Lack of Evidence to Support Claims That Antioxidants Prevent Disease

Most clinical trials have tested the impact of single antioxidant “substances” like vitamins E and C, and beta-carotene – and the results have mostly been negative. There have also been some studies that involved combinations of different types of antioxidant supplements, and these results have also been inconclusive.

One six-year study into age-related eye disease did find that a combination of vitamins C and E, beta-carotene, and zinc helped protect against macular degeneration in older people. It is thought lutein, that a natural carotenoid (a type of antioxidant) found in dark green, leafy vegetables including kale and spinach might have the same effect, but there is no real evidence to support the belief.

While there is lots of evidence that it is beneficial to our health to eat fruit and veg and whole grains that are rich in antioxidants, there is no convincing evidence that antioxidant supplements have any real impact on disease prevention. Also, while it is known that free radicals and oxidative stress contribute to aging and a number of chronic diseases, it doesn’t follow that substances (or molecules) with antioxidant properties are going to fix the problem. 

What Clinical Trials Have Revealed

Clinical trials over the years have focused on the relationship between antioxidants and heart disease and stroke, lung function, cancer, and age-related eye disease.

While my no means comprehensive, here are some examples:

• An early study in China, published in 1993, looked at the effects of antioxidant supplements in relation to risks of cancer. It found that while the antioxidants didn’t lower the risks of developing cancer, or from death as a result of esophageal cancer, they did lower the risks of death as a result of gastric cancer. This positive finding was negated in 2009 when additional deaths were reported.
• In Harvard Medical School’s landmark Women’s Health Study that began in 1993, there was no difference in the rate of cancer and major cardiovascular events between women taking vitamin E and those taking a placebo.
• A US cancer study report published in 1996 found that beta-carotene supplements had no effect on the incidence or mortality of cancer over a 12-year period. Smoking did not have an effect on the outcomes either.
• A German study published in 1997 examined the value of antioxidants and vitamins in the prevention and therapy of cardiovascular disease. It found that low-dose supplementation of antioxidant vitamins for primary prevention seemed feasible, high-dose intervention in secondary prevention needed further study.
• A Swedish study published in 2002 that also considered the role of antioxidant vitamins in preventing cardiovascular disease, took food and supplements into account. This found that increased antioxidant vitamins did lower the risk of stroke and myocardial infarction in people who generally consumed few antioxidants. However it also found that when given in the form of food supplements they had no beneficial effects, and reported that “serious adverse effects” had been reported.
• A Canadian study published in 2005 examined the effects of vitamin E supplements taken long-term on cancer and cardiovascular events. Their finding was that it did not prevent these diseases and might even increase the risk of heart failure.
• Noting in 2007 that previous clinical trials had failed to show that vitamin E reduced the risk of cardiovascular events, an Israeli team of researchers found that it did appear to reduce risks in middle-aged people with type 2 diabetes, who also had a common genetic predisposition that related to oxidative stress.
• A US study report published in 2009 revealed that neither vitamin C or E had make any difference to the incidence of cancer (including prostate cancer) in physicians aged 50 and more.

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Dr Gitte Jensen, founder and research director of the Oregon-based NIS Labs (Natural Immune Systems) is one person who still believes that antioxidants do have value in terms of human health and disease prevention. However she has urged those in the industry to “use our tests and data properly” and to develop improved testing methods. This, she believes could result in the USDA being persuaded to re-open its data banks as a public resource.

Harmful Effects Of Antioxidants

In addition to some of the harmful effects mentioned above, there is a growing body of evidence that shows antioxidants fuel cancer.

At least two studies involving heavy smokers that were carried out to see if beta-carotene could reduce the risks of lung cancer had the opposite effect. A trial in Finland was stopped early when researchers found the group taking the supplement had a higher incidence of lung cancer compared to those in the group taking a placebo. Similarly, research published in 2007 showed that women taking antioxidant supplements had an increased risk of skin cancer – though men didn’t.

It seems that instead of neutralizing free radicals and lowering the risks of cancer, antioxidants can protect some cancer cells from free radicals. For this reason many medics advise those with cancer – particularly lung cancer – to avoid taking extra antioxidants.

Ongoing Research On Antioxidants

Researchers haven’t given up on antioxidants yet and the NIH and its National Center for Complementary and Integrative Health (NCCIH) continue to support studies. Recent topics include:

• Effects of antioxidants on Alzheimer’s
• How oxidation and antioxidants affect risks of breast cancer
• Effects of antioxidants found in pomegranates in the prevention and treatment of prostate cancer
• Whether antioxidants from berries (specifically anthocyanins) can prevent esophageal cancer
• How chemical forms of vitamin E affect inflammation

Ultimately, as the NIH NCCIH points out, concerns do not relate to antioxidants that are found naturally in certain foods, but rather to manmade supplements. As they say, antioxidant supplements should never be used to replace conventional medical care or a healthy diet.