The discovery by researchers that at least 30 percent of the U. S. population is vitamin D deficient has led to a burst of interest in the sunshine vitamin, which has long been known to help strengthen bones and boost the immune system.

“It was a total surprise,” says Stavros C. Manolagas, chief of endocrinology and metabolism at the University of Arkansas for Medical Sciences.

“In retrospect, it turns out to be the way we dress, the way we eat, the way we avoid the sun based on the advice of our dermatologists ... all of that has caused an epidemic of vitamin D deficiency.”

Manolagas said at least 30 percent to 40 percent of the population is vitamin D deficient. Others have estimated that as much as 70 percent of the population is deficient in the fat-soluble vitamin that allows the body to absorb calcium.

Doctors have known the benefits of vitamin D to skin, muscles and bones for more than 50 years. But a growing body of research suggests it may also benefit heart health, reduce high blood pressure, help prevent certain forms of cancer, and curb depression and pain disorders.

“This is probably the hottest topic in nutrition right now from a scientific research standpoint,” says Todd Whitthorne, president and chief operating officer of Cooper Concepts, Inc., part of the Cooper Aerobics Clinic in Dallas. “Over the last three or four years, the research has just exploded.”

But doctors also say much of the research remains inconclusive, and a lot is still unknown about vitamin D. As a result, there is an ongoing conflict between vitamin D experts who say a certain amount of sun exposure is essential to life, and skin-cancer experts who warn about the dangers of sun exposure.

“We’re very concerned about reports that link the health benefits of vitamin D to unprotected ultraviolet exposure, because ultraviolet exposure is a known cause of skin cancer,” says Andrew Kaufman, a dermatologic surgeon in Thousand Oaks, Calif., and a fellow of the American Society for Dermatologic Surgery.

As many as 90 percent of skin cancers are caused by sun exposure, with more than 1 million new cases expected to be diagnosed this year, he adds.

Vitamin D deficiency was called a “pandemic with a heavy public health burden,” in a British Medical Journal article by Gregory Plotnikoff, medical director of the Institute for Health and Healing at Abbott-Northwestern Hospital in Minneapolis.

People who are vitamin D deficient are at increased risk for diabetes, hypertension, infectious and autoimmune diseases, and developing osteomalacia, or a softening of the bones, Plotnikoff wrote.

Harvard Medical School researchers say people who are vitamin D deficient are twice as likely to experience heart trouble, including a heart attack, heart failure or stroke within a five-year period compared with people with normal levels, in a Jan. 8, 2007 report published in Circulation: A Journal of the American Heart Association.