Article by Talkington Bates' Nutritionist, Hebe Richardson
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Vitamin D is a vitamin that has had a lot of press this last year, but what is it, how does it work and where can we get it from?
In the UK, it’s recommended we consume 10 micrograms (400 IU) of Vitamin D a day and during the winter months (October - late March), when there is a lack of sunlight, it is suggested everyone takes a Vitamin D supplement (1).
What is Vitamin D?
Vitamin D is a fat - soluble vitamin, meaning it is absorbed through the gut with the help of fat. Compared to other vitamins, Vitamin D is a little different, as it is classed as a pro-hormone, as it acts as a precursor to a hormone involved in calcium homeostasis.
There are two forms of Vitamin D:
Vitamin D2 - This is a naturally occurring form found in plants and fungi, and synthesised by UVB exposure of ergosterol in plants.
Vitamin D3 - This is synthesised in the skin of humans or animals by the action of sunlight, which contains UVB radiation.
Why do we need it?
The main role of vitamin D is in bone and muscle health. Due to its role in the regulation of calcium and phosphorus in our bodies. Deficiency of Vitamin D in adults can lead to something known as osteomalacia, where muscles are weakened, there is tenderness in the bones and can be pain in the spine, shoulder, ribs or pelvis. In children, a lack of Vitamin D can have implications on growth, bone development and can lead to rickets.
When Vitamin D is synthesised on the skin or obtained through the diet, it’s converted into its circulating metabolite, firstly in the liver and then in the kidneys. Its major role is in calcium and phosphorus metabolism, which is essential for bone mineralisation and neuromuscular function.
There is some evidence which indicates a link between Vitamin D and the immune system, as there are Vitamin D receptors present in the cells of the immune system. However, at this present moment there is insufficient evidence to link vitamin D with conditions such as cancer, infectious diseases, autoimmune disease and cognitive function (2).
Additionally, there is currently research taking place which is considering whether there is a link between vitamin D and coronavirus. The studies are mainly observational, meaning we cannot devise a cause and effect. At this current point in time, there is insufficient evidence to suggest that Vitamin D supplementation has a role in respiratory tract infections, including coronavirus (3).
However, it’s important to note that with a lot more of us spending time indoors, there is a consideration we’re not getting enough Vitamin D from sunlight. Therefore, it’s extra important that we take Vitamin D supplements.
Where can I get it?
Our body makes vitamin D from direct sunlight on our skin when we’re outside. This is influenced by things like the time of day, season, cloud cover, air pollution, clothing and sunscreen use.
In the UK, Vitamin D is only synthesised from sunlight exposure from late March/early April to September. As, during winter, there is only a small amount of UVB radiation, which is insufficient to synthesise Vitamin D in the skin.
There are few very rich dietary sources of vitamin D. The ones we do have are mainly animal based and include; oily fish (such as salmon, sardines, herring and mackerel), red meat, liver, egg yolks and fortified foods (such as some fat spreads and breakfast cereals).
When it comes to plant-based sources of vitamin D, wild mushrooms are a rich source. However, not all mushrooms are as cultivated mushrooms are often grown in the dark, so do not provide any high amount of vitamin D.
Current advice is that, during the autumn and winter months, everyone is advised to take a supplement of vitamin D containing 10 micrograms (400 international units) a day.
For Vegans, it’s worth nothing that not all supplements may be suitable. This is because the D3 form of the vitamin can sometimes be derived from animal sources. If you’re unsure, check the label to see whether it’s suitable for vegans (4).
Am I at risk of deficiency?
As we get most of our vitamin D from exposure to the sun, during the winter months, it is very difficult to get an adequate source from sunlight or food alone, which is why supplements are recommended.
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During the year, some individuals also struggle to get enough vitamin D from sunlight due to having very little or no sunlight exposure. This could be due to spending more time inside than usual, wearing clothes that cover up most of your skin while outdoors or having darker skin. For these groups of people, it is often recommended to take a vitamin D supplement throughout the whole year.
Can I take too much?
The vitamin D we get from the sun is self-regulated, meaning that we cannot over produce it when exposed to the sun. However, when it comes to our diet, excessive intakes can have toxic effects that lead to hypercalcaemia (high calcium in the blood) - which can lead to weakening bones, damage to the kidneys and the heart.
1. Vitamins and minerals - Vitamin D. nhs.uk. 2020. Available from: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-d/
2. Buttriss J. Vitamin D: Sunshine vs. diet vs. pills. Nutrition Bulletin. 2015;40(4):279-285.
3. Insufficient evidence for vitamin D preventing or treating ARTIs. GOV.UK. 2020. Available from: https://www.gov.uk/government/news/insufficient-evidence-for-vitamin-d-preventing-or-treating-artis
4. Vitamin D. The Vegan Society; 2017. Available from: https://www.vegansociety.com/sites/default/files/uploads/downloads/Vitamin%20D%20PDF%20v2.pdf
Other key references:
The Scientific Advisory Committee on Nutrition (SACN). Vitamin D and Health. Public Health England; 2016.