat-soluble vitaminstoggle arrow icon

Overview of fat-soluble vitamins

Name

Active forms Sources Functions Deficiency Toxicity
Vitamin A (retinol)
Vitamin D (calciferol)
  • Cholecalciferol (provitamin D3): exposure to sunlight, fish, milk, plants
  • Ergocalciferol (provitamin D2): plants, fungi, yeast, fortified foods (e.g., milk, cereals, formula)
Vitamin E (tocopherol)
  • Meat, eggs, oils, leafy vegetables
  • Antioxidant
Vitamin K (K1: phytomenadione, phylloquinone, phytonadione; K2: menaquinone)
  • Green vegetables, e.g., broccoli, spinach
  • Synthesized by intestinal flora

Fat-soluble vitamins: The fat cat is in the attic (= “ADEK”).

Vitamin A (retinol)toggle arrow icon

Characteristics

Functions

Retinal is a major component of the retinal pigment rhodopsin in rods, which is necessary for vision, while retinoic acid and retinol are involved mainly in gene transcription and tissue maintenance.

Retinol (vitamin A) nurtures the retinA, acts as an Antioxidant, and can be used for Acne treatment.

Vitamin A deficiency [1]

Vitamin A toxicity

Isotretinoin is highly teratogenic. A negative pregnancy test and two forms of contraception are required before prescribing isoretinoin to women.

Therapeutic uses [3]

Vitamin A is contraindicated in pregnancy (teratogenic): A negative pregnancy test and two forms of contraception must be provided before isotretinoin can be prescribed to women.

Vitamin A should be given to measles patients with vitamin A deficiency to boost their immune system and reduce the risk of complications and mortality, especially in countries where vitamin A deficiency is endemic.

Vitamin D (calciferol)toggle arrow icon

Characteristics

Vitamin D is the only vitamin that the human body can produce entirely on its own!

Functions

Deficiency [6]

Vitamin D toxicity

Therapeutic uses

  • Osteoporosis (prevention and treatment)
  • Paget disease
  • Prenatal supplementation for vegetarian mothers (400 IU per day)

Vitamin E (tocopherol)toggle arrow icon

Characteristics

Functions

Vitamin E deficiency

Vitamin E toxicity

Therapeutic uses

Vitamin K (phytomenadione)toggle arrow icon

Characteristics

  • Synonyms
  • Substance class: naphthoquinones
  • Chemical structure:
    • Vitamin K1
    • Vitamin K2
  • Inactive precursors (provitamins): none
  • Active form: vitamin K hydroquinone
  • Sources
    • Leafy green vegetables (vitamin K1)
    • Eggs, dairy, and meat (vitamin K2)
    • Synthesized in small amounts by intestinal bacteria
  • Transport: via lipoproteins; no specific protein
  • Storage: liver
  • Excretion: bile and urine

Functions

Koagulation requires Vitamin K.

Vitamin K deficiency [10]

Warfarin inhibits the vitamin K-dependent synthesis of clotting factors and proteins.

Vitamin K toxicity

Therapeutic uses

Water-soluble vitaminstoggle arrow icon

Overview water-soluble vitamins
Name Active forms Sources Functions Deficiency Toxicity
Vitamin B1 (thiamine)
  • Whole grain cereals (e.g., whole wheat, brown rice), yeast, pork, legumes
  • No toxicity has been described
Vitamin B2 (riboflavin)
  • Meat, fish, eggs, milk, green vegetables, yeast
Vitamin B3 (niacin, nicotinic acid)
  • Meat (liver), cereals, seeds, legumes
Vitamin B5 (pantothenic acid)
  • No toxicity has been described
Vitamin B6 (pyridoxine)
  • Meat, nuts, whole grains, vegetables

Vitamin B7 (biotin)

  • Eggs, meat, fish, seeds, nuts
  • Cofactor for carboxylases, transcarboxylases, and decarboxylases
  • No toxicity has been described
Vitamin B9 (folate)
  • Green leafy vegetables, dried legumes

Vitamin B12 (cobalamin)

  • Cobamamide
  • Methylcobalamin
  • Meat and dairy products
Vitamin C (ascorbic acid)
  • Fruits: citrus fruits, strawberries, tomatoes
  • Vegetables: potatoes, cabbage, spinach

Vitamin B₁ (thiamine)toggle arrow icon

Characteristics

  • Synonyms: thiamine
  • Inactive precursor (provitamin): none
  • Active form: thiamine pyrophosphate (TPP); activation via intracellular phosphorylation of thiamine
  • Sources: whole-grain cereals (e.g., whole wheat, brown rice), yeast, pork, legumes [12]
  • Resorption: via thiamine transporter-2 (ThTR2)
  • Transport in blood: mainly via blood cells; only ∼10% is free or bound to albumin

Functions

Enzymes thiamine is a cofactor for: Thiamine PATs your Back! (Pyruvate dehydrogenase, Alpha-ketoglutaric acid dehydrogenase, Transketolase, Branched-chain ketoacid dehydrogenase).

