To explain this, a bit of background science is needed. This is quite technical so if you’re not so scientifically minded, you can skip this bit:
The body produces a fat-soluble prohormone called vitamin D3, aka. calcitriol, which has numerous benefits for the body. It is a precursor of other vital hormones, enables proper absorption of calcium & phosphorus from the gut and proper “cementing” of them in the bones, prevents cancer (yes, even skin cancer, as long as you’re not letting yourself burn & expose yourself to the sun regularly), and regulates the immune system.
We can get it from eating mushrooms (they contain ergocalciferol aka. D2, which we can convert to D3), fish (WITH the bones!), beef liver, egg yolk or foods fortified with it such as milk & dairy. However, the absolute best way for us to get it is through being exposed to the sun or by going on a safe tanning bed. The body naturally makes vitamin D3 in the skin from cholesterol* (or more specifically 7-dehydrocholesterol), which circulates in the blood until it reaches the liver where it is converted to calcidiol, then is turned into the active form calcitriol either by the immune system or the kidneys.
* Yes, cholesterol is important for many many functions in the human body. Despite what the media and supermarket ads say, there has never been a proven link between cholesterol and heart disease. Most people don’t even realise that about 75% of the cholesterol in the body is made by the body – meaning only 25% comes from food!
There’s a common belief that the darker your skin is, the more difficult it is for your skin to produce D3, especially in less equatorial/ less sunny climates. The common explanation is that melanin dramatically decreases the amount of solar UV radiation that reaches the layers of the skin where cholesterol is converted, and this is an adaptive response to stop excess UV rays being absorbed and causing sunburn. However, according to the articles below this is flawed. Melanin seemingly bears no correlation to D3 production, as traditionally living African groups (Hadzabe & Maasai were the ones studied) had on average 115nmol/l (nanomoles per litre) of D3 in their blood – well above the currently accepted safe upper limit of 80nmol/l. Also, it seems genetic factors other than pigmentation account for the D3 status of ‘white’ people (ie. variants of genes controlling cholesterol synthesis, hydroxylation & vitamin D transport).
http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=8735073 (note – this one is just an abstract)
http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(10)60635-6/fulltext (note – this one you either have to sign up to The Lancet or pay to view)
It’s little snippets of new research and scientific findings like these that advance human knowledge in the right direction. However, it’ll likely take at least 20 to 50 years for this to become common knowledge. Although new things are discovered by science pretty much daily, it takes ages for that to change the normal social paradigm and propaganda. This is my little contribution to that.