D Is for Defense: Why Vitamin D Is the Ultimate Super Vitamin

Get More Sunshine

The sun is key to D production, and it doesn't take much: Research suggests that 5 to 30 minutes of unprotected exposure between 10 a.m. and 3 p.m. at least twice a week will give you most of the D you need. To guard against skin cancer, "I recommend always wearing sunscreen on your face but not putting it on your arms or legs if you're going to be outside for only 10 or 15 minutes," Dr. Holick says.

For the sun to create vitamin D, however, conditions have to be just right. Overcast skies reduce ultraviolet energy, and thus D production, by half; standing in the shade, even if it's caused by smog, lowers it by 60 percent. Even more critical is the position of the sun. When its rays are at an angle, they must pass through more of the atmosphere, which absorbs energy needed to trigger D formation. "People may think, 'I'm out in the sun at 8 a.m., so I'm making vitamin D,' but they aren't," Dr. Holick says. "The sun needs to be almost directly overhead."

The prime D-forming hours occur when most of us are indoors at work. And if you live above 42 degrees latitude, which runs roughly from Boston to northern California, your body won't produce the vitamin at all from November to February. Researchers now speculate that a lack of D is why we're more prone to flu in winter and the reason people are more likely to survive serious illnesses like cancer if they're diagnosed in summer, when vitamin D levels are higher.