Research suggests that Vitamin D might help build bones, strengthen the immune system, and fight off heart disease, cancer, hypertension, kidney disease, and diabetes. It’s a nutrient that the body makes from sunlight and that is also found in fish and fortified milk. But thanks to skin cancer awareness programs, sunscreens, and poor diets (and likely many other reasons), as many as half of all adults and children are said to have less than optimum levels and as many as 10 percent of children are highly deficient, according to a 2008 report in The American Journal of Clinical Nutrition.

Like a number of my colleagues, I have begun testing Vitamin D levels more frequently. But this is where the confusion begins. There is controversy about what the best blood level of Vitamin D is, as well as what level constitutes a deficiency worthy of treatment. In general, people are considered to be deficient if they have blood levels below 15 or 20 nanograms per milliliter. But many doctors now believe vitamin D levels should be above 30. The ideal level isn’t known, nor is it known at what point a person is getting too much vitamin D, which can lead to kidney stones, calcification in blood vessels and other problems.

People’s vitamin D levels are influenced by the color of their skin, where they live, how much time they spend outdoors and by fish and milk consumption. To raise vitamin D without supplements, you can increase sun exposure for 10 to 15 minutes a day. Eating more fish can help — a 3.5-ounce serving of wild fresh salmon has 600 to 1,000 I.U.’s of vitamin D — but it would take a quart of milk a day to get the recommended dose of vitamin D.

Personally, I’m not taking vitamin D supplements yet. Science needs to catch up with the hype. Although numerous studies have been promising, there are scant data from randomized clinical trials. Little is known about what the ideal level of vitamin D really is, whether raising it can improve health, and what potential side effects are caused by high doses. The data is mostly from observational research, so it may be that high doses of the nutrient don’t really make people healthier, but that healthy people simply do the sorts of things that happen to raise vitamin D.

There is now a major study over the next five years that should provide answers to these questions and more. The nationwide clinical trial is recruiting 20,000 older adults, including men 60 and older and women 65 and older, to study whether high doses of vitamin D and omega-3 fatty acids from fish-oil supplements will lower risk for heart disease and cancer. (If you qualify, consider taking part in the study at www.vitalstudy.org)

In the meantime, I think it’s best to continue checking levels more frequently than we have in the past, eat more fish, and get some outdoor exercise.