That's not what Bruce Armstrong, MD, PhD, of the University of Sydney, Australia, expected to find. Armstrong, an authority on the causes of skin cancer, says he expected to find that the more time people spent in the sun, the more likely they would be to get non-Hodgkin's lymphoma. Non-Hodgkin's lymphoma cases are on the rise in the industrialized world, but nobody knows what causes it.
Instead, Armstrong and colleagues found that women and men who got the most sun exposure during their off-work hours had the lowest risk of non-Hodgkin's lymphoma. Armstrong reported the findings at this week's annual meeting of the American Association for Cancer Research in Orlando, Fla.
"This is an unexpected finding," Armstrong said in a news conference. "What might be causing it? The obvious answer is that vitamin D synthesized in the skin from sun exposure is causing this effect. There is increasing evidence that vitamin D has protective effects against many cancers. The evidence for colorectal cancer protection is pretty solid."
Fun in the Sun
Armstrong's team polled more than 700 20- to 74-year-old Australian men and women with non-Hodgkin's lymphoma and about 700 matched people without cancer. Study participants estimated their working, non-working, and vacation hours of sun exposure at ages 10, 20, 30, 40, 50, and 60. Earlier studies showed that people give accurate estimates of sun exposure.
The researchers found that the people in their study who spent the most time in the sun had 35% less non-Hodgkin's lymphoma than those who spent the least time in the sun. Most of this effect was seen in women.
But it doesn't take much sun to stimulate vitamin D production in the skin. Vitamin D production cuts off after a small amount of sun exposure. So Armstrong's team looked at occasional sun exposure during off-work hours. And that's where they saw the strongest effects. The risk of non-Hodgkin's lymphoma dropped by 60% in women with the most off-work sun exposure. It dropped by 40% in men with the most off-work sun exposure.
Is Sun Safe?
Of course, unprotected sun exposure vastly increases a person's risk of skin cancer. There are other ways to get enough vitamin D than by risking melanoma. Armstrong notes that most Australians think they get plenty of vitamin D from sun -- but in fact, vitamin D deficiency is relatively common there. Unlike the U.S., he says, Australian dairies don't usually add vitamin D to milk.
Armstrong warns that his findings must be confirmed by further studies. He does not advise people to go out in the sun to lower their cancer risk. But he says he wonders whether there might be a happy medium between too much and too little sun.
"I don't think there are two messages in conflict -- get sun to avoid lymphoma but avoid sun to avoid skin cancer," he says. "The amount of sun exposure needed to get vitamin D is not large. Melanoma is from significant overexposure to the sun. So maybe you can get the best of both worlds."