What Is Vitamin D?
Vitamin D is known as ‘The Sunshine Vitamin’ and is well associated with the sun. This is because our bodies can synthesise Vitamin D when our skin is exposed to direct sunlight. This reaction is due to a cholesterol-like compound called 7-DHC in our skin reacting with ultraviolet rays.
Vitamin D is technically better classified as a secosteroid hormone than a vitamin in terms of it’s use by the body. Vitamin D is the collective name for the compounds cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2). These compounds are converted by the liver into a hormone called calcidiol. Some of this calcidiol is then converted by the kidneys in to a compound with a similar name called calcitriol. Calcitriol carries out many functions in the body. 
We can also get Vitamin D from certain food sources, although foods containing high levels of Vitamin D are not common. It is however, very important for our health to ensure that we have adequate stores of Vitamin D.
How The Body Uses Vitamin D
There are many benefits that Vitamin D can provide us, so let’s have a look at four of the most important functions of it in the body.
1. Teeth & Bone Health
One of Vitamin D’s most vital bodily functions is providing us with the foundations for healthy teeth and bones. It has been long known that severe Vitamin D deficiencies causes rickets in children and osteomalacia (softening of the bones) in adults. 
As we discussed in our introduction, Vitamin D is converted in our bodies into a compound called Calcitriol. Calcitriol is used by the body to regulate blood calcium levels, and to ensure that there is enough calcium and phosphorus present to maintain healthy teeth and bones. This process also relies on Vitamin K, which aids this regulation. 
The connection between Calcium and bone health is well documented and spoken about amongst the general public. Less commonly spoken of however, is how Vitamin D comes in to this equation. Vitamin D stimulates calcium and phosphate absorption by the intestine, and thus, regulates bone metabolism. 
Because of this relationship between Vitamin D and bone health, and some research has even suggested that Vitamin D supplementation can help reduce the frequency of fractures. 
2. Immune System Support
Vitamin D also plays a key role in supporting our immune systems. Vitamin D receptors are present in many cell types including various immune cells including T cells, B cells and monocytes. 
This relationship with immune cells is important, and reveals that Vitamin D plays a key role in fighting infections. Vitamin D has been shown to ‘trigger the production of antimicrobial peptides with a direct pathogen-killing capacity’.  It is easy to see how a Vitamin D deficiency could then lead to a decreased immune response, and underscores the importance of maintaining healthy levels in our systems.
This immune support aspect of Vitamin D also plays a crucial role in maintaining a healthy microbiome in the digestive system. “Vitamin D can impact the function of virtually every cell in the gut by binding to its intracellular receptor. Vitamin D ensures an appropriate level of antimicrobial peptides in the mucus and maintains epithelial integrity by reinforcing intercellular junctions.” 
Maintaining healthy levels of Vitamin D in the body is essential for a healthy and fully functioning immune system. In a review of 82 different studies, it was found that Vitamin D supplementation helped to protect against common cold virus, though this effect was more pronounced in individuals who previously had a Vitamin D deficiency before supplementation began.  Similarly, it’s there has been exciting research that has suggested that Vitamin D could even be effective at reducing the severity of COVID-19 symptoms. 
3. Mood Regulation
Could ‘The Sunshine Vitamin’ brighten up our lives by improving our moods too? Vitamin D may even play an active role in regulating certain neurotransmitters, and can help in improving our moods. Whilst the data is not conclusive, some studies have found that a Vitamin D deficiency can lead to depression and brain fog. 
It stands to reason that in cases of a clinically significant deficiency, supplementation may provide a mood enhancing effect by through an improvement to baseline health. This would provide a feeling of wellness and help to reduce the severity of depression. However, there could be other, more complex reasons for Vitamin D’s mood improving effects.
Vitamin D has actually been studied for it’s potential as an anti-depressant. It has been hypothesized that a potential role of Vitamin D in treating depression is through the regulation of calcium at N-methyl-D-aspartate (NMDA) receptor sites in the brain.  Although this is a complex topic worthy of it’s own article, it is worth noting that NMDA receptor sites are regarded as an important research area for Major Depressive Disorder and it’s treatments. 
Regardless of the mechanism, Vitamin D may offer potential relief of depressive symptoms. A 2018 study concluded that “vitamin D deficiency may be associated with an increased risk or severity of depression. Supplementation of vitamin D may confer protection for depressed patients.” 
4. Cancer Prevention
Whilst not a replacement for conventional cancer treatments, Vitamin D has shown clinical potential for it’s anti-tumour activity. Some research has found that Vitamin D may play a key role in the prevention and treatment of certain cancers. 
A 2006 study by Tomasz Beer & Anne Myrthue states “Calcitriol, the principal active metabolite of vitamin D and a naturally occurring hormone, showed significant antineoplastic activity in pre-clinical models of prostate cancer and many other tumour types.” 
Although these results has been observed and measured, the mechanisms by which Calcitriol may exert this influence are not yet conclusively understood. However, research has been trying to answer these questions.
“Several mechanisms of activity have been proposed. These include inhibition of proliferation associated with cell cycle arrest and, in some models, differentiation, reduction in invasiveness and angiogenesis, and induction of apoptosis.”
–Beer TM, Myrthue A. Calcitriol In Cancer Treatment: From The Lab To The Clinic 
Whilst Vitamin D alone may not be enough to singlehandedly beat cancer, the fact that it seems to be a potent and natural weapon in the toolkit against cancer makes it a very important compound. One study even found that ‘Vitamin D supplementation reduced the risk of cancer death by 16%’ compared to placebo and non-treatment groups. 
These studies show exciting promise, but Cancer research is an ever evolving field. Those who have a cancer diagnosis should speak to their specialist healthcare advisor about food supplementation of any kind.
Dosage For Supplementation
Vitamin D plays an essential role in some of the most important biological functions in our bodies, so getting the correct amount is key. As such, Vitamin D is one of the most talked about vitamins to supplement with today. This is especially true in Winter, where it is well known that our Vitamin D levels generally decline for those living on latitudes with very short days during the Winter months. We may benefit from Vitamin D supplementation, especially through the Winter months. If you are considering supplementing with Vitamin D, it is important to ensure you are taking the correct dose.
The recommended daily dosage for Vitamin D3 supplementation is between and 10µg to 100µg per day. Sometimes, the dosage of Vitamin D is presented in International Units (IU), and doses up to a maximum of 4000 IU per day are considered safe. It is safe to take a Vitamin D supplement every day if the doses remain at or below this threshold.  
It is however, possible to take too much vitamin D in circumstances of extreme supplementation. Due to the fact that fat soluble vitamins accumulate in fatty tissue, excessive intake of vitamin D can lead to excessive amounts building up in the body over time. Due to it’s symbiotic relationship with Calcium, excessive bodily levels of Vitamin D can lead to unpleasant effects such as hypercalcaemia – an excessive level of calcium in the blood. This can lead to bone pain, digestive issues, nausea and can increase the risk of kidney stones. 
Fortunately, these issues are rare, and usually only occur in people that have been taking doses of a vitamin D supplement at doses larger than 4000 IU per day for several months. It is also worth considering that Vitamin D deficiency is much more common across the world than cases of vitamin D toxicity. Furthermore, whilst it is possible to intake too much Vitamin D from supplements and foods, we can rest assured that it is impossible to end up with excessive vitamin D from sunlight exposure. Our bodies only make as much vitamin D as we need from sunlight, and there is a natural mechanism that prevents further synthesis of vitamin D if our bodily storage is at full capacity.
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