More than one-third of the 15.7 million Americans with moderate or advanced chronic kidney disease use dietary or herbal supplements, reports new research from Mayo Clinic. Many Americans decide to take supplements on their own, not because of a doctor’s recommendation, the researchers found, most often with the goal of improving their health. However, some of these supplements contain potassium or phosphorous – minerals that are often restricted for that population – or can otherwise be harmful to people with impaired kidney function. The study published recently in the American Journal of Kidney Diseases.

“Many people take supplements without discussing it with their health care providers, likely because they assume these supplements to be safe and potentially beneficial,” says senior author Rozalina McCoy, M.D., a Mayo Clinic endocrinologist and general internal medicine doctor. “Yet some of these supplements may be harmful, particularly if patients have underlying kidney disease, or even if they do not.”

Chronic kidney disease – which affects roughly 1 in 7 Americans, according to the Centers for Disease Control and Prevention – is the gradual loss of kidney function that can lead to kidney failure. People with kidney disease have a harder time filtering medications, wastes, and excess fluids from the body, so it’s especially important for this population to know the safety risks of any supplements they’re taking, the researchers say. When the kidney’s filtering ability is impaired, supplements may accumulate and lead to toxicity. In all people, but especially among those with kidney disease, supplements may cause acute kidney injury or intensify the long-term deterioration of kidney function. Supplements can also interact with other medications, either amplifying or diminishing their effects.

“It is critical that patients share what supplements and herbs they are taking or thinking of taking with their health care providers and pharmacists,” says Dr. McCoy, who also is a researcher in the Mayo Clinic Robert D. and Patricia E. Kern Center for the Science of Health Care Delivery. “And as clinicians, we need to do a better job of routinely asking our patients about what non-prescription medications and supplements they may be taking, and doing it without judgement.”

What do the numbers say?

The multidisciplinary research team studied 10,005 Americans with varying levels of kidney function, using data from the National Health and Nutrition Examination Survey collected between 2011 and 2014. This survey combines both interviews and physical examinations, and is used to study many aspects of health and nutrition across the U.S.

Dr. McCoy and her fellow researchers divided survey respondents into several categories: those with normal or mildly reduced kidney function, those with moderate kidney impairment (stage 3 chronic kidney disease), and those with advanced or end-stage kidney disease (stages 4 and 5 chronic kidney disease). Their study focused specifically on those with stage 3 or worse kidney disease because this population is at highest risk for complications related to supplement use.

Because people often decide to take supplements without consulting their doctors, the research team also looked at whether people whose lab studies revealed the presence of chronic kidney disease were actually aware that they had this disease. Only 12% of people with moderate kidney disease, and 63% of people with advanced kidney disease, reported having kidney disease at the time of the survey.

The Mayo Clinic researchers found between 2.3 and 3.4 percent of Americans with moderate kidney disease took supplements considered as “especially risky” by the National Kidney Foundation. Other supplements may also be harmful for people with kidney disease, the researchers caution, since there is little research in this area and supplements are not regulated in the same way that prescription and over-the-counter medications are in the U.S.

What's in the box could hurt you

“Supplements, in many ways, are a ‘black box,’” Dr. McCoy says. “We need better data to know whether supplements are safe because for most supplements and herbs, we just have no idea. We do not accept such ambiguity for medications, and we should not tolerate it for supplements, either.”

Flaxseed oil, the most commonly used high risk supplement that contains phosphorus, was taken by 16 percent of patients with normal or mildly reduced kidney function. In addition, 1.3 percent of patients who were unaware they had moderate kidney impairment took flaxseed oil, which translates to 167,500 Americans. And while phosphorus is not listed on the flaxseed oil nutrition information label, a tablespoon (about 10 grams) of whole flaxseeds has about 62 milligrams of phosphorus, or about 7% of the daily value for a person without chronic kidney disease. In addition, flaxseed and flaxseed oil may interact with blood-thinning and blood pressure drugs, and may decrease absorption for any oral drug, according to Mayo Clinic.

The good news, Dr. McCoy says, is no Americans surveyed who had advanced or end-stage kidney disease took supplements flagged as potentially risky by the National Kidney Foundation.

“That may mean that awareness translates to caution or to greater odds of discussing supplement use with their clinical team,” she says. “The problem is that many people who have kidney disease are not aware of it.”

The study’s lead author is Shaheen Kurani, Sc.M., a student in the Mayo Clinic Graduate School of Biomedical Sciences.

This research was funded by the Mayo Clinic Robert D. and Patricia E. Kern Center for the Science of Health Care Delivery.

The researchers report no conflicts of interest.

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About the Mayo Clinic Robert D. and Patricia E. Kern Center for the Science of Health Care Delivery
The Mayo Clinic Robert D. and Patricia E. Kern Center for the Science of Health Care Delivery seeks to discover new ways to improve health; translate those discoveries into evidence-based, actionable treatments, processes and procedures; and apply this new knowledge to improve care for patients everywhere. Learn more about the center.

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