You've probably heard that your body needs vitamin D to absorb calcium, a necessary mineral for maintaining bone density. But it now seems that the vitamin's benefits extend far beyond that. It performs multiple critical tasks, including boosting your immune system, which needs all the support it can get, especially during the winter months.

"Most people aren't getting enough," says Walter Willett, M.D., Ph.D., of the Harvard School of Public Health in Boston. Because the sun is our top vitamin D source, the many Americans who spend entire days office-bound often lack the nutrient (technically not a vitamin but a hormone, which our bodies produce in response to sun exposure). In fact, during the winter months, everyone who lives north of the Atlanta-Los Angeles latitude line risks deficiency.

But despite this widespread want for the sunshine vitamin, health experts have long shunned supplements for fear of overdose. During the past decade, however, researchers began looking into senior populations and discovered that those with hip fractures almost always had a deficiency of D. Other studies have shown that at least a third of Americans have a genetic variation that reduces the body's ability to use the vitamin, increasing the risk of disease. Given such findings (along with the conclusion that moderate doses of D are not, in fact, toxic), Willett and others now say adults should get at least 1,000 IU and seniors should aim for 2,000 IU -- well above the current recommended "adequate intake" of 400 IU. "The new recommendations come from the realization that vitamin D affects almost every organ in the body," explains Willett.

"D" Is for "Defense"

Here's a look at some compelling research on the nutrient's health-guarding effects.

Enhancing Immunity

A hundred years ago, the standard treatment for patients with tuberculosis was sun exposure. Doctors have long known that vitamin D, which the skin synthesizes after you spend a few minutes in the sun, revs up the body's immune system. But it took until 2006 for researchers to discover that vitamin D also boosts levels of a highly specialized germ-killing peptide found in cells throughout the body.

Preventing Cancer

Some of the most promising research on vitamin D focuses on cancer. A University of California analysis of 63 studies found that high levels were associated with significant decreases in the risk of breast, colon, ovarian, and prostate cancers, and results from a British study suggest that vitamin D may inhibit the progression of breast cancer. What's more, research from Harvard and Northwestern universities suggests that taking D supplements could slash your risk of pancreatic cancer, a notoriously hard-to-treat illness, nearly in half. Exactly how vitamin D may fend off cancer isn't known, though laboratory studies have shown that it can stop cancer cells from dividing.

Preserving Bone Density

Vitamin D puts calcium to work in our bones. Numerous studies have found that 800 IU of vitamin D daily (or an equivalent periodic dose) significantly lowers the risk of falls and fractures, and that taking 400 IU daily isn't enough to decrease fracture risk.

Keeping Muscles Strong

According to the new medical thinking, it's not just weak bones that lead to falls; weak muscles may play an even bigger role. Vitamin D shines here, too. The nutrient is essential for making muscle, and muscle weakness is a sign of vitamin D deficiency. "Because vitamin D improves muscle function, patients tell me all the time that they simply feel better when they take it," says Boston University's Michael Holick, M.D. Taking 700 to 1,000 IU of vitamin D daily, combined with calcium (500 to 600 mg), has been shown to reduce the risk of falls from 25 to 65 percent

Delivering the D

So how can you get enough? Willett recommends taking 1,000 IU in supplement form daily, increasing to 2,000 IU if you're not getting enough from the sun or have dark skin (which filters out sunlight). "It's pretty hard to overdose," adds Willett, saying you'd have to take extremely high doses (perhaps 40,000 IU) daily for months. Look for supplements containing vitamin D3, since vitamin D2 has two-thirds less biological activity. The most common supplements contain 400 IU per capsule, but some have 1,000 IU and 2,000 IU.

Note that you may also have some stored away from last summer. While winter sun yields minimal vitamin D production for those living in northern areas, during the rest of the year, just 15 minutes of midday exposure at most latitudes can synthesize more than 10,000 IU. Holick, who wrote The UV Advantage, suggests getting 5 to 10 minutes of sun three days each week in the summer, sans sunscreen, wearing no more than a T-shirt and walking shorts. That won't be enough exposure to increase your risk of skin cancer, and, Holick adds, moderate sun exposure actually helps prevent melanoma. "It's true that too much sun will damage the skin, but it's also true that if you don't get any sun, you're losing out on the major source of vitamin D," he says.

Your best strategy for increasing vitamin D, then, is threefold: Take a supplement, enjoy vitamin-D-rich foods, and make judicious sun exposure part of your warm-weather routine. With a comprehensive approach, you'll feel stronger, healthier, and more vibrant.

How much D are you getting?

Summer sun exposure, 15 minutes: 10,000 IU

Cod-liver oil, 1 Tbsp.: 1,360 IU

Salmon, 3.5 oz: 360 IU

Mackerel, 3.5 oz: 345 IU

Sardines, canned, 1.75 oz: 250 IU

Soy milk (fortified), 8 oz: 122 IU

Low-fat or whole milk (fortified), 8 oz: 98 IU

Egg (whole): 20 IU

Beef liver, 3.5 oz: 15 IU

Winter sun exposure, 15 minutes (north of 40 degrees latitude): 0 IU


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