Vitamin D and the Sun Exposure
Vitamin D is also know as 'sunshine vitamin', since human body can produce it on it's own with the help of UVB light - therefore it is not strictly essential dietary vitamin.
How Much Sun Exposure For Vitamin D
Exact amount of time, spent outside on sun, needed for adequate vitamin D production depends on many things: season, latitude, amount of exposed skin, color of the skin, protection factor of used sun block cream (if any), weather, pollution etc.
Season - during summer, intensity of the sun light is much higher than during the winter.
Latitude - higher the latitude, less sun light hits the surface. During summer there is plenty of direct sun light hitting latitudes up to 60°to 70° north. During winter sun light intensity is very low.
Amount of exposed skin - it is not the same if we have only face and arms exposed to the sun light or we are 'frying' ourselves somewhere on the beach.
Color of the skin - people with darker skin produce lower amounts of vitamin D for the same amount of UVB light when compared with people with lighter skin.
Sun block creams - sun creams block both UVA and UVB rays and thus limit production of vitamin D
Weather and pollution - clouds, fog and air pollution can decrease amounts of UVB rays and hamper production of vitamin D in the skin.
Rule of thumb - if you spend 3 times per week, 15-20 minutes each time outside with your hand and face exposed to the sun during summer, on medium latitude, you will produce more than enough vitamin D.
And this is just from sun exposure - if you follow a healthy diet with several fish meals during the week, you should have no worries about vitamin D intake.
People spending plenty of time on the sun during summer, using sun creams with high protection factor (for example PF of 15, 25 or even 50) will also produce more than enough vitamin D, but they risk other problems - skin cancer and similar skin and other problems.
Vitamin D From Sun
So, to produce vitamin D (it's active form), body needs UVB rays.
Body production of vitamin D:
|7-dehydrocholesterol is the precursor of vitamin D3. In the epidermal layer of the skin, 7-dehydrocholesterol reacts due to UVB irradiation, which results in the opening of the B-ring and creation of previtamin D3.|
|Afterwards, previtamin D3 spontaneously isomerizes to form vitamin D3 (cholecalciferol).|
|Cholecalciferol (ingested or made by skin UVB irradiation) is hydroxylated (OH group is added) in the liver at position 25 (upper right of the molecule) to form 25-hydroxycholecalciferol (also known as calcidiol or 25(OH)D). Then, calcidiol is released into the plasma, where it is bound to an α-globulin, vitamin D binding protein.|
|Calcidiol in plasma is transported to the kidneys' proximal tubules, where it is hydroxylated (again is one OH group added) at the 1-α position (lower right of the molecule) to form 1,25-dihydroxycholecalciferol (also know as calcitriol and often abbreviated into 1,25(OH)2D). The conversion of calcidiol to calcitriol is catalyzed by the enzyme 25-hydroxyvitamin D3 1-alpha-hydroxylase, the levels of which are increased by parathyroid hormone (and additionally by low calcium or phosphate).|
Again, (very) long story short - stay away from direct sunlight during summer to the large areas of the skin to avoid skin burns and other skin related problems. Generally, spending 15-20 minutes, three times per week with exposed face and arms will produce enough vitamin D naturally. Combine it with healthy diet including fish several times per week and you will do fine.
If you, however, have problems related to vitamin D levels, after consulting your doctor, vitamin D supplements are needed to treat vitamin D deficiency.