Evidence from multiple studies suggests that at least 40 percent or more of older U.S. adults not living in nursing homes may be deficient in vitamin D. Vitamin D plays a major role in supporting bone health by increasing the amount of calcium and phosphorus the body absorbs from the intestines.

In mild to moderate cases of vitamin D deficiency, there may be no symptoms. In more severe cases, symptoms may include muscle pain, deep bone pain, fractures, and hip pain or muscle weakness.

The importance of getting enough vitamin D may go far beyond bone health.

Adequate vitamin D may also decrease the risk of several forms of cancer, autoimmune diseases, cardiovascular disease and many other chronic diseases and conditions, according to a review article published last year in the New England Journal of Medicine.

Under the right conditions, our bodies make our own vitamin D after exposure to the UV rays in sunlight. After at least 10 to 15 minutes of full sun exposure on the face, arms, hands or back at least twice a week, our skin can make all of the vitamin D we need.

Problem is, for many of us, the conditions aren't right often enough.

Smog, sunscreen and the position of the sun in the sky can affect the amount of light the skin is exposed to and the amount of vitamin D produced. Latitude also makes a difference: Sunlight exposure in North Carolina is greater than the amount people get in Maine or Montana. And a lot of us don't get outside very much.

Food isn't much help either.

Unlike most other vitamins and minerals, few foods are naturally good sources of vitamin D. The exceptions are oily fish such as salmon, mackerel and sardines, as well as cod liver oil and egg yolks. Most people don't eat enough of these to meet their vitamin D needs.

That's why a system was put into place decades ago to add vitamin D to all fluid cow's milk. The thought was that, since most people drank cow's milk on a regular basis, adding vitamin D to it would ensure everyone got enough.

Unfortunately, vitamin D levels in milk are notoriously unreliable, and many people don't like milk or don't drink it because of lactose intolerance. Vitamin D isn't added to milk used to make cheese and ice cream, though some brands of soy milk and breakfast cereals are fortified with vitamin D.

When sunlight and food sources fail, supplements are another option. But even supplements carry a risk.

Too much vitamin D can be toxic, and amounts greater than recommended adequate daily intakes should only be taken under the advice of your health care provider. The Institute of Medicine recommends an adequate daily intake of 200 IU vitamin D for children and adults 50 or younger; 400 IU for adults 51 to 70 years; and 600 IU for adults 71 years and older.

Those recommended levels of intake are under scrutiny, however. Some experts believe the recommended adequate daily intakes should be much higher. Others question the use of supplements at all and suggest they may hinder the body from producing vitamin D the natural way, on its own.

What to do? It's tricky.

If you suspect or want to rule out a vitamin D deficiency, see your health-care provider and follow his or her advice. For many people, judicious, sunscreen-free exposure to sunlight -- enough to get vitamin D benefits without adding undue risk for skin cancer -- may be indicated, in combination with some level of supplementation.