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 Oakland, 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 UC 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 sun rays 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 which 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.