Antioxidants and Cancer - The Link

chillibreeze writer — Shobha Gopalakrishnan

Antioxidants are compounds that protect cells from free-radical induced oxidative damage. Free radicals are produced during natural cellular processes like food breakdown, and also as a consequence of exposure to environmental factors such as cigarette smoke, sunlight, and ionizing radiation.

Free radicals possess an incomplete electron shell. The desire to complete their electron shell renders free radicals highly chemically reactive. They stabilize themselves by stealing electrons from cellular components such as DNA, lipid, and protein. Accumulation of free radical-induced DNA damage over time could lead to cancer. Antioxidants 'mop up' or neutralize free radicals by lending them an electron and, thereby, prevent cellular damage that may otherwise occur.

Antioxidants are of two types: water-soluble (vitamin C) and lipid-soluble (resveratrol, vitamins A and E). Antioxidants may be synthesized in the body or obtained from the diet. Dietary antioxidants may be derived from vegetables, fruits, grains, nuts, meat, fish, and poultry. Alternatively, antioxidant supplements may be taken in the form of pills.

Antioxidants and Cancer
The role of antioxidants as protective agents against cancer has received much attention from the medical community in the past few decades. Considerable evidence from laboratory cell culture and animal studies have indicated that antioxidants may slow or prevent cancer development.

Results from a large scale, long-term study called the European Prospective Investigation of Cancer or EPIC indicated that increased intake of fruits and vegetables was associated with decreased risk of esophagus and stomach cancers (1, 2).

However, trials conducted to study the effects of antioxidant supplements on cancer progression have yielded varied results. A trial conducted in 1993, the Chinese cancer prevention study, concluded that a combination of beta-carotene, selenium, and vitamin E significantly reduced the incidence of gastric cancer (3). Results from a follow-up study of patients who participated in this trial indicated that the reduction in mortality was observed up to 10 years after the cessation of antioxidant supplementation (4).

Clinical trials
Outcomes of other clinical trials investigating the effects of antioxidants on cancer have been contradictory. A trial published in 1994 indicated that beta-carotene and retinol (vitamin A) were associated with increased rates of lung cancer (5). Another recent study analyzing the results of randomized control trials concluded that antioxidant supplements did not have any cancer preventive effects (6). Interestingly, a laboratory study conducted by Joan Brugge's group at Harvard indicates that antioxidants may actually promote survival of cancer cells in culture (7).

While it appears that natural dietary antioxidants may have cancer preventive properties, the role of antioxidant supplements in cancer prevention remains unclear. Antioxidants such as lycopene, green tea polyphenols, selenium and resveratrol are some of the antioxidants that are being aggressively studied.

Given the range of results from various clinical trials so far, it is difficult to assess whether antioxidant supplements are beneficial or harmful. More extensive research followed by carefully controlled clinical trials will be necessary to understand the potential of antioxidants in cancer prevention.

References:

1. Boeing H, Dietrich T, Hoffmann K, Pischon T, Ferrari P, Lahmann PH, Boutron-Ruault MC, Clavel-Chapelon F, Allen N, Key T et al. Intake of fruits and vegetables and risk of cancer of the upper aero-digestive tract: the prospective EPIC-study. Cancer Causes Control. 2006 Sep;17(7):957-69.
   
   
2. Benetou V, Orfanos P, Lagiou P, Trichopoulos D, Boffetta P, Trichopoulou A. Vegetables and fruits in relation to cancer risk: evidence from the Greek EPIC cohort study. Cancer Epidemiol Biomarkers Prev. 2008 Feb;17(2):387-92.
   
   
3. Blot WJ, Li JY, Taylor PR, Guo W, Dawsey S, Wang GQ, Yang CS, Zheng SF, Gail M, Li GY, et al. Nutrition intervention trials in Linxian, China: supplementation with specific vitamin/mineral combinations, cancer incidence, and disease-specific mortality in the general population. J Natl Cancer Inst 1993;85:1483–91.
   
   
4. Qiao YL, Dawsey SM, Kamangar F, Fan JH, Abnet CC, Sun XD, Johnson LL, Gail MH, Dong ZW, Yu B, Mark SD, Taylor PR. Total and cancer mortality after supplementation with vitamins and minerals: follow-up of the Linxian General Population Nutrition Intervention Trial. J Natl Cancer Inst. 2009 Apr 1;101(7):507-18.
   
   
5. Omenn GS, Goodman G, Thomquist M, et al. The beta-carotene and retinol efficacy trial (CARET) for chemoprevention of lung cancer in high risk populations: smokers and asbestos-exposed workers. Cancer Res 1994;54(7 Suppl):2038s–43s.
   
   
6. Myung SK, Kim Y, Jun W, Choi HJ, Bae WK. Effects of antioxidant supplements on cancer prevention: meta-analysis of randomized controlled trials. Ann Oncol. 2010 Jan;21(1):166-79.
   
   
7. Schafer ZT, Grassian AR, Song L, Jiang Z, Gerhart-Hines Z, Irie HY, Gao S, Puigserver P, Brugge JS. Antioxidant and oncogene rescue of metabolic defects caused by loss of matrix attachment. Nature. 2009 Sep 3;461(7260):109-13.

 

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Chillibreeze's disclaimer: This is a contributed article and was published on Chillibreeze in May, 2010. The views and opinions expressed in this article are those of the author(s) and do not reflect the views of Chillibreeze as a company. Chillibreeze has a strict anti-plagiarism policy. Please contact us to report any copyright issues related to this article. The relevance of the facts and figures cited (if any) could change after a period of time.

 

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