Antioxidants and cancer were supposed to be bitter enemies. For years, it was assumed that the benefits of antioxidants also included a reduction in heart disease, memory loss, type 2 diabetes, cataracts and macular degeneration. Antioxidant vitamins (C, E, and beta-carotene) were supposed to help prevent all of them.
So far, the three antioxidants (plus zinc) have succeeded with only one: slowing the pace of macular degeneration in older people who already have the eye disease.
“The randomized trials for antioxidants have been very disappointing,” says Harvard’s JoAnn Manson, who led the Women’s Antioxidant Cardiovascular Study, the Women’s Folic Acid Study, and other major trials.
What’s more, “some risks have been identified that suggest that high-dose antioxidant supplements should be avoided.”
For example, the Physicians’ Health Study II gave roughly 14,600 men aged 50 or older either vitamin E (400 IU every other day), vitamin C (500 mg a day), and/or a placebo. The only significant difference after eight years: “Vitamin E increased the risk of hemorrhagic stroke,” notes Manson.
In a meta-analysis of multiple trials, researchers estimated that high doses of vitamin E (200 to 800 IU a day in most studies) would prevent one ischemic stroke in every 476 people, but would cause one hemorrhagic stroke in every 1,250 people.
A hemorrhagic stroke (caused by a burst artery in the brain) is more devastating than an ischemic stroke (caused by a blocked artery in the brain). But even if the damage were equivalent, the odds wouldn’t justify taking vitamin E.
And in 2005, the HOPE-TOO trial —which gave vitamin E (400 IU a day) or a placebo to roughly 10,000 people with diabetes or a history of heart attack, stroke, or peripheral artery disease— reported a 13 percent higher risk of heart failure in the vitamin E takers.
“Overall, most randomized trials have suggested either a neutral or adverse effect of high-dose vitamin E,” says Manson. “So from a public health standpoint, we can’t recommend taking it at this point.”
Vitamin C hasn’t lived up to expectations either. The Women’s Antioxidant Cardiovascular Study gave vitamin C (500 mg a day), vitamin E (600 IU every other day), and beta-carotene (83,000 IU every other day) to roughly 8,100 women with a history of heart disease. After nine years, the risk of a heart attack, stroke, or other cardiovascular event was no lower in those who took vitamin C (or the other vitamins).
“We have tested vitamin C rigorously,” says Manson. “We’ve seen no benefit for cardiovascular disease or cancer.”
(And, despite Linus Pauling’s predictions from the 1970s, high doses of vitamin C don’t seem to ward off colds. At best, they might shorten a cold by less than half a day.)
As for beta-carotene, high doses actually raised the risk of lung cancer and heart disease in Finnish smokers and American men who had been exposed to asbestos. The only ray of hope: in the Physicians’ Health Study II, which included few smokers, men who got 83,000 IU of beta-carotene every other day for 18 years had better scores on tests of verbal memory (and no higher risk of lung cancer or heart disease).
“Overall, we have not seen benefits for antioxidants and cancer, diabetes, heart disease, and eye diseases,” concludes Manson.
Does that kill the hypothesis for antioxidant benefits the body in general? Maybe not.
“It may still be reasonable,” says Manson. “We don’t know how much is being absorbed and whether it’s actually being delivered to the critical tissues.”
Perhaps the antioxidants never reached their targets. Or maybe the trials didn’t last long enough. “Our trials have been too late and too short,” says Harvard’s Walter Willett.
“Also, if you’re giving antioxidants to people who are eating very well to begin with, then you probably don’t add too much. Trials need to start with people who have low intakes, because they’re the people in whom the benefit is more likely to be seen,” he notes.
“But we do know that these supplements as they’ve been tested—the supplements that people are buying over-the-counter,” says Manson, “have not demonstrated benefits for cardiovascular disease” or taking antioxidants and cancer reduction.
Do you take an antioxidant supplement? Let us know in the comments.
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This article was originally published in 2012 and is updated regularly.