Inside a laboratory at Stanford University, researchers are confidently pursuing evidence that vitamin D plays an important role in breast and prostate cancer prevention.

At Children's Hospital in Oakland, Calif., a famed nutritionist is convinced that widespread deficiency of vitamin D in the U.S. population leads to poor immune system and brain functioning, among other conditions.

And scientists at University of California-Davis this month were awarded $600,000 by the federal government to study the link between vitamin D and major diseases of the day.

For decades, most people paid little attention to vitamin D -- called the "sunshine vitamin," since sunrays absorbed by the skin synthesize the nutrient.

Vitamin D's historic claim to fame has been its role in building and maintaining strong bones and teeth by regulating calcium levels.

But to their surprise, scientists in recent years discovered that vitamin D appears to play an underappreciated role in preventing just about every major disease afflicting Western societies, from cancer and cardiovascular disease to multiple sclerosis and type 1 diabetes.

"There are so many things that vitamin D may do that are beneficial," said Dr. David Feldman, a professor at Stanford University's School of Medicine, who has studied the health effects of the nutrient for 25 years as well as edited a $500 academic tome on vitamin D, now in its second printing.

"I'm afraid if it's hyped too much, people are going to think nothing can be this good, that it works on all these diseases," Feldman added.

But there was no hype at a September conference held by the National Institutes of Health to re-examine federal guidelines on vitamin D.

The gathering was called "Vitamin D and Health in the 21st Century _ An Update," and a conference overview described "a growing prevalence of vitamin D deficiency in the U.S. population," along with "accumulating evidence" of the nutrient's function in supporting sound health, far beyond healthy bones and teeth.

Dr. Michael Holick, a scientist at Boston University Medical Center and a pioneer in vitamin D research, stated that 30 to 50 percent of the U.S. population is chronically or seasonally deficient in the nutrient.

The rate is even higher in those with dark skin, as the melanin that protects them from sunburns also slows, by six-fold or more, vitamin D synthesis.

Vitamin D deficiency is widespread among black Americans, and is suspected to play a role in the high rates of prostate cancer and hypertension in that population.

Holick and a handful of other researchers for more than decade have insisted that vitamin D deficiency is a major public health issue, in a country where many people stay indoors most of the day and cloak themselves in clothes and sunscreen _ which blocks vitamin D synthesis _ when they are outside.

The rickets outbreak in the early 19th century, which was remedied initially with cod liver oil and then later by fortifying milk with vitamin D, was only the most extreme manifestation of vitamin D deficiency, they point out.

Then about a decade ago, scientists first used radioactive material that illuminates cellular details to study vitamin D's role in the body. The researchers were in for a surprise.

Feldman, who was among those researchers, said they first found vitamin D receptors exactly where they'd expect _ in the intestinal tract, where the nutrient shuttles calcium into the blood stream, and in bones. Receptors were also found in the kidneys, which metabolize vitamin D into its active form. The presence of receptors indicates that the nutrient plays a pivotal role in the functioning of the tissue.

But then, scientists unexpectedly found regions throughout the body with the same receptors _ in breast tissue, the prostate, the brain, immune cells and the heart, among many other areas.

"We started finding them in all these other places," Feldman said. "In animals, we found to our surprise, they were everywhere. At low levels, but everywhere."

Dr. Bruce Ames, a scientist with Children's Hospital Oakland, who's well-known for his work on nutrients and disease prevention, pointed out in a statement on vitamin D and brain function that more than 900 different genes, in regions throughout the body, bind to the vitamin D receptor.

Americans typically get more than 90 percent of their vitamin D from the source nature intended _ the sun, Holick stated.

There are very few natural food sources, with cold-water fish like salmon and cod providing the highest levels. Milk is supplemented with it (but not other dairy products like cheese or ice cream).

While this enriched milk is credited with all but ending rickets in the country, it's a voluntary program on the part of manufacturers, and Holick and his colleagues found that actual levels of vitamin D were often lower than the amount listed on the carton.

But with most people now spending the bulk of their days inside, and with rampant warnings about sun exposure and skin cancer, many Americans are now deficient in their vitamin D levels.

Feldman is among the many vitamin D researchers who advocate that people take vitamin D supplements to make up the shortfall. It's especially important during the winter, when at latitudes north of 35 degrees (San Francisco is at latitude 37), little or no UVB rays, which create vitamin D in the skin, reach the ground, as they're absorbed in the atmosphere due to the angle of the sun.

These researchers also suggest taking 1,000 IU (international units) daily, although the federal government advises those up to age 50 to consume 200 IUs daily. Because the vitamin is metabolized less efficiently with age, the levels are upped to 400 IUs for those 51 to 70, and 600 IUs for those 71 and older. Feldman cautions, however, that those prone to kidney stones should take lower levels, since vitamin D increases calcium absorption.

But most vitamin D researchers think current recommended levels are too low, and federal officials have indicated they agree. Many expect in coming years that the guidelines will increase to between 800 to 1,000 IUs per day.

"Based on what we're seeing, 400 IUs is probably not enough to get people up to an adequate level if they don't have enough sun exposure," said Charles Stephensen, Ph.D., a researcher with the U.S. Department of Agriculture's Western Human Nutrition Research Center at University of California Davis.

Holick also advises people to get 10 to 15 minutes of sun exposure, two to three times per week during non-winter months, and after that, put on sunscreen. Vitamin D is stored in fat, so adequate levels built up during the sunny season can get people through the winter.

Even the American Cancer Society recently departed from its long-standing advice to stay out of the sun to protect against skin cancer by stating that "supplementation and small amounts of sun exposure are the preferred methods of obtaining vitamin D."

With so much unknown about the full range of vitamin D's health effects, scientists still couch their expectations for the nutrient in guarded terms.

"We hope there's compelling proof that dietary vitamin D can have a beneficial effect on breast cancer," said Feldman, who is about to launch a clinical trial on the subject.

"There's growing evidence in many areas that vitamin D is protective," Feldman added. "But the evidence is still preliminary in many cases. That's why there's so much research going on."