Scientists are excited about a vitamin again. Unlike fads that sizzled and fizzled, the evidence is strong and keeps growing.

If it bears out, it will challenge one of medicine's most fundamental beliefs: that people need to coat themselves with sunscreen when they're in the sun. Doing that actually may contribute to far more cancer deaths than it prevents, some researchers believe.

The vitamin is D, nicknamed the "sunshine vitamin" because the skin makes it from ultraviolet rays. Sunscreen blocks its production, but dermatologists and health agencies long have preached that such lotions are needed to prevent skin cancer.

Some scientists now are questioning that advice.

The reason is that vitamin D increasingly seems important for preventing and even treating many types of cancer. In the past three months alone, four studies found the vitamin helped protect against lymphoma and cancers of the prostate, lung and, ironically, the skin. The strongest evidence is for colon cancer.

Many people aren't receiving enough vitamin D. Food and fortified milk often aren't enough; 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 healthful.

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."

An American Cancer Society official said the new research almost certainly would affect an ongoing review of the organization's sun-protection guidelines.

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.

The cancer society's chief epideiologist, Dr. Michael 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 that is difficult. Vitamin D occurs naturally in salmon, tuna and other oily fish, and routinely is added to milk. However, diet accounts for little of the vitamin D circulating in blood, he 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 often contain small amounts of D-2 and include vitamin A, which offsets many of D's benefits. As a result, pills might not increase vitamin D levels much at all.

Government advisers can't even agree on the recommended daily allowance for vitamin D. 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.

Giovannucci's research suggests 1,500 IUs might be needed to curb cancer significantly.

Thirty years ago, Dr. Michael Holick helped make the landmark discovery of how vitamin D works. Until last year, he was chief of endocrinology, nutrition and diabetes and a professor of dermatology at Boston University. He then published a book, "The UV Advantage," urging people to get enough sunlight to make vitamin D.

"I am advocating common sense," not prolonged sunbathing or tanning salons, Holick said.

Skin cancer rarely is fatal, he notes. The most deadly form, melanoma, accounts for 7,770 of the 570,280 cancer deaths expected to occur in the United States this year.

More than 1 million milder forms of skin cancer will occur, and these are the ones tied to chronic or prolonged suntanning.

Repeated sunburns — especially in childhood and among redheads and very fair-skinned people — have been linked to melanoma, but there is no credible scientific evidence that moderate sun exposure causes it, Holick contends.

"The problem has been that the American Academy of Dermatology has been unchallenged for 20 years," he said. "They have brainwashed the public at every level."

The drug industry has spent money attacking Holick. One such statement from the Sun Safety Alliance, funded in part by Coppertone and drugstore chains, declared that "sunning to prevent vitamin D deficiency is like smoking to combat anxiety."