How Much Sun Does It Take to Make Vitamin D?

Hint: A lot more than you'd think.


People are puzzling over the new Vitamin D guidelines for children, and readers have sent insightful questions my way. Here's more information on the top two questions: Whether breastfeeding women can increase their Vitamin D so they don't have to give babies vitamin supplements, and why is it that living north of a certain latitude makes it difficult to synthesize Vitamin D in the skin. D is considered essential for bone growth and immune function and may play a role in preventing heart disease and other chronic diseases.

Why can't pregnant women and breastfeeding moms just increase their Vitamin D intake, rather than give supplements to babies?

The amounts that breastfeeding women would have to take is very large—more than 4,000 IU a day—if they wanted to raise the D level in their milk enough to provide 400 IU a day, according to Frank Greer, a professor of pediatrics at the University of Wisconsin who is the point man for the American Academy of Pediatrics on Vitamin D. The safety for lactating mothers of this level of vitamin D has not been established in large numbers of subjects, Greer says. Side effects could include kidney stones. Since pediatricians knew that 400 IU is safe for an infant, they went with that. For pregnant women, 400 IU a day from prenatal vitamins seems to be effective in increasing the vitamin D levels in the fetus, so 400 IU is recommended for all pregnant women. Why can't children get enough Vitamin D from being exposed to the sun?

People living north of Chicago can't get enough sun exposure to make Vitamin D in their skin from November through February, according to the National Institutes of Health's Office of Dietary Supplements. That's based on a calculation of the sun's zenith and the fact that the Earth is tilted. As a result, much less UV light reaches northern latitudes in the winter. People who live south of Los Angeles (34 degrees north) get enough UVB in sunlight to make D in their skin all year. However, even in L.A., there's no simple way to figure out how much UV light exposure a person needs to synthesize a given amount of D. Air pollution, cloud cover, a person's skin color, his or her body mass, altitude above sea level, and how much clothing he or she wears all factor in. The NIH recommends 5 to 30 minutes of sun between 10 a.m. and 3 p.m. at least twice a week on the face, arms, legs, or back without sunscreen, but that's clearly a guesstimate, since it doesn't factor in these many other variables.

The Norwegian Institute for Research has an online calculator to help you figure out if there's enough sunlight for you to start synthesizing Vitamin D at a particular location. Unfortunately, it doesn't include U.S. cities, but Rome is at the same latitude as Chicago (no wonder the pizza's so good there), and London is at the same latitude as Calgary. If I were in Rome today (October17) and it were nice and sunny, I would have nine hours in which the sun is strong enough for my skin to start synthesizing D. (Here's a link to the latitudes of major American cities.)

Alas, I'm at my desk for those nine hours, so my multivitamin had better contain Vitamin D. Americans are estimated to spend 93 percent of their time indoors nowadays, and children are part of that trend. It's as if we're all living in Norway. Scandinavians traditionally have fed their children a spoonful of cod liver oil a day to keep them healthy in the dark winter months. There are about 400 IU of D in a teaspoon. So maybe it's time we all start dosing up, too.