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Taking Certain Supplements Before and During Chemotherapy for Breast Cancer May Be Risky

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A small study suggests that people who took antioxidant supplements before and during chemotherapy to treat breast cancer may have worse outcomes, including a higher risk of recurrence (the cancer coming back).

The research was published online on Dec. 19, 2019, by the Journal of Clinical Oncology. Read the abstract of “Dietary Supplement Use During Chemotherapy and Survival Outcomes of Patients With Breast Cancer Enrolled in a Cooperative Group Clinical Trial (SWOG S0221).”

Dietary supplements

Vitamins, minerals, botanicals, amino acids, and herbs are all considered dietary supplements by the U.S. government. They are intended to supplement the diet, and people take them for a number of different reasons.

It’s important to know that in the United States, the government puts supplements in a special category. They are not considered medicines and are not subject to the same regulations and testing that medicines are.

It’s also important to know that some dietary supplements can interfere with breast cancer treatment and prescription medicines. Taking dietary supplements requires that you do some research, especially if you’re being treated for breast cancer. Before you take anything, make sure you talk to your doctor and possibly a registered dietitian about what you would like to take and discuss all the risks and benefits.

Most pharmaceutical companies and supplement producers do not conduct research on how medicines and supplements interact, so we don’t know all the risks of taking supplements during treatment. This is why the researchers did the study reviewed here.

About this study

Called the Diet, Exercise, Lifestyle and Cancer Prognosis (DELCaP) study, this analysis included 1,134 people who also were part of a phase III trial studying the best dose and schedule for three chemotherapy medicines to treat early-stage breast cancer with a high risk of recurrence after surgery. The medicines were:

  • Adriamycin (chemical name: doxorubicin)
  • Cytoxan (chemical name: cyclophosphamide)
  • Taxol (chemical name: paclitaxel)

For the phase III study, 2,716 people diagnosed with early-stage breast cancer with a high risk of recurrence between 2003 and 2010 participated. They were randomly assigned to receive one of the chemotherapy regimens. The people were followed for about 6 years to see if the chemotherapy medicines caused any side effects, as well as whether the breast cancer recurred.

For the DELCaP study, the researchers asked all the people in the phase III study if they would answer detailed questionnaires about their use of daily dietary supplements twice during the study:

  • when they were assigned to a chemotherapy treatment group
  • 6 months after they completed chemotherapy

It’s important to know that the people in the study were sent the questionnaires and filled them out at home. This means they had to remember which supplements they took and when they took them. If any of the people didn’t report their supplement use accurately, it could have affected the outcome of the study.

Overall, 1,134 people agreed and completed both questionnaires.

The questionnaires asked about use of:

  • vitamin C
  • vitamin A
  • vitamin E
  • coenzyme Q10
  • carotenoids (examples are beta-carotene, lycopene, selenium, and lutein)
  • any antioxidant
  • multivitamins
  • vitamin D
  • vitamin B6
  • vitamin B12
  • iron
  • folic acid
  • calcium
  • omega-3 fatty acids
  • glucosamine
  • melatonin
  • acidophilus

Vitamins C, A, and E, as well as coenzyme Q10 and carotenoids, are all antioxidants. Antioxidants protect your body's cells from free radicals — unstable molecules created during normal cell functions. Pollution, radiation, cigarette smoke, and herbicides also can create free radicals in your body. Free radicals can damage a cell's genetic parts and may trigger the cell to grow out of control. These changes may contribute to the development of cancer and other diseases.

Still, some research shows that despite their benefits, antioxidants such as vitamin E, beta-carotene, and selenium can actually increase the risk of some cancers, cause some cancers to recur, and interfere with the cancer-killing effects of chemotherapy.

During the DELCaP study, 251 people had a cancer recurrence and 181 people died.

The questionnaire results showed that 17.5% of the participants used at least one antioxidant daily during chemotherapy and 44% took multivitamins during chemotherapy. The researchers noted that these rates were low compared to rates of supplement use by people diagnosed with cancer in other studies, especially for antioxidants.

The researchers also noted that supplement use tended to decrease during chemotherapy treatment. For example, 20.5% of the participants took vitamin C before treatment, but only 12.2% took vitamin C during chemotherapy.

When the researchers compared supplement use to the rates of recurrence and mortality, they found:

  • People who said they took any antioxidant both before and after chemotherapy were 41% more likely to have a breast cancer recurrence and 40% more likely to die.
  • Taking antioxidants only before chemotherapy or only during chemotherapy had no effect on outcomes.
  • People taking vitamin B12 both before chemotherapy and during chemotherapy were 83% more likely to have a recurrence and about twice as likely to die.
  • People taking iron supplements both before chemotherapy and during chemotherapy were 91% more likely to have a recurrence.
  • Taking multivitamins had no effect on outcomes.
  • People taking omega-3 fatty acids both before chemotherapy and during chemotherapy were 67% more likely to have a recurrence.

"Although this is an observational study and the number of users of supplements was fairly small, the results are compelling," said Christine Ambrosone, chair of the Department of Cancer Prevention and Control at the Roswell Park Comprehensive Cancer Center and lead author of the study. "Patients using any antioxidant before and during chemotherapy had an increased risk of the breast cancer returning and, to a lesser degree, had an increased risk of death. Vitamin B12, iron, and omega-3 fatty acid use was also associated with poorer outcomes."

The researchers did caution that the study results were not definitive enough to affect how doctors treat people diagnosed with breast cancer.

"People diagnosed with any cancer should talk with their doctors about whether they should be taking vitamins or other supplements," Ambrosone said. "I'd recommend that they try to get their vitamins and minerals — including antioxidants — from food. With a healthy and balanced diet, you can get all the nutrients your body needs, even while undergoing chemo."

What this means for you

Taking dietary supplements requires that you do some homework, especially if you're being treated for breast cancer. Before you take anything, make sure you talk to your doctor and a registered dietitian about what you would like to take and discuss all the risks and benefits.

Keep the following points in mind when you’re considering taking any supplements:

  • Some dietary supplements can interfere with breast cancer treatments and prescription medicines. It’s very important to talk to your doctor about any and all supplements you’re considering.
  • Dietary supplements are not regulated by the U.S. government because they’re not classified as medicine. This means you have no guarantee that the supplement contains only the ingredients on the label. Look for reputable manufacturers you can trust to produce consistently high-quality supplements. Prescription and over-the-counter medicines available in the United States must, by federal law, meet the standards of the U.S. Pharmacopeia (USP). The USP also has created a set of standards for dietary supplements. But supplement makers are not required to follow these standards — it's voluntary. A good rule of thumb is to look for a supplement with the USP notation on the label.
  • Just because it’s natural doesn’t mean it’s safe. Many people believe that any food or supplement in its naturally occurring, unprocessed state is better or safer than something that is refined or manufactured. This is not necessarily true. Some of the most toxic substances in the world occur naturally.
  • Most doctors and dietitians recommend getting the nutrients you need from food, not supplements. This study supports that recommendation.

"Though I take supplements, I still believe the overall health benefits I am receiving from my nine-plus servings of fruits and vegetables and one to three serving of soy foods each day are even more important than the benefits of these supplements," said Diana Dyer, M.S., a registered dietitian and member of the Professional Advisory Board. "If I were forced to choose only one approach, I would put my money on maximizing my diet for the largest potential benefit."

For more information, visit the Dietary Supplements pages.

To talk with others about creative ways to get nutrients from foods, join the Discussion Board forum Healthy Recipes for Everyday Living.

Written by: Jamie DePolo, senior editor

Reviewed by: Brian Wojciechowski, M.D., medical adviser

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