Vitamin D helps build strong bones by promoting the absorption of calcium.

MCT / Todd Sumlin

There’s something simple you can do to help protect yourself against ailments ranging from cancer to depression. Chances are, you’re not taking the cure.

It’s vitamin D — and some experts believe as many as 50 percent of healthy adults and children are deficient in this essential nutrient.

Doctors across the country have begun prescribing high doses to patients who are surprised to learn they’re deficient. Yet doctors and researchers say the vitamin D problem could be solved if we just got out in the sun.

A study released last week found that men deficient in vitamin D have more than double the normal risk of suffering a heart attack. Another study last week found that low levels of vitamin D increase the risk of diabetes, and a study last month linked deficiencies to an increased risk of dying from breast cancer.

The new findings join a growing body of evidence indicating that an adequate level of the vitamin, which most people can get from 20 daily minutes in the sun, is crucial to maintaining good health.

Question: What’s so important about vitamin D?

Answer: For years, doctors have known vitamin D helps build strong bones by promoting the absorption of calcium. (The vitamin was added to milk more than 50 years ago to successfully combat the common childhood bone disease of rickets.) But recent research indicates D is important to almost all body tissues. Low levels of vitamin D have been linked to increased risk of breast and prostate cancer, colon polyps, multiple sclerosis, Type 1 diabetes, muscle weakness — even depression and schizophrenia.

Q: Why are so many people deficient?

A: Two reasons. People aren’t spending enough time in the sun and it’s hard to get enough vitamin D from food. Skin produces plenty of D when exposed to the sun’s rays. But office workers and kids playing video games often don’t spend enough time outdoors to make the D they need. And when they are outside, they’re probably wearing sunscreen. Sunscreen with an SPF of 8 blocks more than 95 percent of the sun’s capacity to make vitamin D in your skin.

Fortified milk, yogurt and orange juice contain about 100 International Units of vitamin D per one-cup serving. Canned salmon contains 300 to 600 IU. Doctors used to think 400 IU daily was enough for most adults. But new research indicates that is too low. Dr. Michael McClung, director of The Oregon Osteoporosis Center recommends adults get 1,000 to 2,000 IU of vitamin D each day — unless their blood calcium is too high or they’ve had kidney stones.

Q: Who is most at risk of vitamin D deficiency?

A: “The farther you live from the equator and the less opportunity you have for regular sun exposure, the more likely you are to be deficient,” says Charlotte, N.C., internist Dr. Charles Rich.

Older adults are more at risk. Obesity and liver or kidney disease also increase your risk. Blacks and people with darker skin are more at risk, because their skin is less able to synthesize Vitamin D from the sun.

Q: What are the symptoms of deficiency?

A: Often there are none. Debra Wilform, 49, of Charlotte, N.C., learned she was low in vitamin D after a blood test during a physical exam.She had no symptoms. Her doctor prescribed 50,000 IU of D2 once a week for six weeks, then once a month for six months.

Q: How much Vitamin D do I need and what are the best sources?

A: There’s still disagreement on how much D is enough. The New England Journal of Medicine says adults and children need 800 to 1,000 IU of vitamin D daily if they’re not getting enough sun exposure. McClung says it’s safe for adults to take 1,000 to 2,000 IU daily. (You’d need to drink 10 glasses of milk to get 1,000 IU.) Cautious sun exposure is the easiest way to get enough vitamin D. The National Institutes of Health says 10 to 15 minutes in the sun at least twice a week with your arms, hands, face or back exposed without sunscreen is usually enough.

The farther north you live, the harder it is to get enough D from sun exposure alone. From November through February, people living north of Atlanta make little or no vitamin D by being outdoors.

Q: Should I be tested?

A: Ask your doctor. The test costs about $100 and is often covered by insurance. WHAT TO DO TO GET MORE D

•Spend 10 to 15 minutes in the sun at least twice a week with your arms, hands, face or back exposed.

•Take 1,000 IU of vitamin D daily.

•Ask your doctor to be tested.