Antioxidants and cancer
What is an antioxidant?
An antioxidant is a molecule that can prevent or decrease the oxidation of other molecules.
What is the relationship between antioxidants and cancer?
Antioxidants are widely used as ingredients in dietary supplements with the hope of maintaining health and preventing illnesses such as cancer. Although initial studies suggested that antioxidant supplements might promote health, later large clinical trials showed no benefit and even suggested that excess supplementation may be harmful.
For example, a study (JAMA, March 16th 2005) showed that long-term vitamin E supplement in patients with diabetes or vascular disease not only did not lower cancer or major cardiovascular events, but may increase the risk for heart failure.
The recent publication in the Journal of American Medical Association (JAMA, November 18th 2009) of two studies conducted in Norway showed that patients who take regular vitamin B12 or folic acid supplement had higher incidence of cancer and increased risk of all-cause mortality.
A large clinical trial called SELECT, showed no benefit of taking vitamin E or selenium for preventing prostate cancer.
Some appropriate amount of vitamin may help reduce risk of cancer. A recent meta-analysis of nine studies showed that taking vitamin B6 may be associated with lower risk of colon cancer.
Vitamin D has been shown in many studies to be beneficial in reducing risk of cancer as well as other diseases.
To learn more about vitamins, follow these links:
Why is not there a clear association between taking antioxidants and lower risk of cancer?
People who enjoy and consume good amount of fresh fruits and vegetables everyday have lower risk of heart diseases, and possibly lower risk of certain cancers.
Since fruits and vegetables contain good sources of antioxidants, it is thought that antioxidants might prevent cancer. But the clinical trials have shown no clear benefit with taking antioxidant supplements. This suggests that other substances in fruits and vegetables such as flavonoids, or a complex mix of substances may be responsible for the benefit, rather than a single antioxidant.
Can oxidants be sometimes beneficial?
Yes. In fact, part of how chemotherapy and radiation therapy kills cancer cells is by producing oxidants inside the cancer cells, which then cause disintegration of DNAs and lead to death of cancer cells. This is called apoptosis, or programmed cell death. Read more about this term in our scientific and medical terms page.
Avoid taking excessive antioxidants during chemotherapy or taking tamoxifen:
There is evidence that taking excessive antioxidants may impact the efficacy of chemotherapy. There is also evidence that taking large amount of vitamin E supplement while taking tamoxifen for treating breast cancer may lower the efficacy of tamoxifen.
Is there evidence antioxidants may be beneficial for treating cancer?
Pomegranate contains rich antioxidants. There is evidence that consuming pomegranate juice might lower PSA in patients with prostate cancer. This study done by a group of researchers from UCLA showed that drinking pomegranate juice slowed down the rise of PSA in patients with prostate cancer.
Definition of pomegranate juice by NCI.
