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Sunscreens can block vitamin D

Many doctors say ultraviolet light from the sun or a tanning machine is dangerous because of the skin cancer risk, but increasingly others disagree.   NBC's Robert Bazell reports.

Tamara Smith visits a tanning parlor when she can’t be outside in the sun. “I’m always indoors. I don’t go outside," says Smith.

Many doctors say ultraviolet light from the sun or a tanning machine is dangerous because of the risks associated with skin cancer. But some health experts, such as Dr. Michael Holick of Boston University, disagree.

“I believe that Americans have gone overboard with their fear of the sun. I think that sensible exposure to sunlight is really important for your overall health and well-being,” says Holick.

The reason for the concern is vitamin D, essential for bone strength and other health needs, which our skin makes through exposure to the sun's ultraviolet rays.

We need 1,000 units of vitamin D a day, but a glass of milk supplies only 100 units and a multivitamin only 400. So most people need the sun in order to avoid deficiency.

Sunscreens can reduce vitamin D production
Now, new research has found that wearing sunscreen continuously can reduce the amount of vitamin D a person is able to make.

"We looked at individuals that always wore a sunscreen before they went outside. ... And we found that, indeed at the end of the summer, they were deficient in vitamin D," says Holick. "And so we have shown over and over again that adults, even if they're on a multivitamin, and drinking milk, if they always wear sun protection, or avoid any direct sun exposure, they're at high risk of developing vitamin D deficiency."

Rooftop measurements of sunlight show that, for most people, getting enough sunlight exposure at this time of year is not easy, even for people who don't regularly wear sunscreen. In the middle of the winter on a very sunny day in a city as far north as Boston, there’s not enough sunlight for people to get sufficient quantities of vitamin D.

The good news is that if you get enough sun during the rest of the year, it carries you through the winter, says Holick.

Or there are machines. In Holick’s lab he put young people in tanning machines and measured their bone density.

“Tanners had higher bone density on average than non-tanners,” says Holick.

Still, he cautions against the dangers of skin cancer and warns people not to go overboard. However, it is critical, he says, to realize the sun’s rays are not always our enemy.