What is Vitamin D

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what is vitamin D

What is vitamin D?

Vitamin D has been well known for its crucial roles in bone health, supporting the immune system, and regulating mood and brain function

Despite efforts from health organizations to increase awareness of low vitamin D, a significant proportion of the Western population is still vitamin deficient.

What does Vitamin D do?

Vitamin D is made from our skin when it is exposed to UV sunlight.

It regulates the level of calcium and phosphate mainly by adjusting the excretion and absorption of this mineral in the kidney and from the intestine.

However, the recent finding of more vitamin D receptors in numerous tissues has led medical scientists to think about exploration into so many other conditions.

Who are at risk of developing Vitamin D deficiency? 

Breastfed infants

Human milk contains around 50IU/L of vitamin D which is not enough daily intake for the infant. Some guidelines suggest maternal vitamin D supplements can increase the infant’s vitamin D level to 2000IU/L.

Older adults

Apart from usual dietary factors, again, the skin of older adults 

Limited sun exposure

Not only people from Western countries, people who wear long robes or head coverings for religious reasons and the use of sunscreen also limit the production of vitamin D in the body.

People with dark skin

People with darker skin tones have more melanin which reduces the skin’s ability to produce vitamin D.

People with absorption problems

People who had conditions in the intestines such as coeliac disease, and people who had surgery on the intestine or stomach are also at risk of developing vitamin D deficiency.

Symptoms of low vitamin D

The symptoms of low vitamin D are usually non-specific, especially in the short term. The most commonly reported symptoms are

  • Tiredness
  • Low mood
  • Difficult concentrating
  • Muscle weakness
  • Generalized aches and pain
  • Hair loss

Medical conditions associated with and benefits of Vitamin D

Osteomalacia and osteoporosis

Calcium is like a brick in bone construction, without vitamin D, our body can not get enough calcium thus causing structural problems in our bones.

Vitamin D has been shown to reduce the risk of fractures in adults and older people.

It also increases muscle strength by preserving muscle fiber thus preventing falls in elderly people.


Vitamin D is used alone or together with steroids in the treatment of psoriasis.

Psoriasis is a skin condition where there are abnormal immune reactions and the proliferation of skin cells.

Vitamin D helps treat psoriasis by maintaining the turnover of the skin barrier thus reducing the proliferation of skin cells.

Obesity and Diabetes

Vitamin D can stimulate insulin secretion by activating vitamin D receptors in the pancreas thus improving overall diabetes control.

In obesity, studies have found using vitamin D along with conventional methods improves overall outcomes.

However, there are still no official guidelines supporting the use of vitamin D in these conditions.

Cardiovascular Health

Vitamin D regulates the Renin Angiotensin Aldosterone system, which is important in controlling the blood pressure and the growth of cells inside the blood vessel and therefore could prevent atherosclerosis ( narrowing of blood vessels in the body).

Following this theory, studies have shown vitamin D reduces death from ischaemic heart disease, and stroke and could potentially prevent the attacks.l 


Animal studies suggest that vitamin D slows the production of cancer cells and enhances the breakdown of a tumor. The actual real use of human subjects and the evidence are still limited. 

Vitamin D2 and D3 what is the difference?

Vitamin D2 is called ergocalciferol and D3 is called Cholecalciferol. D2 is mostly found in plant-based food sources where as D3 is mainly from animal-based foods. For this reason, vitamin D2 is cheaper to produce in supplements.

The main difference is D3 is more stable and can withstand metabolism in the liver than D2 thus twice more effective in raising the level of vitamin D.

Natural sources of vitamin D

Vitamin D3

  • Oily fish
  • Red meat
  • Liver
  • Egg yolk
  • Cereals 

Vitamin D2

  • Mushrooms
  • Fortified foods – cereals
  • Dietary supplements

How much vitamin D do you need per day?

The amount of vitamin D required depends on age. The recommended amount in international units are

  • Birth to 12 months: 400 IU
  • Children 1-13 years: 600 IU
  • Teens 14-18 years: 600 IU
  • Adults 19-70 years: 600 IU
  • Adults 71 years and older: 800 IU
  • Pregnant and breastfeeding women: 600 IU

Vitamin D toxicity

Generally, all forms of vitamin Ds are very safe, and side effects are rare and usually only happen with long-term use.

The main consequence is caused by the build-up of calcium in the blood called hypercalcemia.

Taking 60,000 IU a day of vitamin D for several months has been shown to cause toxicity. To check out more about vitamins and toxicity, this Vitamin Toxicity has a comprehensive explanation.



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Dr Tun Min is s GP working in NHS UK and writing articles about supplements and vitamins based on personal clinical experience and clinical research.

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