What is Vitamin D?

Vitamin D is a fat-soluble vitamin that is important for strong bones, muscles and general good health. There are two main forms of vitamin D, which are D2 known as ergocalciferol and D3 known as cholecalciferol.

The majority of the vitamin D needed by the body is obtained from sun exposure, although some food also contains vitamin D and dietary supplements can also help to increase intake. The vitamin D obtained from these sources is biologically inert and must undergo hydroxylation within the body in order to offer a functional use.


Several organ systems benefit from the presence of vitamin D in the body. The primary role is to support the absorption of calcium from dietary sources into the blood threat and prevent calcium reabsorption in the kidneys. This effect helps to direct calcium towards osteoclasts and osteoblasts to strengthen the bones, as well as to lessen the risk of hypocalcemic tetany associate to high calcium intake.


When there is an inadequate level of vitamin D in the body, the effects tend to appear over the long-term, notably causing bones to become thin and brittle. As a result, it can result in rickets in children, as well as osteomalacia and osteoporosis in adults.

The most common cause for vitamin D deficiency is lack of sunlight exposure, which may be coupled with poor dietary sources. In particular, populations that do not get adequate sun exposure are at risk of deficiency, such as the elderly, full-time office workers and people that cover their skin whilst outside or live in low-sunlight areas.

It is common for pregnant and breastfeeding mothers to be deficient in vitamin D, as well as the infant. Direct sunlight is not recommended for newborn skin and, as a result, the majority of vitamin D required must be obtained from mother’s milk. This often leads to deficiency, although is dependant on the mother’s level of vitamin D.

The Effect of Sunlight

When ultraviolet rays from sunlight come into contact with human skin, 7-dehydrocholesterol reacts and initiates the production of vitamin D3. The ideal wavelength of the ultraviolet light is 295-297 nm, which occurs more commonly in tropical areas and during spring and summer of seasonal climates.

People living in areas that do not receive large amounts of ultraviolet light in this wavelength, such as in the arctic circles, are more likely to be at risk of vitamin deficiency. People with darker toned skin require higher quantities of sunlight in order to spark the production of vitamin D and, as a result, are commonly affected by deficiency.

Despite the positive effect of sunlight on vitamin D production in the body, recommendations to increase sunlight exposure are given with caution due to the risk of skin cancer. Sunscreen inhibits the effects of sunlight on the skin, thus offering protection from harmful ultraviolet rays, but also limiting the production of vitamin D.

Ideally, short 10-15 minute periods of sunlight exposure with direct contact to the face and arms is recommended. However, skin protection is needed for longer periods of time and adequate sun safety practices should always be followed.

Dietary Sources

Vitamin D can be found in some food sources such as fish and eggs, as well as some food products that contain added vitamin D, but obtaining adequate levels from natural food sources is difficult.

Vitamin D supplements are recommended when people are at risk of deficiency or have low levels of the vitamin. Some populations require supplementation of vitamin D during the winter season, when access to sunlight with ultraviolet rays is lower.


  • http://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
  • http://www.nhs.uk/Conditions/vitamins-minerals/Pages/Vitamin-D.aspx
  • http://www.betterhealth.vic.gov.au/bhcv2/bhcarticles.nsf/pages/Vitamin_D?open
  • http://www.hsph.harvard.edu/nutritionsource/vitamin-d/
  • https://www.nrv.gov.au/nutrients/vitamin-d

Further Reading

  • All Vitamin D Content
  • Vitamin D Biochemistry
  • Vitamin D Deficiency
  • Vitamin D Overdose
  • Vitamin D Supplementation

Last Updated: Aug 23, 2018

Written by

Yolanda Smith

Yolanda graduated with a Bachelor of Pharmacy at the University of South Australia and has experience working in both Australia and Italy. She is passionate about how medicine, diet and lifestyle affect our health and enjoys helping people understand this. In her spare time she loves to explore the world and learn about new cultures and languages.

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