DEAR DR. ROACH: Does our skin make vitamin D from sun exposure through a car’s windshield? Sometimes I expose my stomach for about 10 minutes while driving or riding in a car, but I’m not sure if the skin responds the same way through glass as it does with direct sun exposure. — S.R.
ANSWER: Sunlight allows our skin to convert 7-dehydrocholesterol (made from cholesterol) to previtamin D3, which then becomes vitamin D3, the active form. The type of sunlight that is effective for this is in the ultraviolet spectrum, called UV-B. UV-B is effectively filtered out by windshields and ordinary glass, so you are NOT able to get vitamin D through the window: You need to go out into the sun. Ten minutes or so of exposure to your face and arms is roughly equivalent to 200 IU of vitamin D supplement. The exact amount depends on your skin type (lighter skin is much more efficient at using sunlight to convert vitamin D), your latitude, the season and the time of day.