In so doing, researchers are challenging one of medicine's most fundamental beliefs: that people need to coat themselves with sunscreen whenever they're in the sun.

In the past three months, four studies have found that vitamin D, known as the sunshine vitamin, helps prevent lymphoma and cancers of the prostate, lung and, ironically, the skin. The strongest evidence is for colon cancer.

But many people aren't getting enough vitamin D. It's hard to do from food and fortified milk alone. Supplements are problematic, and sunscreen blocks the production of vitamin D, which the skin makes from ultraviolet rays.

Dr. Edward Giovannucci, a Harvard University professor of medicine and nutrition, laid out his case for more vitamin D 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."

The talk so impressed the American Cancer Society's chief epidemiologist, Dr. Michael Thun, that the society is reviewing its sun-protection guidelines. "There is now intriguing evidence that vitamin D may have a role in the prevention as well as treatment of certain cancers," Thun said.

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. Thun and others worry that folks might overdo it.

"People tend to go overboard with even a hint of encouragement to get more sun exposure," Thun said, adding that he would prefer people get more of the nutrient from food or pills.

But this is difficult. Vitamin D occurs naturally in salmon, tuna and other oily fish and is routinely added to milk. However, diet accounts for very little of the vitamin D circulating in blood, Giovannucci said.

Supplements contain the nutrient, but most use an old form -- D-2 -- that is far less potent than the more desirable D-3. Multivitamins typically contain only small amounts of D-2 and include vitamin A, which offsets many of D's benefits.

As a result, pills might not raise vitamin D levels much at all.

Government advisers can't even agree on an RDA, or recommended daily allowance, for vitamin D. Instead, they say "adequate intake" is 200 international units a day up to age 50, 400 IUs for ages 50 to 70 and 600 IUs for people older than 70.

Many scientists think adults need 1,000 IUs a day. Giovannucci's research suggests 1,500 IUs might be needed to significantly curb cancer.

During short winter days, the sun's rays come in at too oblique an angle to spur the skin to make vitamin D. That is why nutrition experts think vitamin D-3 supplements may be especially helpful during winter and for dark-skinned people all the time.

But too much of the pill variety can cause a dangerous buildup of calcium in the body. The government says 2,000 IUs is the upper daily limit for anyone older than a year.

On the other hand, D from sunshine has no such limit. It's almost impossible to overdose when getting it this way. However, it is possible to get skin cancer.