Tag Archives: UVB

Sunshine, skin chemistry, and vitamin D

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The UK is on the same latitude as Northern Canada (Image Source: Wiki Commons)

As I write this it’s the last day of September in the U.K., which means we’re well into meteorological autumn and summer is, at least here, a distant memory. The weather is cooler and the days are getting shorter. Soon, the clocks will go back an hour, and we’ll shift from BST (British Summer Time) to GMT (Greenwich Mean Time).

Seasons in the U.K. are particularly marked because of our northerly latitude. British weather tends to be fairly mild (thanks, Gulf Stream), and it’s easy to forget just how far north we are – but a quick look at a globe makes it clear: London is actually further north than most of the major Canadian cities, while the Polar Bear Provincial Park in Ontario is roughly on the same latitude as Scotland’s capital city, Edinburgh.

Yes, I hear you say, but what on Earth (hoho) does this have to do with chemistry?

Well, a clever little piece of chemistry happens in human skin, and, if you live in the U.K., it’s about to stop. At least, until next spring.

Some clever chemistry happens in human skin.

There’s a substance in your skin called 7-dehydrocholesterol (7-DHC). It is, as the name suggests, something to do with cholesterol (which, despite its bad press, is an essential component of animal cell membranes). In fact, 7-DHC is converted to cholesterol in the body, but it’s also converted to something else.

You will have heard of vitamin D. It helps us to absorb calcium and other minerals, and if children, in particular, don’t get enough it can lead to rickets – which leads to weak bones, bowed legs and stunted growth. Vitamin D deficiency has also been linked to lots of other health problems, including increased risk of certain cancers, heart disease, arthritis and even type one diabetes.

More recently, vitamin D has been linked to COVID-19. It’s estimated that around 80-85% of people who contract COVID-19 experience mild or no symptoms, while the rest develop severe symptoms and, even if they recover, may suffer life-altering after-effects for many months. Early data suggest that patients with low vitamin D levels are much more likely to experience those severe symptoms. There’s a plausible mechanism for this: vitamin D helps to regulate the immune system and, in particular, helps to reduce the production of cytokines.

It’s possible that having inadequate levels of vitamin D may increase your chances of a severe response to COVID-19.

Cytokines are small proteins which are important in cell signalling, but if the body starts to produce too many in response to a virus it can cause something called a cytokine storm, which can lead to organ failure and death.

It’s proposed that having the right levels of vitamin D might help to prevent such cytokine storms, and therefore help to prevent a severe COVID-19 response. This is all early stages, because everyone is still learning about COVID-19, and it may turn out to be correlation without causation, but so far it looks promising.

One thing you many not know is that vitamin D is, technically, misnamed. Vitamins are, by definition, substances which are required in small quantities in the diet, because they can’t be synthesised in the body.

But vitamin D, which is actually a group of fat soluble molecules rather than a single substance, can be synthesised in the body, in our skin. The most important two in the group are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3), sometimes known collectively as calciferol.

Shiitake mushrooms are a good source of vitamin D2.

Vitamin D2 is found in fungi, but it’s cleared more quickly from the body than D3, so needs to be consumed in some form daily. Mushrooms are a good source (especially if they’ve been exposed to UV light), so if you like mushrooms, that’s one way to go. Vitamin D3 is hard to obtain from diet – the only really good source is oily fish, although other foods are fortified – but that’s okay because, most of the time, we don’t need to eat it.

Which brings us back to 7-DHC. It’s found in large quantities in the skin, although exactly how it gets there has been the subject of some debate. It used to be thought it was formed from cholesterol via an enzymatic reaction in the intestine wall and then transported to the skin via the bloodstream. But the trouble with this idea is that the blood would pass through the liver, and 7-DHC would be reconverted to cholesterol, never having a chance to build up in skin. A more robust theory is it’s actually synthesised in the skin in the first place, particularly since higher levels are found in a layer closer to the surface (the stratum spinosum) than in the deeper dermis.

We make vitamin D in our skin when we’re exposed to UVB light from the sun.

Anyway, the important thing is that 7-DHC absorbs UV light, particularly wavelengths between 290 and 320 nm, that is, in the UVB range, sometimes called “intermediate” UV (in contrast with “soft” UVA, and “hard” UVC). When exposed to UVB light, one of the rings in the 7-DHC molecule breaks apart, forming something known pre-D3, that then converts (isomerises) to vitamin D3 in a heat-sensitive process.

In short, we make vitamin D3 in our skin when we’re in the sunshine. Obviously we need to avoid skin damage from UV light, but the process doesn’t take long: 10-15 minutes of midday sunlight three times a week, in the U.K. in the summer, is enough to keep our levels up.

Sun exposure is by far the quickest, and certainly the cheapest, way to get your vitamin D. If you live somewhere where that’s possible.

Here’s the thing, though, if you live in the U.K., for a chunk of the year, it’s just not. I’ve pinched the graph here from my husband, whose work involves solar panels, because it makes a nice visual point.

The amount of sunlight we’re exposed to in the U.K. drops sharply in autumn and winter.

