In the presence of UVB rays from the sun, a precursor, pre-vitamin D is made in the epidermis from a derivative of cholesterol in the skin. This is then made into D3 in the lower layers of the skin. (cholecalciferol)
The liver converts this to calcidiol (25-hydroxy vitamin D), and another conversion to calcitriol 1,25 dihydroxy vitamin D (D3) takes place in the kidneys.
This calcitriol is the active form of vitamin D. It is this form of Vitamin D, which binds to the VDR (vitamin D receptor) to travel in the blood. Together with VDR, RXR (a vitamin A receptor) enters the nucleus of the cell and regulates the expression of hundreds of genes. Inside the genome, there are elements known to respond to Vitamin D, these are called VDREs (vitamin D response elements).
Thousands of VDREs have been identified throughout the genome. VDR activation by 1,25-dihydroxy vitamin D is thought to directly and/or indirectly regulate 100 to 1,250 genes (8).
Recent knowledge has revealed that conversion from 25 hydroxyvitamin D to 1,25-dihydroxy vitamin D has also been demonstrated in various other tissues, including skin, parathyroid gland, breast, colon, prostate, and as cells of the immune system and bone cells. This was previously unknown.
- Can I get Vitamin D in my diet?
- What is the difference between Vitamin D2 and Vitamin D3?
- Are you as intrigued by the science as I am?
Both vitamins D2 and D3 are available in foods. However, the amounts and sources are few. Therefore we do not get sufficient for our daily needs.
Vitamin D3 (cholecalciferol) is found in animal sources in fish, fish oils, (we are all familiar with cod liver oil), eggs, butter and liver. (Cholecalciferol can also be harvested from lanolin in sheep’s wool and from pig’s skin, where it is made like the synthesis in our own skin). Check the source of your vitamin D3!
Vitamin D2 is found in mushrooms exposed to light, a mere 7 IU per 100 grams is usual. However, an interesting fact is if one wants to synthesise vitamin D (D2) in mushrooms, shitake mushrooms that have been exposed, gills up, to sunlight for six hours for two days had a huge 46,000iu per 100grams. D2 is not a naturally occurring form of vitamin D in the human body and needs to be converted by the body to D3.
Interestingly mushrooms are rich in the plant form of vitamin D precursor ergosterol, which ultraviolet B (between wavelengths of 290 nm to 315 nm) converts to ergocalciferols called provitamin D2.
Our own epidermis generates cholecalciferol, which ultraviolet B light (290- 315nm) converts to D3. The supplement form, and our synthesised form, of Vitamin D3 (Calcidiol), has a long life in the bloodstream, 15 days.
Vitamin D2 yields less active vitamin D than D3. Vitamin D2 is also made synthetically in the industry and is the form found in fortified foods and prescriptions. It is a cheaper form, lower quality, and degrades quickly over time. We recommend you use a supplement which contains naturally occurring Vitamin D3. Vitamin D3 is more effective at raising serum levels.
Daily doses appear to be more effective than taking large doses only once a week or once a month.
We suggest you take a natural form of Vitamin D3, minimum 2000iu per day, preferably 4000iu per day, and consume with a fat meal for better absorption. (consuming together with the other fat-soluble vitamins K2 and A is recommended). Nb. If pregnant only take the recommended daily amount of Vitamin A, never exceed the dose.
Testing your serum levels would give you a better idea of the amount you need to supplement. Always discuss with your health care provider.
Other articles about Vitamin D
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