Can Antioxidants Cause Cancer?

Author Jennifer Sacheck, Ph.D., explains the latest research on antioxidants and how they can help and possibly harm your body.

Can Antioxidants Cause Cancer?

About 50% of the American public loads up on nutritional supplements, including those containing high doses of antioxidants – especially vitamin A (or beta-carotene, which is known as “pro-vitamin A”), vitamin C and vitamin E. These antioxidants fight notorious free radicals that are capable of damaging proteins, cell membranes, and our DNA, potentially leading to inflammation and an increased risk of cancer and many chronic diseases.  

But there has been a recent scare in the antioxidant world – with headlines reading “Antioxidants CAUSE Cancer!” After decades of research examining the potential benefits of consuming antioxidants, what brings about this scary news story? Especially when we all thought that loading up on antioxidant foods and supplements was a good thing?

Here’s the scoop. To date there have been nine clinical trials worldwide on antioxidant supplementation and cancer prevention and the results have been surprisingly mixed. In most cases, supplementation had no impact on the risk for a variety of cancers. However, in some studies antioxidant supplementation actually increased cancer risk. The most notable of these studies were the CARET trial and the ATBC Study in which daily supplementation with beta-carotene or a combination of beta-carotene and vitamin A increased lung cancer incidence and all-cause mortality in smokers. There was also the SELECT Trial in which daily vitamin E supplementation increased prostate cancer in older men. We have known about these trial results for a while now, but why all the attention on the negative impact of antioxidants on cancer now?

In a compelling study released in January, researchers bred mice to be prone to lung cancer and treated them with either vitamin E or another antioxidant called N-acetyl-cysteine. Surprisingly, animals given either antioxidant had much more aggressive tumor development and more likely to die from the cancer. Upon closer examination, it appeared that the antioxidants actually inhibited the activity of a gene called p53 – whose job it is to destroy cancer cells! So what this study suggests, is that in high doses, antioxidants may actually help protect the cancer cells by turning off the p53 cancer sweeper, which then enables the cancer to continue to grow. 

Now to be clear – this study studied the effect of antioxidant supplements, not foods containing antioxidants, on increased cancer risk. Antioxidants found in foods are found in natural doses and combinations with other healthful nutrients and even consuming a lot of antioxidant-rich foods, like cantaloupe, oranges and almonds, is unlikely to push your consumption levels much higher than the daily recommended intake.

When antioxidants are consumed in high doses, however, like what is often found in antioxidant supplements, you may put yourself at risk for health problems and toxicity. There are recommended “upper limits” for consumption of these antioxidants. For example, consuming too much vitamin A is associated with liver abnormalities, and more than 2000 mg/day of vitamin C can cause GI distress or increased kidney stones, while vitamin E greater than 1500 IU/day can interfere with blood clotting. Even taking vitamin E at lower doses has been associated with increased risk of certain cancers. So even if you skip your favorite stand-alone antioxidant supplement, but still take a multi-vitamin, make sure it includes antioxidants closer to the daily recommendations (most multivitamins should be in this range).

The take-home message on antioxidants is this: Too much of a good thing is not necessarily a good thing. Taking antioxidant supplements in high doses, especially if you are a smoker, have a family history of cancer, or already have cancer, may actually put you at greater risk. For everyone, antioxidants found naturally in foods still appear to be a wonderful thing – no bad news there – so keep on loading up on foods like fruits and vegetables for a great “natural” and healthy dose of antioxidants.

Jennifer Sachek, Ph.D., Friedman School of Nutrition at Tufts University is the author of “Thinner This Year."

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