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People with cancer should be wary of taking dietary supplements


Cancer dietitian Lisa Cianciotta often finds herself sitting across from a patient who suddenly fishes a bottle of antioxidant supplements from their bag and says, “My friend told me this works really well,” or “I read on the Internet that this is supposed to be really good for cancer.”

Although taking an antioxidant pill sounds harmless, Ms. Cianciotta, a clinical dietitian who works with cancer patients at New York–Presbyterian Hospital, knows well that this popular dietary supplement can interfere with a patient’s radiation or chemotherapy.

But many patients with cancer believe these over-the-counter vitamins, minerals, or herbal remedies will help them, and most use at least one dietary supplement alongside their cancer treatment.

And that leaves Ms. Cianciotta with a delicate conversation ahead of her.

Drug-supplement interactions are complex, often varying by supplement, cancer, and treatment type, and can do more harm than good. Popular dietary supplements may, for instance, cancel the effects of a cancer treatment, making it less effective, or increase serious side effects, such as liver toxicity. But in other cases, supplementation, such as vitamin D for patients who lack the vitamin, may be beneficial, Ms. Cianciotta said.

These drug-supplement interactions can be hard to pinpoint, given that more than two-thirds of doctors don’t know their patients are using supplements.

Here’s what patients need to know about the potential risks of supplement use during treatment, and how oncologists can address this thorny, often poorly understood topic with patients.

The complex drug-supplement landscape

The list of dietary supplements and how they can interact with different treatments and cancer types is long and nuanced.

But certain supplements appear to affect cancer treatments regardless of other things and should be avoided. Any supplement that strongly alters the body’s levels of the protein cytochromes P450 is one example. This group of enzymes plays a key role in metabolizing drugs, including chemotherapy and immunotherapy agents.

Certain supplements – most notably St. John’s wort extract – may decrease or increase the activity of cytochrome P450, which can then affect the concentrations of anticancer drugs in the blood, said William Figg, PharmD, an associate director of the Center for Cancer Research at the National Cancer Institute in Bethesda, Md. Studies show, for instance, that this common herbal supplement can increase the activity of cytochrome P450, resulting in lower levels of cancer drugs.

Outside of drug metabolism, patients with hormone-related cancers, such as breast and prostate cancers, should steer clear of dietary supplements that can alter levels of testosterone or estrogen, Dr. Figg said. The evergreen shrub ashwagandha, for example, is marketed to reduce stress and fatigue, but can also increase testosterone levels – a potential problem for those with prostate cancer receiving androgen deprivation therapy, which lowers testosterone levels.

Many oncologists counsel patients against using antioxidant-based dietary supplements – particularly turmeric and green tea extract – while they have radiation therapy and certain chemotherapies. These therapies work by creating an abundance of highly reactive molecules called free radicals in tumor cells, which increase stress within these cells, ultimately killing them off. Antioxidants, in theory, can neutralize this effect, said Skyler Johnson, MD, a radiation oncologist at Huntsman Cancer Institute at the University of Utah, Salt Lake City. Some studies suggest that antioxidant supplements may lessen the effects of radiation and chemotherapy, although the evidence is mixed.

Some dietary supplements, including high-dose green tea extract and vitamin A, can cause kidney or liver toxicity, and “many cancer patients already have compromised kidney or liver function,” said Jun J. Mao, MD, chief of integrative medicine at Memorial Sloan Kettering Cancer Center in New York. Even herbs that don’t interfere with how well a cancer drug works, such as stevia, can increase treatment-related side effects, such as nausea and vomiting.

Another potential problem with dietary supplements: It’s nearly impossible to know exactly what’s in them. For instance, just last year, the Food and Drug Administration sent nearly 50 warning letters to companies marketing dietary supplements. The issue is that federal regulations governing production are less strict for supplements than for medications. As a result, some supplements contain ingredients not listed on the label.

One historical example was the supplement PC-SPES, a mix of eight herbs, marketed to men with prostate cancer. The supplement was recalled in 2002 after certain batches were found to contain traces of prescription drugs, including diethylstilbestrol, ethinyl estradiol, warfarin, and alprazolam.

To further complicate matters, some dietary supplements can be helpful. Most patients with cancer “are malnourished and missing out on nutrients they could be getting from food,” said Ms. Cianciotta.

Patients are tested routinely for vitamin deficiencies and receive supplements as needed, she said. Vitamin D and folic acid are two of the most common deficiencies in this patient population. Vitamin D supplementation can improve outcomes in patients who have received a stem cell transplant by aiding engraftment and rebuilding the immune system, while folic acid supplementation can help to raise low red blood cell counts and hemoglobin levels.

Although she rarely sees vitamin toxicity, Ms. Cianciotta stressed that more is not always better and supplement use, even when it seems safe or warranted due to a deficiency, should be taken under supervision, and monitored carefully by the patient’s care team.


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