Wouldn’t it be nice if you could pop a vitamin pill and prevent or treat a disease like rheumatoid arthritis? Maybe you could.

Studies have shown that vitamin D levels are often low in people with RA. So, taking a vitamin D supplement should help diminish the symptoms of RA, right?

Not so fast. If it sounds too good to be true, it just might be. And in the case of RA, the jury is still out.

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Strictly speaking, vitamin D is not a vitamin at all. It’s a prohormone, a physically inactive compound that enzymes in the body convert to an active hormone. It is found in a few foods, including fatty fish like salmon, tuna, mackerel and sardines, beef liver, cheese, egg yolks and mushrooms. It is added to other foods, like milk and yogurt, fortified orange juice and some cold cereal.

The body also produces its own vitamin D when ultraviolet light from the sun interacts with the skin.

Vitamin D is an essential component for building and maintaining strong, healthy bones and teeth. It works synergistically with calcium, enabling the body to absorb, and use, the calcium we consume. In recent years, vitamin D has also been linked to several medical conditions, including cancer, fibromyalgia, Alzheimer’s disease, high blood pressure, diabetes and autoimmune disorders like multiple sclerosis and rheumatoid arthritis.

The function of vitamin D in bone metabolism — the ongoing process in which mature bone is removed, or “resorbed,” and replaced by new bone tissue in a process called ossification — is well understood, but the evidence for its role in these other disorders varies, depending on the condition. And in the case of RA, there are a lot of unknowns.

“The [study] data are fairly consistent that people with RA have lower levels of vitamin D than matched controls,” says Dr. Jeffrey Sparks, a rheumatologist and associate physician in the Division of Rheumatology, Immunology and Allergy at Brigham and Women’s Hospital in Boston. “But is this a measure of vitamin D or sickness?”

In other words, does vitamin D deficiency predispose people to get RA or exacerbate symptoms, or does the disease itself affect vitamin D levels?

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It’s a chicken and egg conundrum, according to Sparks, who is also an assistant professor of medicine at the Harvard School of Medicine.

It could be that people with RA have lower levels of vitamin D because they are sicker than the general population, he suggests. “Hence, they may eat poorer diets, be less physically active and get outside less, lowering their exposure to UV light.”

Dr. Susan Goodman, a rheumatologist and professor of clinical medicine at the Weill Cornell School of Medicine in New York, echoes these sentiments.

“While studies have suggested that low vitamin D levels increase the risk of RA, these findings have not been replicated consistently,” Goodman says.

“Part of the problem relates to the difficulty in measuring vitamin D intake, which requires careful food questionnaires as well as careful adjustment for sun exposure. Moreover, there are several ways to measure vitamin D levels,” Goodman adds.

“There is stronger evidence regarding progression of RA or severity of RA, which appears to be associated with vitamin D deficiency,” Goodman says. “Patients with RA who are deficient in vitamin D are less likely to achieve remission. This does not mean that vitamin D is necessary to achieve remission, however, because active RA can impact mobility. One could easily imagine that the patient who feels poorly and has active disease would be less likely to be outside, and would get less sun exposure,” she says.

Further complicating the issue is that some people with RA take corticosteroids, especially during flares of the disease, which are known to interfere with calcium and vitamin D metabolism, Sparks says.

Dr. John J. Cush, a rheumatologist and the director of clinical rheumatology for the Baylor Scott and White Research Institute in Dallas and executive editor of the rheumatology website RheumNow, says, “There is no association” between vitamin D and RA.

The findings of the Institute of Medicine Committee on Dietary Reference Intakes for Vitamin D and Calcium, published in the New England Journal of Medicine in 2016, appear to support Dr. Cush’s viewpoint. The committee reported that fewer than 6 percent of Americans between the ages of 1 and 70 years have a vitamin D deficiency, and only 13 percent are in danger of not getting enough of the vitamin.

These results are at odds with data on vitamin D testing and supplementation. According to Medicare statistics, there was an 8,000 percent increase in testing for vitamin D levels in the decade between 2000 and 2010. The use of vitamin D supplements increased from 5 percent of all Americans in 1999 to 12 percent in 2012.

This increasing use likely represents misunderstanding of the dietary guidelines concerning vitamin D levels and requirements, according to the Institute of Medicine committee, which also cautions that getting too much vitamin D can cause problems like nausea, constipation, kidney stones and abnormal heart rhythms.

Definitive studies are needed to determine what role vitamin D plays, if any, in the development and progression of autoimmune diseases like RA, Sparks says. The Vitamin and Omega-3 Trial, also known as VITAL, underway at Brigham and Women’s Hospital and other centers across the U.S. may eventually provide some more definitive data on the subject.

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The principal endpoint of the VITAL study is to examine whether a daily supplement of vitamin D3 and omega-3 fatty acids reduces the risk of developing cancer, heart attack and stroke in people with a prior history of these diseases. The secondary endpoint of the study is to examine how these supplements affect autoimmune disorders, including RA.

The results of the VITAL study will not be available for a few years. In the meantime, what do physicians recommend?

Sparks does not routinely test vitamin D levels in his patients with RA or recommend that they take a supplement, and he worries that the entire subject may be a distraction.

“At the end of the day, [vitamin D] is another pill, and we don’t understand how or if it works. I would rather give [a pill] that we know is effective,” Sparks says, referring to the available RA medications, including disease-modifying antirheumatic drugs like methotrexate and biologics like adalimumab.

“It’s more important to focus on the medications that will get RA into remission,” he emphasizes.

Goodman does routinely screen for vitamin D levels in her patients with RA, “not to treat the RA, but to address risk factors for osteoporosis, which is common in patients with RA.”

Ultimately, studies on the relationship between vitamin D and autoimmune diseases may help provide an understanding of who gets them and why, and there may one day prove to be a role for vitamin D in the management of these disorders, Sparks says. Until then, “more work needs to be done.”

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Is There a Relationship Between Vitamin D and Rheumatoid Arthritis? originally appeared on usnews.com