Q. Does sunlight through window glass provide vitamin D?
A. Sunlight "does not" provide you with vitamin D. On the contrary, your body produces vitamin D when the skin is exposed to the ultraviolet rays of the sun, which activate vitamin D synthesis. The liver and kidneys convert this biologically inert form of vitamin D into biologically active forms that the body can use to promote calcium absorption and bone health.
But sunlight consists of both ultraviolet A and UVA, which penetrates deep into the skin layers and can cause premature aging; and ultraviolet B, or UVB, which causes the redness of sunburn. It is the UVB rays that activate the synthesis of vitamin D.
Many people can derive the vitamin D that their body needs through direct exposure to sunlight during the summer months. Only 10 minutes per day of exposure to the sun is usually sufficient. But for many people, especially those living in northern climates, the production of vitamin D will be insufficient during the winter months.
And you can not get enough UVB exposure inside or in a car. Virtually all commercial and automotive glass blocks UVB rays. As a result, you will not be able to increase your vitamin D level by sitting in front of a sunny window, although much of the UVA radiation will penetrate the glass and be harmful.
"The it does not matter if it's winter or summer, you will not be taking vitamin D for a window ride, "said Dr. Michael Holick, professor of medicine, physiology and biophysics at the Boston School of Medicine.
Those who are concerned about low vitamin D levels can get more vitamin from the food. The best source of nutrition for vitamin D is old-fashioned cod liver oil. Other food sources are swordfish and salmon and, to a lesser extent, fortified milk, orange juice and yoghurt, as well as canned sardines in oil, egg yolks and fortified grains. Dietary supplements are also available.
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