Two main factors have prompted some doctors to start measuring their patients' blood level of vitamin D. Research has found that more people than previously thought have low levels of this vitamin, and there's growing evidence that those low levels might contribute to a host of illnesses and even premature death. Once thought of primarily as a partner with calcium to form and maintain strong bones, it turns out that vitamin D plays a key role in other systems of the body, too, including muscles, the central nervous system, immune function, inflammation, and cell growth and reproduction.

The strongest evidence is in the area of bone health. In a comprehensive review of 167 studies, the government's Agency for Healthcare Research and Quality found that people who got at least 700 international units of vitamin D daily and took calcium pills had denser bones and experienced fewer falls and fractures. Population studies suggest that higher levels of vitamin D may also protect against certain cancers, including those of the breast, colon, and prostate.

Other research has linked high intake or blood levels of the vitamin with reduced risks of heart attack, both types of diabetes, and multiple sclerosis, as well as stronger immunity. (See Vitamin D from Food.)

Testing guidelines are still evolving, and our experts say it's too soon to say whether everyone should be routinely screened for low blood levels of vitamin D. But testing is probably a good idea for people with conditions linked to abnormal calcium metabolism or vitamin D deficiency such as osteoporosis, hyperthyroidism, and possibly multiple sclerosis. And the higher your risk-based on factors such as your age, location, skin color, and weight-the more sensible vitamin D testing might become. If it reveals a deficiency, your doctor should consider prescribing a very high-dose supplement for two to three months, followed by a maintenance supplement to keep your vitamin D levels in a healthy range.