At some point in the last 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.
In less than ten years, the amount insurers spent on testing rose from $1 million (in 2000) to $129 million (by 2008). Seventy-five percent of Americans are now on vitamin D supplements.
But, as the interest in and testing for vitamin D has become more popular, researchers have been wondering why: the benefits of these increasingly common practices aren't clear.
Those who have looked at the evidence base for vitamin D screening have found that it appears to be pretty useless for healthy adults. What's more, there is no good evidence that treatment for insufficiency (again, in otherwise healthy adults) leads to clinically important health benefits. There is also uncertainty about how to define and measure problematic vitamin D levels, and different countries set different thresholds for healthy levels.
"We really don’t have good data for this intervention," says Dr. Clifford Rosen, one of the country's foremost experts on the health impact of vitamin D screening. "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."
But right now, doctors can't confidently make that case.
That's why, the US Preventive Services Task Force — whose recommendations set the tone for medical practice in this country — has issued new guidelines trying to drive home the point: vitamin D screening for healthy people may not do more good than harm. Here's why.
What is vitamin D and why does it matter?
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, keeping your bones strong, and protecting against osteoporosis and rickets.
In recent years, researchers have found associations between low levels of vitamin D and an 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 too has the demand for testing.
How do you know if you’re getting enough?
Less than 10 nanograms per milliliter of vitamin D in the blood is considered much too little, a clinical 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 their diets and the sun.
They 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 can’t absorb the vitamin as a result of diseases of the liver or bowel.
Why is there so much controversy about vitamin D?
The controversies about the benefits of vitamin D reflect the iterative nature of science, said Dr. Barry Kramer, director of the division for cancer prevention at the National Cancer Institute.
Early research on the health benefits of vitamin D was mostly observational — large-scale, population level studies — and did not look at hard endpoints 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," said Dr. Kramer. "Being wealthier, being health conscious, having health insurance and access to the health care system, low smoking prevalence, increased physical exercise."
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.
Randomized trials — that introduce vitamin D to one group and compare it to a control group — have been disappointing, and showed little or unclear benefit for vitamin D testing and supplementation so far. So until we have more and better studies on vitamin D, related testing and treatment are clouded with uncertainty.
There's also the issue of defining vitamin D levels that are problematic. So 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 cut-off be 30 ng/mL?
According to the US Task Force, this uncertainty led to a lot of inconsistency around how vitamin D insufficiency was defined in studies.
Different professional bodies back different minimum blood levels, usually ranging from 20 and 30 ng/mL. To make matters more complicated, the accuracy of the tests that measure the amount of vitamin D in the blood was "difficult to determine," the Task Force said, because of variations in testing methods among labs and, again, the lack of standard definition of what constitutes unhealthy levels of vitamin D.
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 they 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.
That’s why, they concluded, "the balance of benefits and harms of screening and early intervention cannot be determined."
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 arrive in the next year or two. Maybe then, we'll have a better sense of what, if any, benefit this vitamin holds.
How should you think about vitamin D?
"Perhaps the best way of interpreting the insufficient evidence statement from the Task Force," said the Mayo Clinic's Dr. Victor Montori, "is that clinicians and their high-risk patients should consider the issues together and decide what to do, recognizing that their decision cannot be too confident and may need to be revised as new meaningful evidence appears."
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, like as broken bones, or other illnesses that can cause vitamin deficiencies, such as liver disease or multiple sclerosis.
"For healthy individuals, if you’re tired and weak, but it’s 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.
Dr. 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." Again, there's no good evidence that that's the case.
And there are well-documented costs associated with over-screening: the cost to the health system for all those tests, the personal cost of supplements, and the potential harms from getting too much vitamin D, such as kidney stones and high calcium levels (which can cause nausea, vomiting and loss of appetite).
"Unless you really are truly symptomatic," Dr. Rosen summed up, "it might not be worthwhile to measure vitamin D, and tag you with the diagnosis of deficiency, when it’s not clear those levels make you deficient and you’re not at risk for disease."