And if you're over 60, have dark skin or slather on sunscreen every time you step outside, put yourself in the 85 percent zone. That's bad news for your health. Not just because you need D to build strong bones, but because a steady stream of recent research suggests this familiar nutrient is responsible for more good deeds than a string of superheroes put together—including the biggie that it can even help you live longer. Several studies have found that if people take more vitamin D, they have 25 percent less cancer and heart disease.

If you don't? A just-released study found that people with the lowest levels of vitamin D in their blood are 26 percent more likely to die from any cause (heart disease, cancer, infection, you name it) than folks with respectable amounts.

That's just the beginning. Vitamin D is like the quiet kid in the back of the room who ends up developing the next Google. It's equally underestimated. New benefits of D are being discovered faster than you can say cholecalciferol (that's science-geek speak for the active form of vitamin D, also known as vitamin D3). Here's the latest on how it helps you stay young and healthy.

• It cuts your risk of breast and colon cancer.
  Many cells love to multiply faster than rabbits in the arugula patch. But out-of-control cell growth can lead to cancer. Enter vitamin D. It keeps a lid on the rate that cells reproduce, and it turns on your DNA spell checker, called the P53 gene. This gene checks your DNA for typos and kills cells—like cancer cells—that have errors. Experts now believe this is why women who live in sunny climates, and thus have plenty of D (your body makes it when sunlight hits your skin), are less likely to develop breast cancer. D has also been linked to lower chances of developing ovarian and lung cancers and better odds of beating colon cancer. Recent research found that colon cancer patients with the highest D levels are the most likely to survive.
  
• It deters diabetes and other serious diseases.
  When researchers looked at the link between sun exposure and type 1 diabetes in children, they found fewer cases of diabetes in kids who live closer to the sunny equator (and therefore make more D). And because D improves your ability to produce and use insulin, it may also help protect against type 2 diabetes. The vitamin, which is thought to be an immune system ally, may help prevent autoimmune diseases too, including multiple sclerosis and rheumatoid arthritis.
  
• It keeps your heart healthy.
  Vitamin D helps your ticker by controlling inflammation, moderating blood pressure and keeping your arteries young. That's probably why vitamin D-deficient men are twice as likely to have a heart attack as men with healthy levels—and twice as likely to die from it.

When it comes to getting enough of this high-test health booster, only a handful of foods supply it: primarily oily fish (salmon, herring, sardines) and D-fortified foods (many cereals, nonfat yogurt, skim milk, soymilk and OJ). But you have to eat a lot of them (like 10 glasses of orange juice a day) to get what you need.  True, you can get D from sunshine, but you need to be outside during peak sun hours for at least 10 minutes several days a week. And in the northern two-thirds of the country, that only works from spring through fall. Between October and mid-April, the sun doesn't have enough energy for your body to make D in its active form—even if you sit out in the sun all day.

That's why we recommend vitamin D3 supplements. Aim for 1,000 International Units (IU) daily; 1,200 IU if you're over 60—though check your multivitamin, which probably has around 400 IU of D, so you don't overdo. Limit your daily dose to a max of 2,000 IU. At the same time, don't become D-ficient. Get the best D-fense against cancer and heart disease—about 1,000 IU a day from a bottle.

For more from the YOU Docs, visit RealAge.com.

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