How the sun sees you: The frenemy of our superhero skin

As the end of summer draws ever closer, Dr Nick Knight explores the skin's relationship with sunshine (if the summer weather returns before winter)

If you were going to design a set of superhero skills, you may consider a CV of talents that include protection from danger, super-senses, stretchiness, and regeneration abilities. Although it sounds like the stuff of fiction, you need actually look no further than your own skin. That’s right, your skin, the largest organ in your body, is full of these superhero skills.

Of course we also know that for every superhero, there is the ubiquitous part-time friend, part-time enemy (the modern day ‘frenemy’). For your skin, this frenemy is the sun, who with a surface temperature of 5,500 °C and mass 330,000 times that of Earth, is quite a force to be reckoned with. With increasing exposure to its ultraviolet rays, the creator of bad tan-lines and super-charged good moods, how does your skin really cope with sunshine?

Before exploring this, let us explore what your skin actually is. Your skin is not one but three distinct layers comprising the epidermis, dermis and subcutis. Each has distinct superhero roles, providing nutrition and structural support, while housing hair follicles, sebaceous glands, sweat glands, fat, blood vessels and nerves. Believe it or not, the dermis accounts for 15 per cent-20 per cent of your total body weight! Sadly, no, that does not make ‘excess skin’ a cause of the obesity epidemic.

With that clear, let me show you how your skin responds to sunshine. Firstly, as you warm-up, your brain’s thermoregulation centre triggers an action response to restore your body’s temperature homeostasis (‘balance’). Your skin helps this in three different ways. Firstly, to aid heat loss, tiny superficial arteriolar blood vessels in the skin widen by relaxing the smooth muscles of the blood vessels. This redirects your blood to miniscule skin capillaries and allows for increased heat loss by convection and conduction. That’s why your hands always look so ‘veiny’ in warm weather!

The second trick of your skin is the eccrine glands begin secreting sweat. This speeds up your sweat ducts with verve and gusto, through the sweat pores and onto the surface of your skin, allowing for heat loss via evaporative cooling. On the downside, if you are not drinking plenty of water and replacing lost salts, you can become dehydrated quickly. Finally for those of you, like myself, who have follicularly-endowed limbs, the hairs flatten to prevent heat-trapping in the layer of still air that would otherwise exist between the hairs. This also allows for increased air flow to your skin, thereby increasing heat loss by convection. We can thank tiny erector pili muscles for this since they relax, allowing the hair follicles attached to them to stay flat.

So your skin defeats the sun’s attempt at evil that time. However, as with any frenemy, the tide can turn, and the sun can decide to be a friend, working with our skin - this time to help make strong bones – bizarre as that may sound. It is thanks to a conveyer-belt of biochemical activity between your skin, liver, kidneys, and bowels. It is important to realise that you need vitamin D for making calcium - which promotes strong bones. So, when your skin is exposed, the sun’s ultraviolet light activates the pre-cursor to vitamin D, 7-dehydrocholesterol (the source of 90% of our vitamin D). This activated vitamin D pre-cursor then wanders off to your liver via the blood stream to be modified, before travelling to your kidneys where it becomes biologically ‘active’. Active vitamin D promotes calcium absorption for your gut and helps to maintain adequate levels in the blood to allow for normal bone mineralisation, bone growth, and bone remodelling (our skeleton regenerates approximately every three months).

Of course, in a world of superheroes, the friendliness of the sun may not last long before the enemy surfaces once again. Now, yes, in the short-term, safe levels of sun exposure allows for the tanning process and protects the skin by releasing more melanin from the skin’s melanocytes. However, excess short-term exposure to ultraviolet light directly damages the DNA in your skin causing sunburn. Since we all have different skin types, as identified by the Fitzpatrick Scale (a classification system for skin colour), this, along with our exposure rates, will largely dictate how badly we may burn.

Years of chronic over-exposure is even more dangerous, with the potential to cause a number of different skin cancers - now one of the most common cancer types globally. It is often a result of DNA damage by the sun’s ultraviolet rays to the outermost layer of the skin, the epidermis. These cancers divide into the more common but less serious non-melanoma cancers e.g. basal-cell carcinoma and squamous-cell carcinoma, and the more serious and potentially fatal malignant melanoma cancers.

So let us recap; your skin performs superhero feats to protect, climate control, produce and defend you and your body without hesitation. It exists, however, at the will of your decisions; you can decide to love and nurture your skin, to work in partnership to promote health, or you can decide to rebel like a stroppy teenager, let it burn, not protect it, and let it fail to serve you the way nature intended. Yes, get a nice tan but be safe and think about the long-term exposure risks. As always, I will end with my drier but critical doctor message: if you are concerned about anything on your skin, want advice about sun protection and risks of over-exposure, then visit your GP.

In the meantime, go ahead and give your skin a superhero cape.

Read more: How alcohol affects your liver and heart
Fast food and you: Too much fat, sugar, and salt

Dr Nick Knight is a junior doctor based in London with a PhD background in human performance. His blog on life as a doctor can be read at:

Or follow him via Twitter: @Dr_NickKnight

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