At some point in the past decade, screening blood for vitamin D levels became a routine part of medical care. Feeling a little low this winter? Get a vitamin D test. Think you didn't get enough sun last summer? Check your vitamin D levels.

Between 2000 and 2010, the amount Medicare spent on vitamin D testing rose 83-fold, making the test Medicares fifth most popular after cholesterol. All that screening also led to an explosion in vitamin D supplement use, and millions of Americans now pop daily vitamin D pills.

They mightve been encouraged by media reports over the past few years about the perils of getting too little of the sunshine vitamin. The supplements also seemed to be a cure-all: Many of us are confined to our computers, spending little time outdoors, and may feel we arent eating enough of the foods, like fish, that deliver vitamin D.

But as the interest in and testing for vitamin D has exploded, researchers have been wondering why so many people bother. Most of us actually get enough vitamin D without even trying. No high-quality study has found a benefit to screening asymptomatic adults, and putting people on treatment with supplements has also failed to demonstrably improve health outcomes.

That means when people seek out vitamin D tests and pop the supplement to alleviate the winter blues or prevent cancer, theres no evidence suggesting itll help them.

It would be great if you said the reason we screen is that we find out if a patient is low on vitamin D and we do something about it, we can prevent disease, says Dr. Clifford Rosen, one of the country's foremost experts on the health impact of vitamin D screening. Right now doctors can't confidently make that case.

Vitamin D is an essential vitamin that you get from food, including fatty fish such as salmon and tuna, beef liver, cheese, and egg yolks. Of course, it's also found in fortified foods, such as milk, orange juice, and cereal, and you get it from exposure to UV light.

You need vitamin D to regulate the absorption of calcium and phosphorous in your body, which keeps your bones strong and protects against osteoporosis and rickets.

In recent years, researchers have found associations between low levels of vitamin D and increased risk for a range of health problems, including fractures and falls, cardiovascular disease, diabetes, colorectal cancer, depressed moods, and even cognitive decline. As awareness about the importance of vitamin D for health has spread, so has the demand for testing.

So how much do you need? Less than 10 nanograms per milliliter of vitamin D in the blood is considered much too little, a vitamin deficiency. When your levels hover around there, you might experience symptoms such as muscle weakness, bone pain, and fractures.

Most experts agree that you want your vitamin D blood level to be at least 20 nanograms per milliliter.

The good news: Most of us have this much in our bodies without even trying.

In 2010, the Institute of Medicine brought together an expert committee to review the evidence on the vitamin and figure out whether there was a widespread deficiency problem in North America. According to the 14-member panel, 97.5 percent of the population got an adequate amount of vitamin D from diet and the sun.

The panel did, however, identify a few key populations that seemed to have higher levels of deficiency: people with dark pigmentation (such as African Americans), older folks who live in nursing homes, melanoma patients, and people who cant absorb the vitamin as a result of diseases of the liver or bowel.

The controversies about the benefits of vitamin D reflect how science evolves, said Dr. Barry Kramer, director of the cancer prevention division at the National Cancer Institute.

Early research on the benefits of vitamin D was mostly observational large-scale, population-level studies and did not look at endpoints that are important for long-term health, like whether a high vitamin D intake reduces one's risk for particular diseases or death.

Researchers found associations between higher levels of vitamin D intake and a range of health benefits. "But with the observational studies especially when you're dealing with dietary supplements and diet taking supplements is also associated with many other confounding factors that predict the outcome: being wealthier, being health-conscious, having health insurance and access to the health care system, low smoking prevalence, increased physical exercise," said Kramer.

In other words, the people who were taking these vitamins were doing many other things that might have caused them to have better health outcomes. Still, this early science encouraged people to hop on the vitamin D bandwagon.

Since then, randomized trials that introduce vitamin D to one group and compare that group with a control group have been disappointing, showing little or unclear benefit for vitamin D testing and supplementation in healthy people. That Institute of Medicine report noted that randomized trials had uncovered no health benefit for healthy people with vitamin D blood levels that were higher than 20 nanograms per milliliter.

There are also well-documented costs associated with overtesting and getting too much vitamin D: the cost to the health system for all those tests, and the potential harms from high vitamin D levels, such as kidney stones and high calcium which can cause nausea, vomiting, and loss of appetite.

So until we have more and better studies on vitamin D, related testing and treatment are clouded with uncertainty and a lack of evidence for any benefit.

There's also the issue of defining vitamin D levels that are problematic. Experts agree that anything less than levels of 10 ng/mL of blood is worrisome or a deficiency, but when is someone insufficient? Is 20 ng/mL really enough? Should the minimum cutoff be 30 ng/mL?

According to the US Preventive Services Task Force whose recommendations set the tone for medical practice in this country this uncertainty led to a lot of inconsistency around how vitamin D insufficiency was defined in studies. Different professional bodies also back different minimum blood levels, usually ranging from 20 to 30 ng/mL.

Finally, there's some question of whether healthy (asymptomatic) adults who undergo routine screening for vitamin D actually see any health benefit as a result. The task force points out that there were no studies on the benefit of screening otherwise healthy adults, but it did find that putting them on treatment with supplements did not improve health outcomes for a range of issues, including cancer, Type 2 diabetes, and fractures.

"Although the evidence is adequate for a few limited outcomes, the overall evidence on the early treatment of asymptomatic, screen-detected vitamin D deficiency in adults to improve overall health outcomes is inadequate," the task force authors write in their latest guidance.

To clear up some of the uncertainty, the NIH has funded one of the largest randomized trials on vitamin D, with the results expected to be ready next year. Maybe then we'll have a better sense of what, if any, benefit this vitamin holds.

One of the authors on that study, Dr. JoAnn E. Manson, recently told the New York Times, A lot of clinicians are acting like there is a pandemic, of vitamin D deficiency. That gives them justification to screen everyone and get everyone well above what the Institute of Medicine recommends.

It's important to be clear that the task force is highlighting uncertainty around screening and treating asymptomatic people who don't have real signs of illness, such as broken bones, or other illnesses that can cause vitamin deficiencies, like liver disease or multiple sclerosis.

"For healthy individuals, if youre tired and weak, but its nondescript, this is a really tempting thing to do: measure vitamin D and then treat," Dr. Rosen, who is based at the Maine Medical Center Research Institute, warned. "But there just isn't enough evidence it does anything."

So, for example, if you were feeling a little low this winter and you ask for a vitamin D test, then find out your levels are hovering around 20 or 30 ng/mL, you can go on supplements. And there's no doubt that those supplements will raise your vitamin D levels, since researchers have found they are absorbed by the body very efficiently. Doctors just don't know whether that change actually has any health benefit.

Rosen also cautioned that the biggest misconception about vitamin D is the association between low vitamin D levels and disease risk. "There's this idea, if we treat you, not only will some of your symptoms get better but also your long-term health benefit will be enhanced," he said. Again, there's no good evidence that that's the case.

"Unless you really are truly symptomatic," Rosen summed up, "it might not be worthwhile to measure vitamin D, and tag you with the diagnosis of deficiency, when its not clear those levels make you deficient and youre not at risk for disease." In other words, beware of the vitamin D test.

Read this article:

Your vitamin D tests and supplements are probably a waste of money - Vox