Science & Technology in Action

7th Edition

Vitamin D and Parathyroid Hormone


This lesson discusses vitamin D and its function in maintaining human health. Its sources are discussed and its various vitamers are identified. The impact of a deficiency and the conditions resulting from deficiency, such as rickets, are discussed. RDAs are also explained. The functions of parathyroid hormone, the symptoms of hyperparathyroidism and hypoparathyroidism, the effect of chronic dialysis on calcium and parathyroid hormone levels and the use of Doxercalciferol as a control are also discussed.
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Lesson excerpt

Vitamins are organic compounds that are essential (or vital) for the body in tiny amounts. The name vitamin was originally spelt with an "e" at the end, being derived from combination of the words vital and amine (since the first ones to be identified belonged to this chemical family). However, it was later found that not all vitamins are amines, but by then the name vitamin(e) was so well established that it was too late to change it, so the "e" at the end was dropped in order to de-emphasise the amine part. Vitamin D can be photosynthesised by humans and other organisms, so it is not strictly a vitamin (vital food substance) unless one is unable to synthesise it due to lack of exposure to sufficient sunlight. For this reason it is often called 'the sunshine vitamin'.

In 1913 Elmer McCollum and Marguerite Davies discovered a compound in cod liver oil which came to be called vitamin A. Edward Mellanby, an English vet, noticed that dogs fed cod liver oil did not develop rickets so he concluded, incorrectly, that vitamin A prevents rickets. In 1921 E. McCollum did further research using cod liver oil where the vitamin A had been destroyed. It still prevented dogs developing rickets so he concluded that anothervitamin was at work. He called it vitamin D as it was the fourthvitamin to be discovered.

You may have heard of vitamins E and K and so might wonder what has happened to vitamins F, G, H, I and J. The simple explanation is that substances which at the time they were discovered were thought to be vitamins, and had been allocated letters in order of their discovery, were later found not to be vitamins. Thus F to J disappeared from the lists leaving vitamin K as the last one.

Vitamers are different forms of a particular vitamin. Vitamin D has five vitamers [D1to D5]. Of these five, two are important: Vitamin D2 which is called ergocalciferol and is synthesised in invertebrates, fungi and plants, and Vitamin D3, cholecalciferol which is synthesised in vertebrates: both in response to ultra violet light (UV). Vitamin D usually refers to both the D2 and D3 vitamers.

Vitamin D is fat soluble and is stored in the liver as the pro-hormone calcidiol. Vitamin C, on the other hand, is water soluble and cannot be stored, so any excess is excreted in the urine.

True or False?

  1. Vitamin D causes rickets. false
  2. Vitamin D is photosynthesised in skin. true
  3. Endocrine glands have ducts. false
  4. Synthetic compounds are normally found in nature. false
  5. Apoptosis is a form of cell division. false
  6. Calcium and vitamin D are used to treat osteoporosis. true
  7. Doxercalciferol is a synthetic precursor of vitamin D. true
  8. Differentiation is the change in form and function of a cell as it matures. true
  9. Doxercalciferol is used to reduce levels of parathyroid hormone in the blood of patients undergoing chronic renal dialysis. true
  10. Vitamin D is stored in the liver as the prohormone calcitriol. false
  11. Rickets still occur in Ireland. true