Study: Do antioxidants encourage the spread of cancer cells?


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Scientists do not yet know why some cancers spread to other parts of the body (a process called metastasis). A study in mice suggested that high doses of some antioxidants (chemicals that can protect cells from damage) might actually make it easier for cancer cells to spread. (12/01/2015)

Contents

At a glance In-depth
Findings     limitations            
Guidelines Resources and references
Questions for your doctor  


STUDY AT A GLANCE

This study is about:

The effect of antioxidants on the spread of cancer cells in mice.

Why is this study important?

Learning what causes cancer cells to spread can help scientists design better ways to stop this process. Antioxidants include some vitamins and nutrients that can be found in foods or taken as supplements. They protect our cells from some of the damage that occurs from day-to-day life; for example, exposure to the environment, sun, hormones, etc. Research has looked at whether taking antioxidant supplements can prevent cancer in people, and results have been mixed. In some studies, taking large amounts of certain antioxidant supplements has led to poorer outcomes among patients. 

Study findings: 

The study found that very large doses of antioxidants promoted spread of melanoma cells in mice. 

What does this mean for me?

People often wonder if eating certain foods or taking supplements can help them prevent or treat their cancer. Research on the relationship between antioxidant supplements and cancer is complicated and has produced mixed results. Results vary by the population studied, the type of cancer, the antioxidant used, and even the dose. Studies in people have shown that increasing dietary antioxidants does not reduce cancer risk. In some research studies vitamin E supplements have been linked to an increase in the chance of being diagnosed with or dying from prostate and lung cancers.

The doses of antioxidants used in this study are much higher than what people would normally get from their diet. According to experts, eating a healthy diet should provide most people with enough of the antioxidants needed for good health. You should speak with your doctor before taking any antioxidant dietary supplements.

Expert Guidelines

The National Comprehensive Cancer Network (NCCN) has guidelines on survivorship that include the following recommendations on supplement use:

  • Supplement use is not recommended for most survivors, except in instances of documented nutritional deficiencies, inadequate diet, or other indications (eg., osteoporosis).
  • Little data exist to support the use of vitamins or other dietary supplements for the purposes of cancer control, recurrence, or prevention. 
  • Taking vitamin supplements does not replace the need for a healthy diet. All efforts should be made to obtain nutrients from food and beverages.
  • Providers should assess supplement use at regular intervals. Ask about reasons for supplement use and supplement ingredients. 
  • Survivors of certain cancers (eg., gastrointestinal cancers) may be at risk for vitamin deficiencies based on their cancer treatment. Deficiencies should be asessed and addressed as needed. 

The American Cancer Society recommends speaking with your oncologist before taking any supplements. 

The American Institute for Cancer Research (AICR) is an organization that translates research on diet and cancer risk into practical information for the public. AICR recommends that people try to meet their nutritional needs through a healthy diet. They do not discourage people from taking a multivitamin supplement, but they warn people not to rely on supplements alone to protect from cancer. 

Questions To Ask Your Health Care Provider

  • Are there foods I should eat or avoid after I have had cancer?
  • What level of antioxidants should I include in my diet?  
  • Are there supplements I should take or avoid?
  • Can you refer me to a nutritionist?

IN DEPTH REVIEW OF RESEARCH

Study background: 

Cells undergo a process called metastasis to spread from a primary tumor to other sites in the body. Few cancer cells can survive the metastasis process to spread and grow at another site in the body. It is not fully understood how cells that manage to metastasize are able to do so. Dr. Sean Morrison and colleagues at the University of Texas Southwestern Medical Center looked at the capability of metastasized melanoma cells to withstand oxidative stress. (Oxidative stress, which can damage cells, occurs when an imbalance develops between reactive oxygen species (ROS) and the body’s ability to remove them.) ROS occurs naturally in the body as oxygen is processed; they are damaging, but our bodies usually have a sufficiently strong system response to handle it. ROS increase during times of environmental stress, such as exposure to ultraviolet (UV) light or ionizing radiation. Molecules called antioxidants play a role in controlling ROS.  Antioxidants are produced by the body and are also found in many different foods. Some antioxidants are sold as over-the-counter dietary supplements.

Researchers of this study wanted to know:

How oxidative stress and antioxidants affect the ability of melanoma cells to metastasize.

Population(s) looked at in the study:

Cancer cells from four patients with non-metastatic melanoma and four patients with metastasized melanoma were injected into mice that lacked functioning immune systems.

Study findings: 

  1. Changes in melanoma cells in the blood and at distant points from the primary tumor indicated oxidative stress that was not found in the rodent’s primary tumors. 
  2. Melanoma cells that successfully metastasized withstood oxidative stress. 
  3. Large doses of the antioxidant N-acetyl cysteine (NAC) promoted metastasis.

Limitations:

This study was done in mice that did not have functioning immune systems, allowing researchers to inject and study human tumor cells in a mouse model. Though the researchers used melanoma cells from human patients samples, observing what happens when they were transplanted into immunodeficient mice does not exactly correlate to what happens in human cancer patients with functional immune systems. In addition, the number of different patient samples used in this study was relatively low, which means that there is a possibility that the results the researchers see are unique to those tumors. Researchers did note that melanoma metastasis in the study mice is predictive of clinical outcomes—in other words, the cancer cells that metastasized in human patients also metastasized in the mice. While this may be true, it does not mean that the mechanism that the cells use to metastasize is exactly the same in the two species, or that a functional immune system may affect the process.  

Conclusions:

Not too long ago, the idea that antioxidants are beneficial was well received, and clinical trials looked at giving cancer patients supplementary antioxidants. Recently, however, this idea is being questioned. This study indicates that oxidative stress limits the ability of melanoma cells to metastasize; when researchers added antioxidants to relieve oxidative stress, more metastasis occurred. The antioxidant treatments used in these mouse studies may help us understand how cancer acts, but the path from antioxidant treatments in genetically altered mice to clinical application is a long one, and not all study results in mice hold true in human patients. More work needs to be done to fully understand this process and how it might impact cancer patients.

However, the data from previous clinical trials prompted the U.S. Preventive Services Task Force (USPSTF) to recommend against the use of Vitamin E and Beta-carotene (antioxidants) for the prevention of cardiovascular disease or cancer. The USPSTF also says that there is not enough data to recommend for or against the use of multivitamins (which include antioxidants) for the prevention of cardiovascular disease or cancer.

It is also important to note that the antioxidants injected into mice in this study were pure, supplemental treatment antioxidants. They included much greater antioxidant levels than what a person would get from a normal, healthy diet. In the context of this study, a greater dose of treatment antioxidants promoted metastasis. People with cancer, and those who have a high risk of cancer, should continue to eat a healthy diet that includes a variety of fruits and vegetables.

posted 12/01/15

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