For decades we've been warned to lather on sunscreen or pay the health price. The American Academy of Dermatology categorizes the sun's rays as a carcinogen, saying that no matter how few minutes you spend outside, there is "no scientifically proven safe threshold of sun ex­posure." Yet mounting research links sunlight and its physiological by-products – including vitamin D – with everything from cancer prevention to improved sports performance to weight loss. Many experts are adamant that our heliophobia has gone too far, and it's time to swing the pendulum back.

"This alarmist motto that you should never be exposed to one ray of sunshine without wearing sunscreen has led to a pandemic of vitamin D deficiency and health problems," says Dr. Michael Holick, a vitamin D researcher with Boston University School of Medicine. He believes 60 percent of us may be affected. Other studies have conservatively pegged that number at 40 percent.

Which begs the question: What's the best way to get the benefits of sunshine without raising our skin cancer risk?

"It's about being sensible," says Dr. Frank Lipman, founder of Eleven Eleven Wellness Center in New York City. "Recently, numerous studies have shown that modest, sunscreen-free exposure to sunlight helps the body produce the vitamin D it needs for good health and disease prevention." His recommendation: Step outside without sunblock for 20 to 30 minutes a few times a week.

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Here's what you get during those minutes, according to new reports: an instant endorphin surge and drop in blood pressure, regulation of hunger hormones that may help you keep weight off, and key vitamin D synthesis that alters the expression of 291 genes responsible for everything from controlling how quickly your bones age to how fast you bounce back from tough workouts to how fertile you are.

The bit about bone health is critical. Men can start losing bone mass at 30, but maintaining vitamin D levels may slow that decline, preventing fractures later in life and fending off the muscle pain and weakness that come with soft bones. "There is no question that if you don't get enough D, you are putting your bones at risk," says Dr. John Swartzberg, editor of a new 50-page report on vitamin D by the University of California Berkeley School of Public Health.

Pro sports teams are now wise to the benefits of vitamin D, too. Because of studies proving that it strengthens fast-twitch muscles, quells swelling, and can stop respiratory infections, teams such as the Chicago Blackhawks started testing players' levels. Recreational athletes, take note. "We know lack of vitamin D can compromise athletic performance," says Enette Larson-Meyer, associate professor and vitamin D researcher at the University of Wyoming. "The week before a fit event, you may want to get some sun."

The science is young, but researchers also suspect poor D levels could lower your sperm count. One recent study found that vitamin D–deficient rats were 73 percent less likely to impregnate their mates, and in human studies on countries with long, dark winters, researchers find that conception rates are higher in summer months, when people are outside and synthesize more sun.

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Despite all of this, dermatologists insist that no amount of unprotected sun exposure is safe. "If you have low D – but not so low that your bones are breaking – it's still better than getting melanoma, which can kill you," says Dr. Kenneth Mark, a clinical assistant professor at New York University's department of dermatology, who hears sunburned patients say, "I was just getting vitamin D." He believes we should rely on D-rich foods and supplements.

This may not be so realistic. Foods naturally rich in D aren't likely to be your supper-time staples. (Care for a can of sardines, eel, or cod liver oil?) And to get the minimum amount of prescribed D via commonly fortified foods alone, you may need to drink up to 10 glasses of milk or eat 10 bowls of fortified cereal – daily.

Meanwhile, supplements are expensive – and may not be effective. "There is no medical evidence that for healthy people D supplements treat, or ward off, diseases," says Dr. Marc Gillinov, a cardiac surgeon at the Cleveland Clinic, who points to several recent reviews confirming their negligible effects. Gillinov would recommend supplements for people with osteoporosis or gastro­intestinal disorders that keep them from absorbing D, and also for nursing-home residents who rarely go outside. If that's not you, "you're better off taking a walk outside in the sun a few times a week," he says.

Maybe it's just that America's behind the research curve. The Cancer Council Australia – the Aussie version of the American Cancer Society – now advises southern Australians to get as much as three hours of sun during winter weeks, and even more if they have darker complexions.

No one's advocating that we return to tinfoil sun reflectors and dousing ourselves in baby oil, though. Lipman, an integrative-medicine specialist, and Holick – who, it's worth noting, is a pale scientist who cycles, gardens, and plays tennis outside regularly – stress that this isn't about "tanning." They just think it's time people give the sun some credit.