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, otherwise known as ergocalciferol, and D3, which is also known as cholecalciferol.

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The majority of the vitamin D needed by the body is obtained from sun exposure, although some food products also contain vitamin D. Dietary supplements are also available to help increase intake of vitamin D. 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 of vitamin D is to support the absorption of calcium from dietary sources into the bloodstream and prevent calcium reabsorption in the kidneys. This helps to direct calcium towards osteoclasts and osteoblasts to strengthen the bones, as well as to lessen the risk of hypocalcemic tetany associated with 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, vitamin D deficiency can result in rickets in children, as well as osteomalacia and osteoporosis in adults.

The most common cause for vitamin D deficiency is a lack of sunlight exposure, which may be coupled with poor dietary intake of vitamin D. 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 who 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 sources for the infant must be obtained from the mother’s milk. This often leads to deficiency, although it is dependent on the mother’s vitamin D levels.

The effect of sunlight

When ultraviolet (UV) rays from sunlight come into contact with human skin, 7-dehydrocholesterol reacts and initiates the production of vitamin D3. The ideal wavelength of UV 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 UV light in this wavelength, such as in the arctic circles, are more likely to be at risk of vitamin D 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 the 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 UV 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 are 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 UV rays is lower.


Further Reading

Last Updated: Jan 13, 2023

Yolanda Smith

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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