• Clinical science



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 vitamins

Fat-soluble vitamins Water-soluble vitamins
  • Mainly diet
  • Intestinal flora: small amounts of vitamin K are synthesized by intestinal bacteria
  • Vitamin D is predominantly synthesized in the body
  • Mainly diet
  • Intestinal flora: small amounts of vitamin B7, B9, and B12 are synthesized by intestinal bacteria
  • Absorption depends on intestinal and pancreatic function
  • Require lipids for absorption
  • Absorption in the intestine via specific luminal transporters
  • Not stored in the body, except vitamins B9 and B12, which are stored in the liver
    • Hepatic stores of B9 last for approx. 3-4 months, whereas hepatic stores of B12 last for approx. 3-4 years.
  • Function primarily as
  • Excess accumulation is possible (due to oversupplementation) → toxicity
  • Accumulation and subsequent toxicity are exceedingly rare
  • 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 vitamins


Active forms Sources Functions Deficiency Toxicity
Vitamin A (retinol)
  • Retinal
  • Retinoic acid
Vitamin D (calciferol)
  • Cholecalciferol (provitamin D3): fish, milk, plants, exposure to sunlight
  • Ergocalciferol (provitamin D2): plants, fungi, yeast, fortified foods (e.g., milk, cereals, formula)
  • Calcium and phosphate homeostasis
  • ↑ Absorption of Ca2+ and PO43- in the intestine
  • ↑ Bone resorption at high levels
  • ↑ Bone mineralization at low levels
Vitamin E (tocopherol)
  • Meat, eggs, oils, leafy vegetables
  • Antioxidant
Vitamin K (phytomenadione)
  • Green vegetables, broccoli, spinach
  • Synthesized by intestinal flora

To remember the fat-soluble vitamins, think: “The fat cat is in the ADEK” (pronounced “attic”).

Vitamin A (retinol)


  • Synonyms: retinol
  • Active forms: retinal, retinoic acid
  • Sources
    • Plant sources; : as inactive provitamin (esp. beta-carotene) in yellow and leafy vegetables, e.g., spinach, kale, carrots
    • Animal sources: in storage form; , e.g., in liver, kidney, fish, eggs, butter
  • Storage: : in hepatic cells; (Ito cells) within the perisinusoidal space (of Disse)


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) for your retinA.

Vitamin A deficiency [1]

Vitamin A toxicity

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

Therapeutic uses

Vitamin A should be given to patients with measles 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)


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


Vitamin D deficiency [6]

Vitamin D toxicity

Therapeutic uses

Vitamin E (tocopherol)



Vitamin E deficiency

Vitamin E toxicity

Therapeutic uses

Vitamin K (phytomenadione)



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

Koagulation isn't the same without Vitamin K!

Vitamin K deficiency [10]

Vitamin K toxicity

Therapeutic uses

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
  • No toxicity has been described
Vitamin B3 (niacin)
  • 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
  • No toxicity has been described

Vitamin B12 (cobalamin)

  • Cobamamide
  • Methylcobalamin
  • Meat and dairy products
  • No toxicity has been described
Vitamin C (ascorbic acid)
  • Citrus fruits, strawberries, tomatoes, potatoes, cabbage, spinach

Vitamin B₁ (thiamine)



To remember the enzymes that thiamine is a cofactor for, think: “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.

References: [15]

Vitamin B₂ (riboflavin)



Vitamin B2 deficiency [16]

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

References: [17]

Vitamin B₃ (niacin)



Vitamin B3 deficiency [12]

The four D's of Pellagra: Dermatitis, Diarrhea, Dementia, Death

Vitamin B3 toxicity

Therapeutic use

Vitamin B₅ (pantothenic acid)



Vitamin B5 deficiency [12]

Vitamin B5 is pento-thenic acid!

Vitamin B₆ (pyridoxine)



Vitamin B6 deficiency [19]

Vitamin B6 toxicity

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

Therapeutic use

Vitamin B₇ (biotin)


  • Synonyms: