Vitamin D deficiency has become a much-diagnosed condition in recent years, but very few Americans are actually vitamin D deficient, Julia Belluz reports for Vox.
Moreover, according to the U.S. Preventive Services Task Force (USPSTF), there are no studies showing benefits of screening asymptomatic adults—and giving supplements to vitamin D deficient, asymptomatic adults didn't improve outcomes for an assortment of health issues, such as cancer, fractures, and Type 2 diabetes.
You likely have all the 'sunshine vitamin' you need
According to Belluz, humans need vitamin D, the so-called "sunshine vitamin," to regulate the absorption of calcium and phosphorus, which protect against conditions such as osteoporosis and rickets. We get the vitamin naturally from certain foods—such as fatty fish, beef liver, cheese, and egg yolks—and our bodies can manufacture it from sunlight.
But what exactly is the right amount of vitamin D? Well, that's not quite clear, according to Belluz. While experts agree that less than 10 nanograms per milliliter of vitamin D in the blood qualifies as either a deficiency or at least a concern, there isn't a consensus about when a person should be considered formally insufficient. And while experts generally agree that humans' blood level of vitamin D should be at least 20 nanograms per milliliter, various professional bodies advise different minimum levels, ranging range from 20 to 30 nanograms, Belluz reports.
Yet the Institute of Medicine (IOM) in 2010 determined that 97.5 percent of Americans got enough Vitamin D from natural sources. That said, IOM noted that deficiency is more prevalent among certain populations, including individuals with darker skin pigmentation, seniors living in nursing homes, individuals with melanoma, and individuals who have trouble absorbing vitamin D because of liver or bowel conditions.
But even though most of us have all the vitamin D we need, Belluz writes, Medicare payment for screening for the vitamin D levels jumped 83-fold between 2000 and 2010.
The science around vitamin D deficiency—and its limitations
So why are so many Americans getting tested for a deficiency of vitamin D even though most of us have enough? According to Barry Kramer, director of the cancer prevention division at the National Cancer Institute, it stems at least partly from the incomplete science around the benefits of the vitamin.
Belluz explains that early research has suggested a link between low vitamin D levels and increased risk of certain health problems, such as fractures, cardiovascular disease, diabetes, cognitive decline, and depressed moods. The research also has shown links between higher level of vitamin D intake and various health benefits.
According to Belluz, as awareness about those findings spread, so did patient demand for testing and treatment. But the early evidence on benefits of the vitamin was largely observational, Belluz reports. The research was done on a large scale and did not look at key long-term health endpoints, such as the relationship between vitamin D intake and the risk of certain diseases or death.
As Kramer noted, "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."
Moreover, since that early research, randomized trials into screening for and supplementing vitamin D in healthy individuals have shown little or ambiguous benefit, Belluz reports. According to IOM, randomized control trials have found no benefit for healthy individuals whose blood level of vitamin D exceeded 20 nanograms per milliliter.
And there are downsides to too much testing and vitamin D, Belluz writes. Not only do tests rack up costs, Belluz writes, but excessive vitamin D is linked to kidney stones and high calcium, which can lead to vomiting and loss of appetite.
According to Belluz, NIH is hoping to clear up some of the murkiness around vitamin D with a large randomized trial, with results expected next year. "Maybe then we'll have a better sense of what, if any, benefit this vitamin holds," she writes.
In the meantime, Belluz cautions that the UPSTSF conclusions involve asymptomatic adults. Those presenting with symptoms, such as broken bones or conditions that can cause a vitamin D deficiency, should have their levels checked and treated.
But for everyone else, Clifford Rosen of Maine Medical Center Research Institute, who's an expert on the health impact of vitamin D screening, said, "Unless you really are truly symptomatic ... 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"(Belluz, Vox, 6/20).
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