Getting Vitamin D from the Sun
Researchers say that the human body can generate 10,000 to 12,000 IUs of vitamin D from a half-hour of summer-sun exposure. However, just because we can do this, does not mean that it actually happens. Although most of us have been told that we can get enough vitamin D simply by exposing our arms and legs to sunlight for ten minutes a day, this is not necessary true. The real story is far more complicated than that.
First of all, it appears that it is only the UV-B rays that our bodies use to manufacture vitamin D when we are out in the sun. This means that most tanning salons that use UV-A will not help your body manufacture D. Furthermore, there is only enough UV-B to produce adequate D when the rays are at their strongest in northern climates – this means in the summer (or perhaps spring and fall, depending on your latitude*), and also during the hours of 10 to 2 pm, exactly when we have been told to stay out of the sun! At other times, your skin will burn before you produce enough D.
* Both latitude and altitude affect the amount of UV-B rays that reach us. At higher altitudes, more UV-B gets through, so more vitamin D can be produced. At latitudes higher than 45 degrees, it is very difficult for your body to produce optimum levels of D even in the summer! At levels between 30 and 45 degrees, for 2 to 8 months or more you will not be able to produce optimum D, even being out in the midday sun.
In addition, because body oils that contain cholesterol are crucial for absorbing the D you have manufactured, and it take 30-60 minutes to absorb it fully, you need to wait before showering or swimming, especially in chlorinated pools. Also, because you product 100-200 IU of D for each 5 percent of body surface exposed, if you're sunning in a bikini, you will produce a lot more D than if you are out walking with just your arms, face and perhaps legs exposed.
If you are light skinned, you will only need 10-20 minutes of exposure to reach the maximum your body will produce for the day. If you are dark skinned, you may need closer to 90-120 minutes. If you use a sunscreens with a sun protection factor (SPF) of 8 or more, you will be blocking the UV-B rays that produce vitamin D, so you need to be out in the sun without sunscreen.
Last, but not least, UV-B rays, for the most part, do not travel through glass, clouds, fog or smog, so in order to make the most vitamin D, you need to be outside on a sunny, smog-free day.
Note: There are a couple mechanisms that prevent us from getting too much vitamin D from sunlight. One is that vitamin D production will shut down after a certain amount is made from any particular exposure. The other is that as we get more vitamin D, we also produce more melanin pigment (which gives us the "tanned" look), and this melanin prevents some of the UV-B rays from getting through.
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