Can sunlight do you some good when it comes to cancer? The theory has been around for decades, fueled in part by statistics showing that people in sunnier climates are less likely to die from cancer than those in gloomier areas. Now two new studies published in the Journal of the National Cancer Institute (Vol. 97, No. 3: 195-199 and 199-209) are adding to the evidence.

One study, led by researchers at the Karolinska Institutet in Sweden, found that people who got lots of sun exposure through sunbathing or vacations in sunny places were less likely to develop lymphoma than people who stayed out of the sun. The other, led by researchers at the University of New Mexico, Albuquerque, found that people with melanoma were less likely to die from the disease if they had had lots of sun exposure.

The findings were somewhat surprising because ultraviolet (UV) radiation from sun exposure is known to raise the risk of developing melanoma, and is also thought to play a role in the development of lymphoma.

Both groups of researchers say sunlight's influence on vitamin D may provide one possible explanation for the results. But the theory is still just that: a theory.

"It raises a lot of questions, more than answering any," said Martin A. Weinstock, MD, PhD, professor of dermatology at Brown Medical School and chairman of the American Cancer Society's Skin Cancer Advisory Board. He was not involved with either of the studies.

Vitamin D is a crucial nutrient that is found in certain foods like oily fish (salmon, tuna, mackerel, and sardines), liver, and fortified milk, cereals, and orange juice. Sunlight is another source; the UV rays stimulate the skin to produce it.

We all need vitamin D to keep our bones healthy and strong. But it also seems to have an effect on the immune system and on how cells grow and change -- and that action may influence the development of some types of cancer.

"Vitamin D is what everyone's really interested in right now," said Marianne Berwick, PhD, MPH, head of Epidemiology and Cancer Prevention at the UNM Cancer Research and Treatment Center and first author of the melanoma study. "I think [our study] points to a genetic or functional relationship with the sun that we hadn't thought about before."

It also suggests that sunshine may not be the only thing that triggers melanoma.

"I think that what happens is that some people are more genetically predisposed than others, so even with low, low levels of sun exposure they will get melanoma -- or maybe it's not due to sun exposure," Berwick said.

Weinstock agreed. "It may be that the types of melanomas in people who have lots of sun exposure are not as aggressive as those that arise in people with less sun exposure," he said. "Or it might be that something about sun exposure affects prognosis. [The researchers] suggest vitamin D as a protective factor, but we just don't know."

The same applies to the relationship between sun and lymphoma. Sun exposure was thought to raise lymphoma risk because people who have had some types of skin cancer are also more likely to get lymphoma. But the study showed just the opposite: Sun exposure seemed to have a protective effect.

"It's really not clear why this is," Weinstock said. "It could be any number of factors. Vitamin D was suggested, but it could be totally unrelated to that, so basically it means [we need] more research to clarify what's going on."

Until more is known, it's important to keep protecting yourself from too much sun.

"Sun protection should definitely be practiced," Berwick said. "There is no doubt in most people's minds that excessive sun exposure… can lead to the development of skin cancer, including melanoma."

Getting enough vitamin D is also important, Weinstock said.

"There are many people whose [vitamin D] levels are less than optimal in the US," he said, "so taking a multivitamin with vitamin D is a reasonable step. Whether taking that multivitamin will improve your survival from melanoma or reduce your risk of lymphoma is an open question. These studies say maybe, but lots more work is needed."

Still, the findings do open new research directions that may turn out to be very helpful for both cancers, Weinstock said.

"We don't have good therapies [for melanoma] once surgery has failed, so if this ends up being a clue to new therapy, that would be great," he said. "And in terms of lymphoma, if this gives us a clue on how to prevent them, that would be great because we don't have a means of preventing them."