What is Vitamin D?

Vitamin D is often advertised as being ‘the vitamin for strong bones’, and regularly makes an appearance in dairy adverts- but how does it help our bones? This article...

Vitamin D is often advertised as being ‘the vitamin for strong bones’, and regularly makes an appearance in dairy adverts- but how does it help our bones? This article will explore what vitamin D is, how it benefits us, and how we can obtain enough to maintain a healthy body.


Vitamin D

Vitamin D is not actually a vitamin by definition, because our body is able to synthesise it in sufficient quantities when exposed to sunlight. It is also sometimes called a hormone, which is closer to the truth, but still not true. The term ‘Vitamin D’ refers to a fat soluble group of compounds called secosteroids, which exist in 5 forms; D1, D2, D3, D4 and D5. When talking about vitamin D, people usually are referring to vitamin D2 and/or D3, which are the 2 major forms and the most biologically relevant for humans.


Vitamin D in the Body

The full extent of vitamin D in the body is not completely understood as it is involved with a large number of important biological functions. It has been shown that low levels of circulating vitamin D in the body is directly related to an increased risk of mortality by a variety of causes from heart disease to cancer.

Bone Health

We know that vitamin D is strongly linked to bone health, and a severe deficiency of vitamin D can lead to rickets, which is the softening and weakening of bones. Vitamin D in needed to absorb calcium in the intestines, and helps the body to regulate the levels of many minerals in the body, including calcium. Sufficient levels of vitamin D are required to prevent the body reabsorbing calcium from the bones which makes them porous. This means that vitamin D alone cannot strengthen bones, as it works closely alongside calcium.

Immune System

Vitamin D interacts with a number of immune cells in the body, in-particularly T-cells. T-cells are important for recognising foreign cells, such as virus cells, and destroying them. Vitamin D promotes the activity of these cells, which allows the body to rapidly identify and destroy any threats.


Sources of Vitamin D

Vitamin D is most abundant in dairy, fish and eggs, with 100ml of whole milk providing approximately 10.5% your RDA (42.6IU). This is usually in the form of D3, whereas plant sources (such as mushrooms) are usually in the form of D2. Animal sources of vitamin D (vitamin D3) are better than vegetable sources (vitamin D2) as our body can utilize vitamin D3 as it is, but must convert vitamin D2 into D3 before we can use it. This process is not 100% efficient, and so 10mg of vitamin D2 will not equate to 10mg of vitamin D3.

Humans are also capable of producing vitamin D3 in their skin when in contact with sunlight. It is possible, that when conditions are right humans are able to obtain all the vitamin D they need just from sun exposure, although, with modern lifestyles this is unlikely – especially in the winter months.


Vitamin D Requirements & Dosages

In Europe the RDA for vitamin D is 250 µg/day (10,000 IU) for adults, and this is usually obtained from a combination of diet, and synthesis  in the skin when exposed to sunlight. This makes mild deficiency relatively common in winter months when there is less sunlight and people spend less time outside. Deficiency of vitamin D can be difficult to recognise, and include a weak immune system, impaired cognitive function and  an increased risk of cardiovascular disease; which are difficult to identify.

It is also possible to overdose on vitamin D, although this is difficult to do and its occurrence is rare. Excessive vitamin D causes hypercalcemia, and this can cause weakness, vomiting, insomnia and can lead to renal failure.



Vitamin D plays a very complex and essential role in human health, and affects a number of biological systems. It works closely alongside calcium to ensure strong bones, but also is needed to regulate a number of minerals in the body, and supports the immune systems. The best dietary sources of vitamin D are animal sources (such as milk and eggs), but it can also be found in some vegetables. When exposed to sunlight we are able to produce vitamin D in our skin, which reduces the importance of obtaining vitamin D form our diet, especially during the summer.

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