Vitamin B1 deficiency [13]

In malnourished or alcohol-dependent patients, always administer thiamine before giving dextrose to decrease the risk of precipitating or exacerbating Wernicke encephalopathy.

Vitamin B1 deficiency causes Ber1Ber1.

Vitamin B₂ (riboflavin)toggle arrow icon

Characteristics

  • Synonyms: riboflavin
  • Substance class: flavins
  • Inactive precursor (provitamin): none
  • Active forms: : flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD)
  • Sources: meat, fish, eggs, milk, green vegetables, yeast[12]
  • Resorption: flavoproteins are cleaved into riboflavin in the intestine
  • Transport in blood: via albumin and immunoglobulins

Active forms of riboFlavin are FMN and FAD.

Functions

Vitamin B2 deficiency [14][15]

The 2 C's of Vitamin B2 deficiency: Corneal vascularization and Cheilitis!

Vitamin B₃ (niacin)toggle arrow icon

Characteristics

Active forms of Niacin are NAD+ and NADP+.

Functions

Vitamin B3 deficiency [12]

3 typical features of severe vitamin B3 Deficiency: Dermatitis, Diarrhea, and Dementia.

Vitamin B3 toxicity

Therapeutic use

Vitamin B₅ (pantothenic acid)toggle arrow icon

Characteristics

Functions

Vitamin B5 deficiency [12]

Deficiency is rare.

Vitamin B5 is pentothenic acid, as in “pente,” the Greek word for “five.”

Vitamin B₆ (pyridoxine)toggle arrow icon

Characteristics

  • Synonyms: pyridoxine
  • Substance class: pyridoxine
  • Inactive precursor (provitamin): none
  • Active form: pyridoxal phosphate (PLP)
  • Sources: nuts, whole grains, vegetables, yeast, meat (esp. liver and poultry) [12]
  • Resorption: cleavage by phosphorylases and subsequent resorption by the intestine
  • Transport in blood: partially free, partially bound to albumin

Functions

Vitamin B6 deficiency [17]

Vitamin B6 toxicity

Although rare, excess pyridoxine can lead to irreversible sensory neuropathy.

Therapeutic use

Vitamin B₇ (biotin)toggle arrow icon

Characteristics

  • Synonyms: biotin
  • Inactive precursor (provitamin): none
  • Active form: biotin
  • Sources
    • Plants (e.g., soy products, nuts), animal products (e.g., liver, egg yolk, dairy products)
    • Small amounts are synthesized by intestinal flora
  • Resorption: pancreatic enzyme biotinidase cleaves protein-bound biotin into free biotin active intestinal resorption
  • Transport in blood: mainly free

Functions

Biotin is a coenzyme in all carboxylase enzyme complexes that are not vitamin K-dependent.

Vitamin B7 deficiency (biotin deficiency)

Biotin binds to avidin, which is found in raw egg whites: Biotin loves avidin!

Vitamin B₉ (folate)toggle arrow icon

Characteristics

  • Synonyms: folic acid, folate
  • Substance class: pteridines
  • Inactive precursor (provitamin): none
  • Active form: tetrahydrofolic acid (THF)
  • Sources
    • Leafy green vegetables, fortified foods (e.g., bread, flour, and cereal)
    • Small amounts are synthesized in intestinal flora
  • Resorption: jejunum via specific transporters
  • Transport in blood: via folate-binding transport proteins
  • Storage: small reserve in the liver (enough for approx. 3–4 months)

Foliage (leafy green vegetables) contains Folate.

Function

Vitamin B9 deficiency [18]

Unlike vitamin B12 deficiency, folate deficiency is not associated with neurologic symptoms.

Therapeutic uses

Vitamin B₁₂ (cobalamin)toggle arrow icon

Characteristics

Vitamin B12 is the only water-soluble vitamin that is stored in the body in significant amounts.

Functions

Vitamin B12 deficiency

Therapeutic uses

  • Prenatal supplementation for vegetarians (2 μg per day)

Vitamin C (ascorbic acid)toggle arrow icon

Characteristics

  • Synonyms: ascorbic acid, ascorbate
  • Substance class: lactones
  • Inactive precursor (provitamin): none
  • Active form: ascorbate
  • Sources: fruits and vegetables
  • Resorption
    • Passive resorption via oral mucosa
    • Active resorption in the intestine (especially jejunum)
  • Transport in blood: mainly free, only very small amounts as dehydroascorbate
  • Storage: no specialized vitamin C stores; high concentrations in organs that require vitamin C as a cofactor (e.g., adrenal gland)

Functions

Think of vitamin C as “absorbic acid” since it promotes the intestinal absorption of iron.

Vitamin C deficiency [12]

Vitamin C deficiency results in sCurvy due to impaired Collagen synthesis.

