Sunlight triggers the synthesis of vitamin D within the body. Stanford researchers found that this action causes immune cells to travel to the outer layers of the skin where they are available to protect and help repair damage such as that caused by sun exposure.
There is a growing body of research suggesting that vitamin D deficiency increases the risk for a host of human cancers, as well as other disorders including type 1 diabetes, rheumatoid arthritis, tuberculosis, and multiple sclerosis.
But the idea that sunlight, in small doses, may actually protect the skin from damage is bound to be controversial. A dermatologist who spoke to WebMD was skeptical.
"Everyone agrees that vitamin D is useful, even though most of the benefits that have been attributed to it are still theoretical," New York University dermatology professor Darrel Rigel, MD, says.
"What isn't theoretical are the more than 8,100 Americans who will die of melanoma this year and the 1 million Americans who will get skin cancers. The vast majority of these cancers are caused by UV exposure."
Researcher Hekla Sigmundsdottir, PhD, tells WebMD that the findings are preliminary and that clinical trials are needed to confirm the findings.
The study is published in the January issue of the online journal Nature Immunology.
Don't Abandon Your Sunscreen
The researchers found that sunlight exposure was the most efficient way to induce the receptors that direct protective cells to the skin. But nobody is suggesting that people abandon their sunscreen.
"We obviously aren't saying that it is a good idea to go out and lie in the sun all day," Sigmundsdottir says. "This paper doesn't prove that sunlight can be good for your skin. It is still a hypothesis."
Rigel says even if the hypothesis proves true, people can get all the vitamin D they need through vitamin supplements, the foods they eat, and incidental exposure to sunlight.
"Even if you wore sunscreen as directed, which means reapplying every few hours, you would probably still get enough sun exposure to more than meet your needs for vitamin D conversion," he says.
How Much Is Enough?
The answer to the question, "How much vitamin D do I need?" depends on who you are and whom you ask. Age, skin type, where you live, and the season of the year all affect vitamin D levels. Federal guidelines say adults should get between 200 and 600 international units (IU) of vitamin D each day, with recommended levels increasing with age.
Vitamin D researcher Michael F. Holick, MD, PhD, believes the optimal daily dosage for preventing disease is closer to 1,000 IU. Holick heads the Vitamin D Research Lab at Boston University and is the author of the book The UV Advantage.
Holick tells WebMD that he takes a 1,000-IU vitamin D supplement each day.
"It is not easy to get the vitamin D you need from foods unless you make a special effort," he says. "A glass of milk or vitamin D-fortified orange juice has only about 100 IU of vitamin D, and a serving of salmon has only about 500 IU."
Holick says most people can get more than enough vitamin D in the spring, summer, and fall by engaging in what he calls "sensible sun exposure" -- no more than five to 10 minutes of direct sun to unprotected legs and arms two or three times a week.
"We are not talking about burning in the sun," he says. "No one is saying that is good for you."
SOURCES: Sigmundsdottir, H. Nature Immunology, online edition, January 2007. Hekla Sigmundsdottir, PhD, post doctoral fellow, Stanford University. WebMD Feature: "Vitamin D: Vital Role in Your Health." Michael Holick, MD, director, Vitamin D Research Lab, Boston University Medical Cente. Darrel Rigel, MD, clinical professor of dermatology, New York University, New York City.
By Salynn Boyles
Reviewed by Louise Chang