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So vitamin D is a sort-of vitamin, this is because although we get it from our food we can also make it in our bodies. Because of this some refer to vitamin D as a prohormone. It exists as vitamin D2 (ergocalciferol) - the form found in plants, produced by the action of sunlight on the plant sterol ergosterol - or vitamin D3 (cholecalciferol) which is the form we produce using sunlight in our skin from 7-dehydrocholesterol.
It never fails to amaze me how UVB light from the sun acts to help us make something which we need for our body to function properly. It makes me really feel our connection with plants on a whole new level. Bare skin needs to be exposed to sunlight without sunscreen for 10-15 minutes for fair skin or up to 45 minutes for darker skin a few times a week to get enough. But remember UVB light is the kind that burns our skin, so this is no excuse not to sunscreen up!
Food sources of vitamin D include...
* Oily fish
* Fortified foods (e.g. cereals and spreads)
Fun fact - you can increase the amount of vitamin D in mushrooms by leaving them out in the sun! Vitamin D is also a fat-soluble vitamin, meaning it needs fat to be absorbed in our body. Fried mushrooms are here to stay.
We can only make vitamin D in our skin if we’re exposed to intense enough light, which is hard to come across in the Autumn and Winter if you live somewhere like the UK. Because it’s hard to get enough from our diet it’s advised that during the darker months (or when you can’t get outside much AKA COVID lockdown) we should all take a 10mcg (400iu) supplement.
If you live very North (like I do in northern Scotland!) we’re advised to start thinking about taking a supplement at the end of October and taking it until the Spring (end of March time). And this goes for everyone aged over 4 years of age.
Pregnant and lactating people should also take a 10mcg supplement every day along with those with limited exposure to the outdoors, darker skin colours and those aged over 65. This is because as we age our kidney and liver function can sometimes reduce which impacts on the amount of active vitamin D in our blood, as these organs help us ‘activate’ vitamin D.
You really don’t need a fancy supplement though, you can get them from Boots or even supermarkets for very cheap and these will do just fine. Beware of super-high strength doses (several thousand times the recommended level) as we just don’t know what consequences there could be for our health – remember, there really is too much of a good thing. To put this in perspective the safe upper limit is set at 100µg/day (or 4000iu) by the European Food Safety Authority.
In terms of the type of vitamin D you should be okay taking either vitamin D2 or D3. You may see claims that vitamin D3 is more potent (which is mostly true) but it’s not absolutely necessary to take this form, especially if you eat fortified foods, eggs or oily fish regularly. Vitamin D2 supplements will always be vegan whereas vitamin D3 can sometimes come from animal origins (e.g. sheep’s wool).
The major function of vitamin D is to work as a hormone regulating calcium and phosphorus homeostasis (balance) in the body, which is important for bone and neuromuscular function. Vitamin D is probably most well-known for its effect at reducing the bone disease rickets, which can occur in children who are deficient; sometimes resulted in weak bones and bow-leggedness as bones don’t harden properly. It’s less of an issue since we discovered the connection between the vitamin and the disease (roughly 3 cases per 100,000 people in 2011) but it’s for this reason that it’s recommended to give a vitamin D supplement of 7-8.5µg per day (along with vitamin C and A) to infants and children aged under 4.
In adults vitamin D is important for maintaining good bone health and deficiency is linked with osteomalacia, where bones and muscles become weakened which can cause a lot of pain and difficulties walking normally. There's some debate as to whether vitamin D supplementation can help with osteoporosis (weak and brittle bones) and prevent fractures but it doesn’t hurt to take a supplement if you’re at risk or have been diagnosed.
There are vitamin D receptors on cells all over the body, which is leading scientists to question what else this vitamin gets up to in the body, and what else it can do to keep us in good health. Epidemiological evidence (the kind which only shows us links not causation) shows an association between vitamin D deficiency and chronic diseases such as cancer, multiple sclerosis, thyroid autoimmunity, high blood pressure, diabetes, metabolic syndrome and cardiovascular disease.
Because it’s been suggested that vitamin D has a role in the body’s response to respiratory infections (due to the presence of vitamin D receptors on immune and some lung cells, plus its role in reducing the inflammatory response) I’m going to focus on why it’s getting so much attention during the current COVID-19 pandemic. Which leads me to the question...
It’s important to know that there has only been one randomised controlled trial (RCT) looking at this so far – this means the kind of study where we give COVID patients vitamin D and see what happens. The bulk of the current evidence comes from observational studies (and one case-control study) which show a potential link between low vitamin D status and risk of developing COVID-19; but the quality of the studies and therefore the evidence is low. For example, in a paper published in September involving 489 COVID-19 patients, those who tested deficient for vitamin D (admittedly a year prior) had a 1.77-times increased risk of testing positive for COVID-19 than those with historically sufficient levels.
Going back to the RCT researchers found that 50% of patients who just received normal care (the control group) were admitted to the ICU, and two went on to die. But only 2% of patients receiving normal care plus calcifediol (a more potent version of vitamin D3) required ICU admission, and none died. However, it’s important to note that calcifediol is different to what you’ll find in vitamin D supplements and no status testing was done, so we don’t know if these patients were deficient when they got sick or not.
What this early evidence might be telling us is that vitamin D supplementation could be most beneficial for those who are already deficient in vitamin D. The UK Government are even in talks to discuss their messaging around vitamin D supplements, with some MPs calling for it to be placed on prescription for the most vulnerable. So really what it’s doing is underscoring just how important it is, as we head into Winter, so make sure you’ve got some supplements at the ready!
Castillo ME et al. (2020). Effect of calcifediol treatment and best available therapy versus best available therapy on intensive care unit admission and mortality among patients hospitalized for COVID-19: A pilot randomized clinical study. doi: 10.1016/j.jsbmb.2020.105751
COVID-19 rapid evidence summary: vitamin D for COVID-19 https://www.nice.org.uk/advice/es28/chapter/Factors-for-decision-making
Kearns et al. (2016). The impact of vitamin D on infectious disease: a systematic review of controlled trials. doi:10.1097/MAJ.0000000000000360
Meltzer DO (2020). Association of Vitamin D Status and Other Clinical Characteristics With COVID-19 Test Results. doi:10.1001/jamanetworkopen.2020.19722