And now for some science… the marvels of skin

By Siouxsie Wiles 06/06/2014

Apologies for the lack of actual science posts recently. Let’s see if we can remedy that!

Last month I had the great privilege of interviewing skin cancer surgeon Dr Sharad Paul* for a session at the Auckland Writers Festival. We talked about his recent book Skin – A Biography, published in 2013 by Fourth Estate. Here’s what I found out:

Having skin is more important than having a brain!

Sharad is as enthusiastic about skin as I am about nasty microbes and makes this assertion based on the fact that there exist creatures that have done away with their brains but not their skin! Sea squirts are a group of bag-like marine filter feeders that are actually closely related to humans – they belong to the same phylum, Chordata, and start life out as a little tadpole like larvae with a primitive backbone called a notochord, which allows them to navigate in response to light. It’s what happens next though that’s quite amazing. The tadpole wiggles and twitches around until it settles headfirst onto a suitable surface. Next it cements itself to that surface and then starts to transform, losing it’s notochord, gills and twitching tail to become the ‘brainless’ bag of ‘skin’ that is an adult sea squirt. As Sharad put it, the sea squirt eats its own brain but has to keep its skin!

Picture of Halocynthia sp. taken by Yuri A. Zuyev, Hydrometeo. Univ., St. Petersburg - NOAA Photo Library.
Picture of Halocynthia sp. taken by Yuri A. Zuyev, Hydrometeo. Univ., St. Petersburg – NOAA Photo Library.

Skin colour is down to one single pigment – melanin

Melanin is the pigment that is responsible for producing all shades of all human skin colours and is found in our melanocytes. What I found fascinating is that regardless of skin colour, we all have the same number of melanocytes! That’s 10,000 for every square centimeter of skin (at least on our arms). The reason we humans come in different shades it that our melanocytes contain different amounts of melanin. In dark skinned people the melanin deposit in each melanocyte is huge, whereas those with white skin have lots of tiny little deposits. Sharad used the analogy of umbrellas to describe the melanin deposit in each melanocyte: people with black skin have the equivalent of a large solid umbrella whereas those with very pale white skin have an umbrella that is full of holes! This explains why those with very pale white skin freckle rather than tan.

Light-skinned early humans turned into dark skinned Africans to protect their folic acid

The last common ancestor humans and chimps shared 6 million years ago was light-skinned with dark hair. Apes in Africa are still like this whereas Africans are dark-skinned and relatively hair free. When our early ancestors started walking upright and lost their layer of hair, they needed to protect the folic acid in their skin from being broken down by the sun. Folic acid is important for normal neural tube function and a lack of folic acid can result in birth defects like spina bifida. This is why it is recommended that women take folic acid supplements during pregnancy. Melanin acts like a filter, preventing the penetrating UV light from damaging folic acid. Interestingly, spina bifida is much less common in Africa and the Tropics.

Humans who migrated out of Africa lightened to prevent rickets

When humans migrated out of Africa and into Europe 100,000 years ago, the shorter days meant that dark-skinned people would have likely have suffered from rickets due to a lack of vitamin D. Vitamin D is required for proper calcium absorption from the gut. Rickets causes skeletal and bone deformities and infertility so its likely that people’s skin lightened to allow better penetration of sunlight so they could produce sufficient vitamin D. This is supported by the fact that people who had a cereal-based diet low in vitamin D were lighter than those living at similar latitudes but who had a fish-based diet high in vitamin D. This also explains why Inuits are quite dark skinned, despite living somewhere with so little sunlight for such large parts of the year. Meanwhile back in Africa, black-skinned people were developing mechanisms which gave them higher levels of vitamin D to compensate. People in Tanzania have around 115 nmol/L of serum 25-hydroxyvitamin D, compared to 30-60 nmol/L for Westerners. Interestingly, Indian people tend to have very low levels of vitamin D, about half that of Westerners. Their darker skin colour emerged again to preserve folic acid as the lighter-skinned people moved out of Europe and into sunnier climates. Sharad says many Indians who move to New Zealand and Australia end up with vitamin D deficiency despite being exposed to plenty of sun.

Know your skin type and how quickly you will burn in the sun

How long you can safely spend out in the sun depends on three things: your skin type, the UV index and your sunscreen. In 1972 Thomas Fitzpatrick developed his scale for grading skin types: from the Celtic red-head who always burns and never tans (type I) to the black African skin that does not burn (type VI). The UV index was developed in the early 90’s by Canadian scientists and takes into account the thickness of the ozone layer, cloud cover and altitude. The scale originally went from 1 to 11 but it soon became apparent that scale wasn’t sufficient – New Zealand routinely sees a UV Index of 12 in summer while Western Australia has recorded a peak of 17! People with type I skin can spend 67 minutes/UV Index unprotected in the sun which would be less than 6 minutes in the NZ summer. For type II (usually blonde and blue-eyed) it is 100 minutes/UV Index, for type III (usually brown/black haired and brown-eyed) it is 200 minutes/UV Index and for type IV (Mediterranean, Spanish or lighter Indian skin) it is 300 minutes/UV Index.

Using factor 50 sunscreen is a bad idea!

Wearing sunscreen allows you to stay out in the sun longer but probably not for as long as you think! A sunscreen with a sun protection factor (spf) of 15 will block 93% of the UV falling on your skin allowing you to stay out in the sun 15 times longer, so about 75 minutes for the person with type I skin in an NZ summer. A sunscreen with an spf of 30 will block 97% of the UV giving you 2 and a half hours in the sun, while an spf of 50 will block 98% of the UV allowing you to stay in the sun for just over 4 hours. Sharad said the US Food and Drug Administration now inhibits sunscreens and cosmetics from claiming an spf of 50 as it gives users a false sense of security and means they end up spending much longer in the sun than they should.

I’ll finish with two of my favourite passages from Sharad’s book. This quote by Aristotle: “Touch is the one sense that the animal cannot do without. The other senses which it possesses are the means, not to its being, but to its well-being”, which I think is a lovely sentiment. And lastly: “skin wears its health for all to see – everything is unashamedly laid bare”. Nothing could be further from the truth as I approach the big four o!

About Dr Sharad Paul
*As a little background, Sharad (@DrSharadPaul) is skin cancer surgeon who runs a busy practice in Auckland where he offers free skin cancer checks. As well as having worked as a surgical consultant and GP, he also has a degree in medical law and ethics. In 2007 he pioneered a new skin graft technique which reduces costs, pain and healing time for patients and has also developed a range of skincare products designed for brown skin. He single-handedly brought Waitemata Health’s waiting times for skin cancer treatment down from a year to a month and won a Health Innovation Award for this in 2003.He also teaches at the University of Auckland and for one week a month at the University of Queensland in Brisbane. In 2012 Sharad was awarded the New Zealand Medical Association’s highest honour, the Chair’s Award which goes to an individual or organisation which has made a substantial contribution to the health of New Zealanders. He has also featured in Time magazine and was a finalist for New Zealander of the Year in 2012 – he lost out to Weta’s Sir Richard Taylor. He has also appeared at Goa’s THiNK festival alongside Robert De Niro and Bianca Jagger. To keep him sane he says, Sharad writes, and has had 3 novels published as well as his non-fiction book on skin. His love of literacy has seen him start his own book shops, first in Newmarket and then in Brisbane, and once a week he teaches creative writing in low decile schools around Auckland.