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Re-activateYou've reported that people generally need some sun exposure to maintain adequate vitamin D levels. But does sunlight passing through windows trigger production of vitamin D? —D.P., Baie Verte, Newfoundland, Canada
No. Glass blocks ultraviolet B (UVB) radiation, the type that stimulates vitamin D synthesis by the skin. To get enough of the vitamin, you probably need to briefly expose some skin to direct sunlight a few days a week during the warmer months––unless you take a multivitamin containing D or consume lots of D-fortified milk plus some fatty fish. (Avoid high fatty-fish intake, which may supply excessive amounts of mercury and other toxins.)
However, you can get sunburned through a window. That's because windows don't block UVA, the other harmful type of radiation from sunlight, unless the glass is specially coated or tinted. Like UVB, UVA can cause sunburn and skin aging as well as an increased risk of skin cancer and possibly cataracts and macular degeneration. So apply sunscreen if you're driving or just sitting in front of a window in bright sunlight for more than about 20 minutes in most parts of North America, and less than that in southern states in the U.S.
Find out if it makes sense for you to take a vitamin D supplement and read the latest research on vitamin D and heart disease prevention.
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