What factors can affect how much vitamin D my body makes from sunlight?

Where you live, the season, and the time of day affect how much ultraviolet B (UVB) reaches your skin. The farther you live from the equator, the less UVB radiation you receive. People who live north of about 37 degrees latitude (picture an imaginary line between San Francisco, CA, and Richmond, VA) can't make any vitamin D from sunlight from November to March, even if they were to stay outside all day. This phenomenon has to do with the angle of the sun; during the winter months, the earth tilts away from the sun, increasing the angle at which the sun's light reaches the earth's surface. More UVB radiation is absorbed by the ozone layer, which lowers or eliminates the amount that can reach a person's skin. As for time of day, the sun's rays are most direct between 10 a.m. and 3 p.m.

What's more, your age, your skin color, how much skin you expose, and your sunscreen use also influence your production of vitamin D. Many people also avoid the sun because they fear skin cancer, a valid concern given that ultraviolet radiation is a known contributor to most of the estimated 1.5 million skin cancers that occur each year in the United States. Any of these factors can combine to limit vitamin D production, which is why a surprisingly large number of Americans -- including half of those age 65 and older -- have relatively low levels of vitamin D.