A concerned reader writes to the science section of the New York Times to ask: “Am I still getting vitamin D when I’m outside on a gray, cloudy day?” The answer from the Times explains that your skin needs exposure to ultraviolet-B rays in order to synthesize vitamin D. Unfortunately, this is the same ultraviolet-B that causes sunburn and skin damage. Finding the optimal exposure time is complicated, especially when the amount of UV-B energy is affected by factors such as cloud cover and latitude.
To strike a balance between useful exposure and protection, the N.I.H. recommends an initial exposure of 10 to 15 minutes, followed by application of a sunscreen with an S.P.F. of at least 15. The institutes say this much exposure, at least two times a week, is usually sufficient to provide adequate vitamin D, though some researchers suggest it may not be enough. At the earth’s northern latitudes for much of the year, and at the midlatitudes in winter, the sun does not stay far enough above the horizon (45 degrees) for the angle of the sun’s rays to guarantee an efficient ultraviolet-B bath.
So even if you follow the NIH recommendations to the letter, the resulting UV-B exposure still may be too little or too much? Sorry, but I’m not going to waste my time on a process as inconvenient and unreliable as this.
Fortunately, I don’t have to. Vitamin D is available in pill form in any grocery store. Yes, your skin can synthesize it, but it doesn’t have to. The pills are inexpensive and convenient; why not use them? You get exactly the right dosage every time (regardless of cloud cover or latitude) and there’s no risk of sunburn or skin damage.
There are plenty of good reasons to go outside and let the sun shine on you, but nobody should feel obligated to do so in order to get enough vitamin D. It simply isn’t necessary.