For years the message has been simple: less sun, less skin cancer. But for melanoma, by far the deadliest form of skin cancer, the story's more complex. Some researchers have found that vitamin D may help prevent melanoma, and other cancers, from spreading (metastasis). Indeed, melanoma patients with higher blood levels of vitamin D at diagnosis seem to have a higher rate of surviving the sometimes fatal disease. We spoke with leading skin cancer expert Marianne Berwick, Ph.D., M.P.H., chief of the Epidemiology and Biostatistics division in the Department of Internal Medicine at the University of New Mexico in Albuquerque.

Is the sun to blame for melanoma?

The relationship between the amount of sun exposure and the development of melanoma is not clear. What is clear is that people who get daily sun exposure -- like people who work in the sun -- are not at increased risk for melanoma. People who get intermittent sun exposure, large blasts that cause you to get sunburn, are at a raised risk.

How much sun is enough?

The appropriate dose of sunlight each person should get is hard to say. A little bit of sun won't hurt you, but we really aren't sure. Researchers are working on the normal relationship. The wisdom at this point is 15 minutes a day will give you adequate vitamin D for about three days. I don't think that is going to give you melanoma. We don't know for sure if you can get significant amounts of vitamin D wearing sunscreen or not.

Can you get too much vitamin D?

You cannot get too much vitamin D in the sun because UV exposure is self-regulating. Once you get to a top level, it starts to degrade so it won't become toxic. But if you take too many supplements they can become toxic.

Should we exercise caution with sun exposure?

It's always wise to protect yourself. People should wear hats and long sleeves and not go out at midday. I myself prefer long, loose clothing. If you are going to be outdoors for many hours, apply sunscreen. Follow your instincts -- if it doesn't feel good, get out of the sun.