From April – September, there’s plenty of energy available from sunlight. But look at what happens from October – March. The numbers drop drastically. And here’s the thing: it turns out that vitamin D production in human skin only occurs when UV radiation exceeds a certain level. Below this threshold? Well, no photocoversion takes place.

In short: if you live in the U.K. you can’t make vitamin D in your skin for a few months of the year. And those few months are starting… round about now.

The NILU has a web page where you can calculate how much vitamin D you can synthesise in your skin on a given day.

If you want to experiment, there’s a website here, published by the Norwegian Institute for Air Research (NILU), where you can enter various parameters – month, longitude, cloudiness etc – and it will tell you how many hours during a given a day it’s possible to synthesise vitamin D in your skin.

Have a play and you’ll see that, for London, vitamin D synthesis drops off to zero somewhere around the end of November, and doesn’t restart until sometime after the 20th of January. In Edinburgh, the difference is even more marked, running from the first week or so of November to the first week of February.

It’s important to realise that it tails off, too, so during the days either side of these periods there’s only a brief period during midday when you can synthesise vitamin D. And all this assumes a cloudless sky which in this country… is unlikely.

The skin pigment, melanin, absorbs UVB. (Image Source: Wiki Commons)

The situation is worse still if you have darker skin because the skin pigment, melanin, absorbs UVB. On the one hand, this is a good thing, since it protects skin cells from sun-related damage. But it also reduces the ability to synthesise vitamin D. In short, wimpy autumn and winter sunshine just isn’t going to cut it.

Likewise, to state the obvious, anyone who covers their skin (with clothing or sunblock), also won’t be able to synthesise vitamin D in their skin.

Fortunately, there’s a simple answer: supplements. The evidence is fairly solid that vitamin D supplements increase blood serum levels as well as, if not better than, sunshine – which, for the reasons mentioned above, can be difficult to obtain consistently.

Now, as I’ve said many times before, I’m not a medical doctor. However, I’m on fairly safe ground here, because Public Health England do actually recommend everyone take a vitamin D supplement from October to May. That is, from now. Yes, now.

I do need to stress one point here: DO NOT OVERDO IT. There always seems to be someone whose reasoning goes along the lines of, “if one tablet is good, then ten will be even better!” and, no. No. Excessive doses of vitamin D can cause vomiting and digestive problems, and can lead to hypercalcemia which results in weakness, joint pain confusion and other unpleasant symptoms.

If you live in the U.K. you should be taking a vitamin D supplement from October-May.

Public Health England recommend everyone in the U.K. take 10 micrograms per day in autumn and winter. Babies under one year should also be given 8.5–10 micrograms of vitamin D in the form of vitamin drops, unless they’re drinking more than 500 ml of infant formula a day (because that’s already fortified).

Amounts can get a little confusing, because there are different ways to measure vitamin D doses, and in particular you may see IU, or “international units“. However, if you buy a simple D3 supplement, like this one that I picked up at the supermarket, and follow the dose instructions on the label, you won’t go far wrong.

So, should you (and everyone else in your family) be taking a simple vitamin D supplement right around now? If you live in the U.K., or somewhere else very northerly, then yes. Well, unless you’re really keen to eat mushrooms pretty much every day. At worst, it won’t make much difference, and at best, well, there’s a chance it might help you to avoid a really unpleasant time with COVID-19, and that’s got to be a good thing.

But, look, it’s not toilet roll. Don’t go and bulk buy vitamin D, for goodness sake.

Until next time, take care, and stay safe.

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Where did our love of dairy come from?

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The popularity of the soya latte seems to be on the rise.

A little while ago botanist James Wong tweeted about the myriad types of plant ‘milk’ that are increasingly being offered in coffee shops, none of which are truly milk (in the biological sense).

This generated a huge response, probably rather larger than he was expecting from an off-hand tweet. Now, I’m not going to get into the ethics of milk production because it’s beyond the scope of this blog (and let’s keep it out of the comments? — kthxbye) but I do want to consider one fairly long thread of responses which ran the gamut from ‘humans are the only species to drink the milk of another animal’ (actually, no) to ‘there’s no benefit to dairy’ (bear with me) and ending with, in essence, ‘dairy is slowly killing us‘ (complicated, but essentially there’s very little evidence of any harm).

Humans have been consuming dairy products for thousands of years.

But wait. If dairy is so terrible for humans, and if there are no advantages to it, why do we consume it at all? Dairy is not a new thing. Humans have been consuming foods made from one type of animal milk or another for 10,000 years, give or take. That’s really quite a long time. More to the point (I don’t want to be accused of appealing to antiquity, after all), keeping animals and milking them is quite resource intensive. You have to feed them, look after them and ensure they don’t wander off or get eaten by predators, not to mention actually milk them on a daily basis. All that takes time, energy and probably currency of some sort. Why would anyone bother, if dairy were truly detrimental to our well-being?

In fact, some cultures don’t bother. The ability to digest lactose (the main sugar in milk) beyond infancy is quite low in some parts of the world, specifically Asia and most of Africa. In those areas dairy is, or at least has been historically, not a significant part of people’s diet.

