Cancer is a term used to describe a group of diseases in which cells divide abnormally and acquire the ability to invade other parts of the body. Conventional cancer therapies include surgery, radiation therapy and chemotherapy. Some cytotoxic drugs used in chemotherapy induce cell death by increasing oxidative stress -- production of chemically unstable molecules that can react with and cause damage to cellular components like DNA and protein. Although chemically unstable molecules called free radicals occur during normal processes in the body, an excess of free radicals increases the likelihood of irreversible cellular damage. Antioxidants are substances that can react with free radicals, arrest their damage-causing potential and reduce oxidative stress. The opposing effects of antioxidants and some cytotoxic drugs raise concerns about their concurrent use during cancer chemotherapy.
Cytotoxic Chemotherapy
Cytotoxic chemotherapy refers to the use of a drug or drug combination that typically causes the death of cells that divide rapidly. Rapid cell division is a feature common to cancer cells. Cytotoxic drugs used in chemotherapy include platinum-containing complexes, alkylating agents and some antibiotics. Many of these cytotoxic drugs increase oxidative stress within cancer cells, which, along with other mechanisms, contributes to their anti-tumor activity. The antibiotic doxorubicin, for instance, forms oxygen free radicals, which causes lipid peroxidation of lipids in the membranes enveloping cells.
Dietary Antioxidants
Antioxidants are substances that can react with free radicals and prevent them from causing damage to cells. A wide variety of antioxidants are commonly present in fruits and vegetables, as well as in some nuts, meat, fish and grains. Vitamins C and E and forms of vitamin A have antioxidant properties. In addition, beta-carotene, resveratrol, lycopene and flavonoids are just a few examples of phytochemicals that have antioxidant properties. People consuming more fruits and vegetables, which are rich in antioxidants, have been shown to have a reduced risk of certain cancers, including cancers of the mouth, stomach and lung. The American Cancer Society suggests a balanced diet for inclusion of antioxidants to reduce cancer risk, rather than supplements.
Oxidative Stress
Free radicals, specifically reactive oxygen species, or ROS, are normal byproducts of metabolic processes inside our body. They are also part of the mechanism by which your immune system attacks pathogens. However, increased or unregulated exposure to ROS, through environmental exposure such as ultraviolet radiation, can increase oxidative stress and cause damage to cellular molecules. Dietary antioxidants and internal regulation mechanisms cooperate to maintain a balance between the normal and detrimental effects of free radicals. Many dietary antioxidants also have other properties, including reduction of inflammation and causing changes in proteins that repair damaged cell membranes. While it is generally true that dietary antioxidants play a protective role by reducing cancer risk, the use of antioxidants, especially supplements, during cancer therapy is a subject of continued investigation and debate.
Concurrent Use
In principle, antioxidants decrease oxidative stress and can potentially interfere with the mechanism by which some cytotoxic drugs induce cell death in tumors. In practice, data suggests that antioxidants can aid, by ameliorating undesirable side effects, and hinder chemotherapy. Whether the effect is synergistic or prohibitive is likely dependent on many factors, including the specific pair of antioxidant and cytotoxic drug in question, as well as the characteristics of the cancer itself. For instance, one clinical study published in 2010 in "Cancer Epidemiology, Biomarkers and Prevention" found that breast cancer patients who took vitamin supplements, especially vitamins C and E, had a reduced risk of mortality and recurrence, with both concurrent and nonconcurrent chemotherapy, but not radiotherapy. Another study, published in "Journal of the American College of Nutrition" in 2005, found no tumor response or survival benefits of adding vitamin C and E to chemotherapy regimens in advanced lung cancer patients. Further studies are required to examine the potential of dietary and pharmaceutical antioxidants to assist specific chemotherapy regimens.
References
- "Alternative Medicine Review"; Davis W. Lamson and Matthew S. Brignall; 1999
- National Cancer Institute; Antioxidants Factsheet
- "Cancer Epidemiology, Biomarkers and Prevention"; Sarah Nechuta, et al; December 2010
- "Journal of the American College of Nutrition"; Ashutosh Kumar Pathak, et al.; February 2005
