Q. Does sunlight through window glass provide vitamin D?
A. Sunlight doesn’t actually “provide” you with vitamin D. Rather, your body produces vitamin D when skin is exposed to the sun’s ultraviolet rays, which trigger vitamin D synthesis. The liver and kidneys convert this biologically inert form of vitamin D into biologically active forms the body can use to promote calcium absorption and bone health.
But sunlight consists of both ultraviolet A, or UVA, which penetrates deep within the skin layers and can cause premature aging; and ultraviolet B, or UVB, which causes the redness of sunburn. It’s the UVB rays that trigger the synthesis of vitamin D.
Many people can derive the vitamin D that their bodies need through direct exposure to sunlight during the summer months. As little as 10 minutes a day of sun exposure is typically adequate. But for many, particularly those living in northern climes, production of vitamin D will be inadequate during the winter months.
And you can’t get adequate UVB exposure sitting indoors or in a car. Virtually all commercial and automobile glass blocks UVB rays. As a result, you will not be able to increase your vitamin D levels by sitting in front of a sunny window, though much of the UVA radiation will penetrate the glass and may be harmful.
“It doesn’t matter if it’s winter or summer, you will make no vitamin D sitting in front of a window — zip,” said Dr. Michael Holick, a professor of medicine, physiology and biophysics at Boston University School of Medicine.
Those concerned about low vitamin D levels can get more of the vitamin through foods. The best dietary source for vitamin D is old-fashioned cod liver oil.
Other dietary sources include swordfish and salmon and, to a lesser extent, fortified milk, orange juice and yogurt, as well as sardines canned in oil, egg yolks and fortified cereals. Dietary supplements are also available.