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Sunny days not only lift the spirit, they provide us with a real physiological benefit: when you're out in the sun, your skin is busy making vitamin D. Or at least, a cholesterol-based compound gets converted to something that's almost vitamin D in the skin, then the liver and the kidneys add hydrogen and oxygen to activate the vitamin. While you can get some vitamin D in your diet, most of us get around 90% of our vitamin D by making it, in our skin, in the presence of sunlight.
But what does it actually do? Like most vitamins, we only found this out when people started to suffer from a lack of it. The industrialisation of Europe may have been a great technological step forward, ultimately leading to all sorts of improvements in people's lives, but there were casualties along the way. Crowded cities, working in factories, skies full of smog – all left their mark on the children of the industrial revolution. They didn't grow properly and their soft young bones set in awkward curves. In 1919, a German researcher called Kurt Huldschinsky found that shining ultraviolet light at children with rickets could cure them. Not long after, the chemical identity of vitamin D was discovered and eventually researchers found out how it worked: the vitamin acted like a hormone. Once activated by the kidneys, it travelled in the bloodstream to the gut, carrying a message: get calcium!
But vitamin D is a busy little chemical. By the 1980s, it was becoming clear that, as well as its important job in calcium metabolism and building bones, it also played a crucial role in the immune system. A lack of vitamin D means that you're more likely to develop autoimmune diseases (where the armies of your immune system start to engage in friendly fire), cancers, diabetes and heart disease.
Perhaps the most surprising finding in recent years relates to skin colour. For a long time, it's been accepted that people with dark skin struggle to make enough vitamin D in northern countries. Skin colour in indigenous populations maps rather neatly on to latitude. If your ancestors lived close to the equator, you probably have dark skin. Sunlight may help us to make vitamin D, but it can also be very damaging. The skin pigment melanin acts as a natural sunscreen, providing protection from sunburn and skin cancer – it's easy to see why natural selection would favour darker skin in the tropics.
As you go further north, skin colour gets progressively paler. This has been explained as an adaptation to weaker sunlight, allowing people to make enough vitamin D, to avoid problems such as rickets, in less sunny parts of the world. This story all seems to add up nicely. Except that some new studies are now showing that people with dark skin seem to be able to make vitamin D just as efficiently as pale people. What a spanner in the works! At first glance, this looks like it could bring all our theories about skin colour and human evolution tumbling down.
But evolutionary changes don't just happen because a certain mutation confers an advantage. Sometimes, they come about because a particular mutation doesn't make much difference in terms of survival. Perhaps what happened as our ancestors moved north was that a strong selection pressure for dark skin – as defence against sunburn and skin cancer – eased and mutations for paler skin could happen without being weeded out.
But then why do people with dark skin today, in northern Europe and North America, tend to suffer from vitamin D deficiency? One study found a clue, by asking people to fill in detailed questionnaires about what they did. It turns out that the pale-skinned people in the study tended to rush out in the sun when it appeared, while the darker-skinned people stayed inside. So the difference in vitamin D levels could be down to culture and behaviour rather than skin colour. Staying out of the sun might be a good strategy in a tropical place with plenty of strong sun, but in Britain, with weaker sun and less of it, we need to make the most of sunny days.
At the same time, we're all warned to protect ourselves from sunlight, to prevent sunburn and skin cancers. Every summer, health campaigns remind us of the importance of covering up and slathering sunscreen on ourselves and on our children. But have we overdone it? Might all this sunscreen drastically reduce our skin's ability to make vitamin D?
A few early studies suggested this might be the case. People who regularly wore sunscreen had only half as much vitamin D in their blood compared with those who didn't. But several more recent experiments have suggested that wearing sunscreen makes very little difference to vitamin D levels. If anything, people who used sunscreen regularly seem to have higher vitamin D levels – perhaps because they were also getting out in the sun more
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