But it is in European diets. Particularly northern European diets. Northern Europeans are, generally, extremely tolerant of lactose into adulthood and beyond.

Which is interesting because it suggests, if you weren’t suspicious already, that there IS some advantage to consuming dairy. The ability to digest lactose seems to be a genetic trait. And it seems it’s something to do, really quite specifically, with your geographic location.

Which brings us to vitamin D. This vitamin, which is more accurately described as a hormone, is a crucial nutrient for humans. It increases absorption of calcium, magnesium and phosphate, which are all necessary for healthy bones (not to mention lots of other processes in the body). It’s well-known that a lack of vitamin D leads to weakened bones, and specifically causes rickets in children. More recently we’ve come to understand that vitamin D also supports our immune system; deficiency has been meaningfully linked to increased risk of certain viral infections.

What’s the connection between vitamin D and geographic location? Well, humans can make vitamin D in their skin, but we need a bit of help. In particular, and this is where the chemistry comes in, we need ultraviolet light. Specifically, UVB – light with wavelengths between 280 nm to 315 nm. When our skin is exposed to UVB, a substance called 7-dehydrocholesterol (7-DHC to its friends) is converted into previtamin D3, which is then changed by our body heat to vitamin D3, or cholecalciferol – which is the really good stuff. (There’s another form, vitamin D2, but this is slightly less biologically active.) At this point the liver and kidneys take over and activate the chloecalciferol via the magic of enzymes.

We make vitamin D in our skin when we’re exposed to UVB light.

How much UVB you’re exposed to depends on where you live. If you live anywhere near the equator, no problem. You get UVB all year round. Possibly too much, in fact – it’s also linked with skin cancers. But if you live in northerly latitudes (or very southerly), you might have a problem. In the summer months, a few minutes in the sun without sunscreen (literally a few minutes, not hours!) will produce more than enough vitamin D. But people living in UK, for example, get no UVB exposure for 6 months of the year. Icelanders go without for 7, and inhabitants of Tromsø, in Norway, have to get by for a full 8 months. Since we can only store vitamin D in our bodies for something like 2-4 months (I’ve struggled to find a consistent number for this, but everyone seems to agree it’s in this ballpark), that potentially means several months with no vitamin D at all, which could lead to deficiency.

In the winter northern Europeans don’t receive enough UVB light from the sun to produce vitamin D in their skin.

In the winter, northern Europeans simply can’t make vitamin D3 in their skin (and for anyone thinking about sunbeds, that’s a bad idea for several reasons). In 2018, this is easily fixed – you just take a supplement. For example, Public Health England recommends that Brits take a daily dose of 10 mcg (400 IU) of vitamin D in autumn and winter, i.e. between about October and March. It’s worth pointing out at this point that a lot of supplements you can buy contain much more than this, and more isn’t necessarily better. Vitamin D is fat-soluble and so it will build up in the body, potentially reaching toxic levels if you really overdo things. Check your labels.

Oily fish is an excellent source of vitamin D.

But what about a few thousand years ago, before you just could pop to the supermarket and buy a bottle of small tablets? What did northern Europeans do then? The answer is simple: they had to get vitamin D from their food. Even if it’s not particularly well-absorbed, it’s better than nothing.

Of couse it helps if you have access to lots of foods which are sources of vitamin D. Which would be…  fatty fish (tuna, mackerel, salmon, etc) – suddenly that northern European love of herring makes so much more sense – red meat, certain types of liver, egg yolks and, yep, dairy products. Dairy products, in truth, contain relatively low levels of vitamin D (cheese and butter are better than plain milk), but every little helps. Plus, they’re also a good source of calcium, which works alongside vitamin D and is, of course, really important for good bone health.

A side note for vegans and vegetarians: most dietry sources of vitamin D come from animals. Certain mushrooms grown under UV can be a good source of vitamin D2, but unless you’re super-careful a plant-based diet won’t provide enough of this nutrient. So if you live in the north somewhere or you don’t, or can’t, expose your skin to the sun very often, you need a supplement (vegan supplements are available).

Fair skin likely emerged because it allows for better vitamin D production when UVB levels are lower.

One thing I haven’t mentioned of course is skin-colour. Northern Europeans are generally fair-skinned, and this is vitamin D-related, too. The paler your skin, the better UVB penetrates it. Fair-skinned people living in the north had an advantage over those with darker skin in the winter, spring and autumn months: they could produce more vitamin D. In fact, this was probably a significant factor in the evolution of fair skin (although, as Ed Yong explains in this excellent article, that’s complicated).

In summary, consuming dairy does have advantages, at least historically. There’s a good reason Europeans love their cheeses. But these days, if you want to eat a vegan or vegetarian diet for any reason (once again, let’s not get into those reasons in comments, kay?) you really should take a vitamin D supplement. In fact, Public Health England recommends that everyone in the UK take a vitamin D supplement in the autumn and winter, but only a small amount – check your dose.

By the way, if you spot any ‘diary’s let me know. I really had to battle to keep them from sneaking in…

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