Vitamin D3 (cholecalciferol) is synthesized in the skin as a result of Ultraviolet light exposure. It is produced from cholesterol. It occurs in two steps. First 7-dehydrocholesterol is converted to Previtamin D3 by the suns light. Then the Previtamin D3 turns into the D3 isomer quickly.
This production and conversion occurs in the outer-most region of the skin known as the Dermis. The Dermis is made up of 5 layers. The outermost layer that you can touch is the Stratum Conrneum. The sole job of this layer is to protect the skin from penetration of any foreign matter. The two deepest layers are the ones we are interested in now. They are the Stratum Spinosum and the Stratum Basale. These two layers are where 7-dehydrocholeterol are synthesized by the UV rays of the sun.
It’s often said that spending as little as 30 minutes of whole-body sun exposer will produce 10,000 IUs of vitamin D3. This is a misconception. It produces 10,000 IUs of 7-dehydrocholesterol. The 7-dehydrocholesterol still must convert to Previtamn D3 and then to D3. This does not always occur for a variety of reasons. For instance, the skin has a feedback loop and when large amounts of D3 are being synthesized it begins to degrade faster than it synthesizes it to maintain equilibrium. Another issue is that the 7-dehydrocholesterol can be washed out of the skin before it even converts into Previtamin D3 if you don’t allow it to stay put long enough. This can occur from swimming while sun bathing or showering immediately after sun bathing. Needless to say, not all sun exposure will lead to a surge in D3 levels.
So even if you’re not sun-phobic you may be surprised to find that all your time gardening or sun bathing is not having the effect you want on your D3 levels. And if you are sun-phobic and slather sunscreen on at every possible time you’ll be surprised to know that a sunscreen with an SPF of as little as 8 will reduce D3 synthesis by a whopping 95%.
D in the Diet
Our ancestors got Vitamin D2 primarily form the diet. D3 was synthesized by their days in the sun. We will not get into D2 here, but suffice it to say that we have never gotten the large doses of D3 in out diet that we force down our throats today ever in our evolutionary history. Today the scientific research on the benefits of optimal D3 levels has reached the mainstream. This has clinicians recommending and people taking as much as 10,000 IUs a day of D3 supplements. These ultra-high levels of oral D3 is not something we have ever experienced before. Research now shows that getting D3 in the diet is treated differently by the body than the D3 you get from sun exposure.