RE: Getting Vitamin D from the sun
Posted Thursday, January 7, 2010 at 2:29 PM
Again, great question. I'm not sure this specific issue has been tested - 15 mins at once, 5 minutes 3x per day, run out side 15 times a day for a minute. But here are some details as we do know them as stated by the National Institutes of Health (this is a long post, sorry):
Most people meet their vitamin D needs through exposure to sunlight. Season, geographic latitude, time of day, cloud cover, smog, skin melanin content, and sunscreen are among the factors that affect UV radiation exposure and vitamin D synthesis. The UV energy above 42 degrees north latitude (a line approximately between the northern border of California and Boston) is insufficient for cutaneous vitamin D synthesis from November through February; in far northern latitudes, this reduced intensity lasts for up to 6 months. In the United States, latitudes below 34 degrees north (a line between Los Angeles and Columbia, South Carolina) allow for cutaneous production of vitamin D throughout the year.
Complete cloud cover reduces UV energy by 50%; shade reduces it by 60%. UVB radiation does not penetrate glass, so exposure to sunshine indoors through a window does not produce vitamin D. Sunscreens with a sun protection factor of 8 or more appear to block vitamin D-producing UV rays, although in practice people generally do not apply sufficient amounts, cover all sun-exposed skin, or reapply sunscreen regularly. Skin likely synthesizes some vitamin D even when it is protected by sunscreen as typically applied.
The factors that affect UV radiation exposure and research to date on the amount of sun exposure needed to maintain adequate vitamin D levels make it difficult to provide general guidelines. It has been suggested by some vitamin D researchers, for example, that approximately 5-30 minutes of sun exposure between 10 AM and 3 PM at least twice a week to the face, arms, legs, or back without sunscreen usually lead to sufficient vitamin D synthesis.
Susan Levin, MS, RD
PCRM Director of Nutrition Education