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What is Vitamin D?

Updated: Jun 28, 2020

Vitamin D is a fat-soluble nutrient required for numerous roles in the body, it’s often referred to as the ‘sunshine vitamin’. Despite its name, it’s not technically a vitamin and is actually considered a pro-hormone instead, as unlike other vitamins, we are able to produce it ourselves so it doesn’t necessarily have to be consumed through the diet.

Why do we need it?

Bone Health

Vitamin D is essential in the developing and maintaining of healthy bones due to the role it plays in absorbing other important nutrients such as calcium and phosphate, both of which are critical for bone health. Studies have shown there to be a significantly positive correlation between vitamin D status and bone mineral density (1), highlighting its importance in relevance in reducing the risk of bone fractures, particularly important for athletes and the elderly population who are at a higher risk.

Immune Health

The importance of vitamin D on immune health has been demonstrated across numerous studies. For instance, He and colleagues (2) found that individuals with deficient levels of Vitamin D, not only suffered from more frequent upper respiratory illnesses, but the severity and duration of these were also increased when compared to those with optimal levels of vitamin D. Why is this? Well, it appears that deficient levels of vitamin D correlate with reduced levels of salivary SIgA, in addition to lower levels of monocyte and lymphocyte cytokine production (these cells play a major role in your immune health).

Muscle Function

So can vitamin D also play a role in muscle function? Well, there is a growing body of evidence supporting this claim, particularly in elderly populations where vitamin D levels are typically sub-optimal. A study conducted by Agergaard et al. (3) found that following 12 weeks of supplementation, elderly men saw improvements in muscle quality, which in-turn may have the potential to improve their quality of life and independence.

Regarding athletic performance, Owens et al. (4) showed that by elevating an individual’s vitamin D status, they incurred a positive effect on muscle force recovery following a bout of damaging eccentric exercise. Additionally, Close and colleagues (5) demonstrated that following an 8-week vitamin D supplementation protocol, individuals were shown to significantly improve their 10-metre sprint and vertical jump performances. However, the research on young athletic populations is limited, with some reporting positive effects and others non at all. What seems likely though, is that individuals with low levels of vitamin D will observe health and performance benefits should they address the issue and improve their vitamin D status.

Where do we get it from?

The Sun


We know there are clear benefits to having optimal levels of vitamin D, so how can this be achieved? The main source of vitamin D comes from the sun. As the UVB rays hit our skin, a sterol called 7-dehydrocholesterol is converted to vitamin D3, where after undergoing numerous different processes via the liver and kidneys, is converted to an active form where it can be utilised by the body. However, modern society has limited the time most people get to spend in the sun, and even when we do get the opportunity, how much of our skin are we really exposing? Probably not much. As result, vitamin D deficiency has become a major global public health issue.



We are able to acquire vitamin D from the diet, however, this is only in small quantities

(~10% of the required amount). Some vitamin D containing foods include oily fish, liver, eggs, milk, mushrooms and fortified food products.



Supplementing with vitamin D3 is an easy way to ensure you reach optimal levels (providing the product is from a safe and reliable source). In terms of dosage, the recommended daily intake is currently set at 400 IU in the UK, however, this is up for debate. Research suggests in order to maintain optimal levels in certain individuals (6), up to 4,000 IU may be required, 10 times the recommended amount! Remember, before supplementing, it would be well advised to speak with your doctor and have your levels checked.


So, to summarise, we now understand the importance of vitamin D in regards to our bones, immune health and muscle functioning. We know that we get the majority of our intake from the sun, small amounts from food, and if necessary, supplementation may be advised (after consulting with your doctor!).

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