Vitamin D is nicknamed the "sunshine vitamin" because the skin makes it from ultraviolet rays. Sunscreen blocks its production, but dermatologists and health agencies have long preached that such lotions are needed to prevent skin cancer. Now some scientists are questioning that advice. The reason is that vitamin D increasingly seems important for preventing and even treating many types of cancer.

In the last three months alone, four separate studies found it helped protect against lymphoma and cancers of the prostate, lung and, ironically, the skin.

Many people aren't getting enough vitamin D. It's hard to do from food and fortified milk alone, and supplements are problematic.

So the thinking is this: Even if too much sun leads to skin cancer, which is rarely deadly, too little sun may be worse.

No one is suggesting that people fry on a beach. But many scientists believe that "safe sun" -- 15 minutes or so a few times a week without sunscreen -- is not only possible but helpful to health.

One is Dr. Edward Giovannucci, a Harvard University professor of medicine and nutrition who laid out his case in a keynote lecture at a recent American Association for Cancer Research meeting in Anaheim, Calif.

His research suggests that vitamin D might help prevent 30 deaths for each one caused by skin cancer.

"I would challenge anyone to find an area or nutrient or any factor that has such consistent anti-cancer benefits as vitamin D," Giovannucci told the cancer scientists. "The data are really quite remarkable."

Even some dermatologists may be coming around. "I find the evidence to be mounting and increasingly compelling," said Dr. Allan Halpern, dermatology chief at Memorial Sloan-Kettering Cancer Center in New York, who advises several cancer groups.

The dilemma, he said, is a lack of consensus on how much vitamin D is needed or the best way to get it.

No source is ideal. Even if sunshine were to be recommended, the amount needed would depend on the season, time of day, where a person lives, skin color and other factors. Doctors worry that folks might overdo it.

Dr. John Kirkwood, director of the Melanoma Center at the University of Pittsburgh, is among those who oppose a suggestion that there can be levels of "safe" sun exposure for the purpose of getting enough vitamin D.

"Everbody in this society drinks vitamin D through milk," he said. "The amount that you need is easily possible to take in through the food that you eat or as a vitamin tablet. There is no excuse for sun exposure to prevent illness."