26. Fat-soluble vitamins

Page created on February 27, 2019. Last updated on June 17, 2020 at 10:53

I include the updated values for RDA as presented by the Institute of Medicine, the government of USA. The values taught by the university are marked in italic.

Vitamin A

Name: Vitamin A, retinol or axerophthol

The name axerophthol comes from the Greek words for not (“a”), dry (“xero”) and eye (“opthal”), meaning that this is the vitamin that prevents dry eyes. This name is not widely used anymore though.

Function in the body:

Vitamin A is an essential cofactor for the normal function of rhodopsin in the rods, which is that which senses light in the retina.

It’s a transcription factor that binds to intracellular RAR and RXR receptor to regulate transcription. Especially important in embryonic development.

It’s essential for normal function of the epithelium.

β-carotene is a precursor that the body can convert into vitamin A.

Store in the body: In hepatic stellate (Ito) cells

Typical sources: Yellowish fruits and vegetables like pumpkin, tomato and carrots. Liver.

RDA: 900 µg as retinol, 12 times that for β-carotene.

Typical causes for deficiency:

Regions where rice is main source of calories. Increased requirement during pregnancy, lactation or fever. Chronic diarrhoea. Liver cirrhosis. Lack of bile (which is necessary for absorption).

Hypovitaminosis condition:

Xerophthalmia (dry eyes) and increased susceptibility for infections which may cause blindness. Nyctalopia (night blindness). Dry and hyperkeratotic skin. Squamous metaplasia in the upper respiratory tract. Abnormal embryonic development.

Hypervitaminosis condition:

Acute intoxication leads to increased intracranial pressure. Chronic intoxication causes mildly increased ICP, liver damage, joint pain and hair loss.

Vitamin A hypervitaminosis is highly teratogenic. High dose vitamin A (or vitamin A-like compounds) are used as pharmaceutical drugs for treatment of acne.

Vitamin D

Name: Vitamin D, 1,25-dihydroxycholecalciferol or calcitriol

Function in the body:

A steroid hormone that binds to intracellular steroid receptors to regulate gene transcription. Increases Ca²⁺ and PO₄^3- absorption from the GI tract, enhances Ca²⁺ resorption from bone and enhances Ca²⁺ reabsorption in the kidney. Increases serum Ca²⁺ and enhances bone formation in the long run.

Store in the body: In liver and adipose tissue. Enough for 1 year.

Typical sources:

Vitamin D can be synthesized in the body. Cholesterol is converted into cholecalciferol in skin exposed to UV radiation. Cholecalciferol is converted into 25-hydroxycholecalciferol in the liver. 25-hydroxycholecalciferol is converted into 1,25-dihydroxycholechalciferol in the kidney. The last step requires PTH (parathyroid hormone).

It’s said that 30 minutes of sun exposure while in a t-shirt is enough to cover the vitamin D requirement for 1 day.

Cholecalciferol is present in fish, meat and milk.

RDA: 15 µg (10 µg)

Typical causes for deficiency:

Lack of skin exposure to sunlight. Chronic diarrhoea. Lack of bile. Liver or renal failure. Lack of PTH. Rapid growth, pregnancy, lactation.

Hypovitaminosis condition:

Stunted growth and bone deformities, called rickets in children and osteomalacia in adults. Hypocalcaemia. Caries. Bone pain.

Rickets: Muscle weakness. Soft skull. Bell-shaped chest. Widening of wrist. Bow legs. Rickety rosary.

Osteomalacia: Softening of bones. Proximal muscle weakness. Fragile bones.

Hypervitaminosis condition:

Hypercalcaemia. Metastatic calcification.

Vitamin E

Name: Vitamin E or tocopherol

Function in the body:

Vitamin E is an antioxidant. It also stabilizes membranes of erythrocytes, the nervous system and of muscles. It may play a role in fertility. It may prevent cancer and atherosclerosis, but that’s not completely proven.

Store in the body: In adipose tissue.

Typical sources: Nuts and seeds

RDA: 15 mg (10-30 mg)

Typical causes for deficiency:

Lack of bile. Chronic fasting. Increased requirement in rapid growth and preterm infants with IRDS. Deficient when there is deficiency of betalipoproteins, which carries it in the plasma.

Note that vitamin E is stored, so a simple biliary obstruction would not cause a deficiency as these obstructions are usually resolved before the stores are depleted.

Hypovitaminosis condition:

Haemolytic anaemia. Increased risk for infections. Muscle weakness.

Hypervitaminosis condition:

Hepatosplenomegaly, jaundice, ascites. Competes for vitamin K absorption and therefore causes vitamin K hypovitaminosis if overdosed.

Vitamin K

Name: Vitamin K or phylloquinone

Function in the body:

Essential for the synthesis of clotting factor II, VII, IX, X and anticoagulant factors protein C and S.

See pharmacology 2 for more details.

Store in the body: Not significant (only for a week)

Typical sources: Produced by enteric bacteria

RDA: 120 µg

Typical causes for deficiency:

Infants are regularly deficient, as they’ve not had time to build up their stores (their gut is sterile until birth), and the breast milk doesn’t contain vitamin K. Infants therefore receive 0.5-1 mg vitamin K intramuscularly as prophylaxis to prevent vitamin K deficiency bleeding (VKDB).

Chronic diarrhoea. Lack of bile. Coumarins (warfarin).

Hypovitaminosis condition:

Coagulopathy. Prolonged prothrombin time.

Hypervitaminosis condition:

Increased risk of thrombosis. Because vitamin K competes with bilirubin for albumin-binding can increased levels of vitamin K displace free, unconjugated bilirubin in the serum, causing kernicterus.