It is now well-accepted that vitamin D deficiency is strongly associated with worse Covid-19 outcomes, and that in the UK, many people have low levels of vitamin D; so we must look at why UK levels are so low and what we can do about it.

We obtain most vitamin D from sunlight and supplementation, plus small amounts from food. UV-B Ultra violet radiation from the sun on our skin helps us produce vitamin D. The radiation you receive depends on both the amount of sunlight received, and the angle that it comes from. So the latitude of where you are in the world is hugely important. The UK, despite its mild weather (thanks to the Gulf Stream) is at a surprisingly high latitude, in line with southern Canada.

The season also affects how much you receive, since the sun is lower in the sky during the winter than summer; similarly, the time of day affects it, with noon the optimal time when the sun is highest in the sky. And of course, the more you get direct sun, rather than overcast skies, the more UV-B you will get.

Research has shown that seasonal surges in Covid-19, are not correlated with temperature, humidity or even school opening dates. Instead there is a significant correlation with latitude and the time when the daily dose of UV-B drops below 34% of strength at optimal latitude.

For many years we have known that avoiding the midday sun is key to not getting burnt, but for many people in the UK’s latitude and climate, there is more risk of becoming vitamin D deficient in the winter than getting sunburnt in the summer!

People with fair skin tend to produce the most from the sun, and darker skinned people tend to produce less. This seems to be an evolutionary thing: where sunlight is abundant, skin has probably evolved to be resistant to burning, and where it is limited, skin has evolved to maximise UV-B available.

This means that darker skinned people need more sun to get the same amount of vitamin D. And in place like the UK, it’s not surprising that a lot of BAME people are dangerously deficient in vitamin D. But the rest of us can also easily become deficient by the end of the winter if we haven’t got out much.

Because of increasing evidence about low vitamin D levels, including some evidence suggesting causation in Covid-19 outcomes, many people are taking vitamin D supplements; and some medical professionals are advocating quite high doses.

What should you do about it?

Since vitamin D from the sun is free and getting out and about has other health benefits too, you should definitely prioritise this. When you have limited time outside and the sun is weak, you can maximise your vitamin D by exposing lots of skin, and going out in the middle of the day.

UV-B from the sun is 10x stronger at noon, than before 9am or after 3pm, and 70% of all the day’s UV-B is between 10am and 2pm. So do try to go out in the middle of the day to get the best of it.

The time of year also makes a lot of difference. There is maybe around 3x more UV-B at noon in July, than there is from November to February. We’re starting to get useful UV-B by the end of March, but it’s still only half the July measure.

There is evidence we can store up vitamin D from the summer in our body’s tissues for wintertime. And even though things are brighter by the spring, this is the time when many people are at their lowest vitamin D reserves because they have been used up; so you may need to carry on topping up with a vitamin D supplement.

Vitamin D is useful for many other things other than Covid, and that is the main reason the NHS recommends supplementation. I would also recommend supplementation from October through to April, and depending on your skin shade and how much you and your family get sun exposure from May through to September, you could consider year-round supplementation.

A very few foods are already fortified in the UK, notably baby formula amd some plant-based milks. More foods are fortified in other high latitude countries. Without fortification, most food has no vitamin D, and a few foods such as oily fish, grass-fed red meat, liver, and eggs have varying amounts. But you’d have to eat quite a lot daily to get enough just from food.

For adults, taking more than 4000 IU of vitamin D is not advised, except for short periods to address a known deficiency from testing. There is evidence that much higher doses can be tolerated, and it is generally safe, but toxicity from excessive oral vitamin D can and does happen. I haven’t seen any medical professionals advise taking more than that routinely, and some who state that is the level they supplement at personally. So for adults, I would suggest 3,000 to 4,000 IU; children 1-12 can take 1,000 to 2,000 IU, but the maximum for babies under 12 months is 1,000 IU.

Vitamin D from sunlight is quite different in the way the body produces it, as a lot can be produced in summer in a very short time. But the body appears to regulate overproduction better than oral supplementation.

In terms of getting out, do try to do it at lunchtime, between 10am and 2pm, especially if it is sunny. Strip off as much as you reasonably can, so your skin is exposed. Just be careful not to spend too much time in the sun during the summer, and slap on sun cream after you have had a nice dose of vitamin D.

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