WASHINGTON — Don't be surprised if your doctor orders a vitamin D test during your next physical.

Blood tests to check levels of the so-called sunshine vitamin are on the rise as doctors and patients react to headline-grabbing research that suggests having too little may not only hurt your bones — it might increase your risk of certain cancers or heart disease.

But there are problems with deciding next steps: As intriguing as the research is, it's far from proof that vitamin D really is that powerful. Also, it's not clear just how much is enough — and megadoses can harm.

Nor are there guidelines on exactly who should be tested, or how. Test during winter, for example, and in much of the country people will harbor considerably less vitamin D than if they were tested in the sunny summer.

Still, "the hope is so high that it will have some effect that everybody's asking for it," says Dr. Clifford Rosen of the Maine Medical Center, who is helping government researchers evaluate the research. "It's pretty much the wild, wild West right now."

There is no count of how many people get their vitamin D checked. But at testing giant LabCorp, the volume of vitamin D tests doctors order has, on average, doubled every year for the past four, says spokesman Eric Lindblom. So far this year, test orders are up another 90 percent. At competitor Quest Diagnostics, the volume of D tests approximately tripled between May 2006 and last May.

Dr. James Underberg, a New York University internist, once checked vitamin D levels mostly in people at risk of thinning bones. Over the past year, he's begun screening more patients, especially those at risk of heart disease, as he closely watches the evolving research.

"We don't have any data yet that says taking an otherwise healthy adult who's vitamin D deficient and supplementing them prevents cancer, reduces the risk of heart disease," Underberg acknowledges.

"You just have to keep your eyes and ears open to make sure something doesn't show up counterintuitive to what people thought," he adds, noting that other once-touted heart protections — estrogen therapy after menopause, for example — failed when more rigorously researched.

Dr. Ann Marie Gordon, a Washington, D.C., internist, isn't hesitating. She has made a vitamin D test a routine part of every physical, and she estimates that 60 percent of her patients are low.

"Any kind of deficiency needs to be addressed. Whether patients are convinced or the medical world is convinced that vitamin D goes beyond bones is irrelevant," she says.

Vitamin D and calcium go hand in hand. You need a lifetime of both to build strong bones. We get D in three ways: sun exposure, dietary supplements or certain foods, particularly D-fortified milk, orange juice and cereals.

Scientists have been interested in vitamin D's possibly broader effects for decades, since noticing that cancer rates between similar groups of people were lower in sunny southern latitudes than in northern ones. In recent years, studies have linked low levels of vitamin D with breast, prostate and colon cancer, heart disease, diabetes and certain other ailments — as well as an overall increased risk of death.

Much of the evidence is circumstantial. There's a chicken-or-egg question: Does correlating how much vitamin D is in someone's blood at a certain time really mean it triggered or worsened a disease — or did the disease, or other risk factors, trigger the low vitamin D? Being a couch potato, for instance, is a key risk for heart disease, and also keeps you out of the sun.

Moreover, not all vitamin D studies find that the nutrient helps. The disappointing ones seldom make headlines.

But the increasing interest in vitamin D parallels increasing concern that people aren't getting enough — and increasing confusion about how much that might be.

Currently, the government and other health authorities recommend consuming anywhere from 200 to 600 international units a day from food or supplements, depending on your age — levels that many vitamin D proponents say are too low.

There's no consensus on how to balance the risk of skin cancer from sun exposure with vitamin D needs. Some specialists recommend 10 to 15 minutes daily without sunscreen. Others say sunscreen doesn't completely block vitamin D production so sunscreen users will get enough. Regardless, time of day, season and geography play a role.

There's not even good agreement on what's a low level — different studies use different definitions, notes a newly published research review sponsored by the National Institutes of Health.

The government has begun discussions with the Institute of Medicine to determine if the daily recommended intake needs changing.

Meanwhile, because megadoses may be toxic, the government considers 2,000 IUs a day the upper limit, although doctors may recommend 10,000 or even 50,000 IUs for a short period if someone needs a rapid boost.