Sunshine Vitamin D

Q. Does sunlight through window glass provide 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.

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.

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.