Fort Lauderdale Ocean Rescue Lt. Jim McCrady spends 10-hour days patrolling the beach. Most people would worry that's too much sun. But to some, McCrady, 41, is soaking up critical ultra-violet rays. McCrady may be slightly pink at the end of his four-day work week, but there's one thing he's not suffering from: lack of vitamin D.

Many of us cannot say the same.

Vitamin D comes primarily from the sun, not food. It's best known for helping calcium build strong bones. But an exploding area of research is raising theories about vitamin D's vital role in staving off colon cancer, multiple sclerosis, diabetes and heart disease.

For years it was assumed people got enough D from fortified milk and everyday sun exposure. Not so. Today, even by conservative estimates, at least half of all Americans have insufficient levels of vitamin D. Among those most at risk: breast-fed infants, adults over 50 and people with darker complexions.

Researchers call it ''an unrecognized epidemic,'' but its consequences could become apparent soon enough for this generation and its children.

''`We're seeing [weaker bones] not only in women, but men, too. As they get older, more and more men are having fractures, and it's the men who have two or three greater chances of dying after a hip fracture than women,'' says Dr. Silvina Levis, director of the University of Miami Osteoporosis Center.

South Floridians -- despite living in a sun-drenched climate -- are not immune. A University of Miami study found men and women here are just as likely to be D-deficient as those who live in the North. Although 97 percent of the study's participants reported taking multi-vitamins, which contain some D, 39 percent were vitamin D deficient.

''Unless you're walking to your car in your bikini, you may not get enough sun,'' says UM's Levis, who co-authored the study, published in 2005 in The Journal of Clinical Endocrinology & Metabolism.

MEDICAL DEBATE

How did it get to this point? Too many people have replaced fortified milk with soft drinks and juice. Too many are tethered to indoor lives, and when they spend time outside, they cover themselves -- and their children -- with sunscreen and clothing, for fear of skin cancer.

''Many dermatologists have thrown the baby out with the bath water,'' says Dr. James Dowd, a Michigan-based rheumatologist and author of The Vitamin D Cure (Wiley, $24.95), published this year. 'Their idea is `No sun at all. Always cover up. Sun is evil.' That's an extreme message. We need enough sun to make vitamin D.''

It can be daring for a doctor to prescribe even a short burst of sun without sunscreen, which can block D absorption if it has SPF 8 or higher. Just ask Dr. Michael Holick, who was asked to resign from the dermatology department at Boston University's medical school shortly after he published his 2004 book The UV Advantage (IBooks, $19.95). Holick's book, which touts the health benefits of limited UV exposure, drew loud protests from dermatologists and the sunscreen industry.

Too much sun is indeed harmful. Evidence that ultraviolet radiation causes skin cancer is so strong the U.S. Department of Health and Human Services lists UV radiation as a known carcinogen. At current rates, one in five Americans will develop skin cancer.

''Under no circumstances should anyone be misled into thinking that natural sunlight or tanning beds are better sources of vitamin D than foods or nutritional supplements. The only thing they are proven to be better at is increasing your risk of developing skin cancer,'' says a statement by the American Academy of Dermatology.