Vitamin C toxicity

Therapeutic use

  • Supportive treatment for methemoglobinemia: Vitamin C reduces Fe3+ to Fe2+

Referencestoggle arrow icon

  1. Sassan Pazirandeh, MD. Overview of water-soluble vitamins. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/overview-of-water-soluble-vitamins. Last updated: February 7, 2019. Accessed: March 26, 2019.
  2. Pazirandeh S, Burns DL. Overview of vitamin A. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/overview-of-vitamin-a#H10. Last updated: October 26, 2016. Accessed: December 14, 2016.
  3. Ansstas G. Vitamin A Deficiency. In: Griffing GT, Vitamin A Deficiency. New York, NY: WebMD. http://emedicine.medscape.com/article/126004-clinical#b5. Updated: October 12, 2016. Accessed: December 14, 2016.
  4. West KP. Vitamin A deficiency disorders in children and women. Food Nutr Bull. 2003; 24 (4_suppl2): p.S78-S90.doi: 10.1177/15648265030244s204 . | Open in Read by QxMD
  5. D’Souza RM, D’Souza R. Vitamin A for treating measles in children. Cochrane Database Syst Rev. 2001.doi: 10.1002/14651858.cd001479 . | Open in Read by QxMD
  6. Committee on Infectious Diseases. Vitamin A treatment of measles. Pediatrics. 1993; 91 (5).
  7. Drezner MK. Patient education: Vitamin D deficiency (Beyond the Basics). In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/vitamin-d-deficiency-beyond-the-basics#H9. Last updated: June 9, 2015. Accessed: December 14, 2016.
  8. Pazirandeh S, Burns DL. Overview of vitamin E. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/overview-of-vitamin-e#H7. Last updated: October 3, 2016. Accessed: December 12, 2016.
  9. Le T, Bhushan V. First Aid for the USMLE Step 1 2015. McGraw-Hill Education ; 2014
  10. Rosenbloom M. Vitamin Toxicity. In: Mille MA, Vitamin Toxicity. New York, NY: WebMD. https://emedicine.medscape.com/article/819426-overview#showall. Updated: December 26, 2017. Accessed: July 4, 2018.
  11. Pazirandeh S, Burns DL, Lipman TO, Motil KJ, Hoppin AG. Overview of Vitamin K. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/overview-of-vitamin-k. Last updated: April 13, 2016. Accessed: December 14, 2016.
  12. Pazirandeh S, Burns DL. Overview of water-soluble vitamins. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/overview-of-water-soluble-vitamins?source=search_result&search=niacin%20deficiency&selectedTitle=1~32. Last updated: November 9, 2016. Accessed: December 16, 2016.
  13. Nguyen-Khoa D-T. Beriberi (Thiamine Deficiency). In: Khardori R, Beriberi (Thiamine Deficiency). New York, NY: WebMD. http://emedicine.medscape.com/article/116930-overview#showall. Updated: July 1, 2016. Accessed: December 15, 2016.
  14. Gill RS. Riboflavin Deficiency. In: Griffing GT, Riboflavin Deficiency. New York, NY: WebMD. http://emedicine.medscape.com/article/125193-overview. Updated: August 9, 2016. Accessed: December 16, 2016.
  15. Lane M, Alfrey CP Jr. The anemia of human riboflavin deficiency. Blood. 1965; 25: p.432-42.
  16. Richard L. Dunbar and Joel M. Gelfand. Seeing red: flushing out instigators of niacin-associated skin toxicity. Journal of Clinical Investigation. 2010.
  17. Frye RE. Pyridoxine Deficiency. In: Griffing GT, Pyridoxine Deficiency. New York, NY: WebMD. http://emedicine.medscape.com/article/124947-overview#a4. Updated: September 15, 2016. Accessed: December 15, 2016.
  18. Schrier SL. Clinical Manifestations and Diagnosis of Vitamin B12 and Folate Deficiency. In: Post TW, ed. UpToDate. Waltham, MA: UpToDate. https://www.uptodate.com/contents/clinical-manifestations-and-diagnosis-of-vitamin-b12-and-folate-deficiency. Last updated: July 15, 2016. Accessed: December 15, 2016.
  19. Morkbak AL, Poulsen SS, Nexo E. Haptocorrin in humans. Clinical Chemical Laboratory Medicine. 2007; 45 (12).doi: 10.1515/cclm.2007.343 . | Open in Read by QxMD
  20. E. Wu. Peter. Medication Effects on Fecal Occult Blood Testing. JAMA Internal Medicine. 2019; 179 (4): p.591.doi: 10.1001/jamainternmed.2019.0132 . | Open in Read by QxMD
  21. Alhatem A, Cai D. Behind the Skin: A Rare Case of Scurvy-Associated Megaloblastic Anemia. Clinical Medicine Insights: Case Reports. 2019; 12: p.117954761984903.doi: 10.1177/1179547619849036 . | Open in Read by QxMD
  22. Vitamin B5 (Pantothenic acid). http://umm.edu/health/medical/altmed/supplement/vitamin-b5-pantothenic-acid. Updated: July 16, 2013. Accessed: December 15, 2016.

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