What are antioxidants, and are they good for us? (part 2)

Category: Science blog June 25, 2009 Henry Scowcroft Comments are closed

This entry is part 2 of 3 in the series Antioxidants

There’s no proven benefit in taking antioxidants during cancer treatment

Yesterday, in the wake of an ASA ruling over antioxidants in green tea, and a US research paper that suggested that some cancer patients were taking them in large quantities, we looked at what antioxidants were, and how they occur in our bodies.

And we showed that the idea that ‘antioxidants = good’ is a huge oversimplification.

Now we’re going to look at the evidence that trying to boost the levels of antioxidants in our diets is good for us, particularly while undergoing cancer treatment.

But first, we need to make a clear distinction between antioxidants in the foods we eat – ‘dietary’ antioxidants – and antioxidant pills such as vitamin pills, taken to supplement our diets.

Antioxidants in our foods

There have been hundreds, if not thousands of trials looking at how antioxidants in our everyday diet affect our health – and many of these have focused on cancer prevention.

This sort of research faces significant hurdles. Firstly, over the course of a day, we all eat a hugely complex mix of different chemicals in our foods, so it’s incredibly difficult to work out the effects of any particular one.

There’s also the problem that some chemicals can affect the way our gut absorbs others. So some chemicals may only be effective when eaten with (or without) others.

The best evidence available comes from long-term ‘prospective’ studies that follow lots of people over long periods of times, such as the EPIC study that we’re helping fund.

The results from EPIC have been mixed. It’s found evidence that some cancers are less common in people who eat a diet high in fruit and vegetables. And when the team have done further analysis, they found that there’s a weak but measurable effect with foods rich in some some antioxidants, namely vitamin C and lycopene, on the risk of developing some types of cancer.

Looking at the overall picture, it’s reasonable to say that it’s likely that a diet high in foods containing some antioxidants might protect against some cancers under some circumstances as part of some diets. But equally, if there was a substantial net benefit to be gained, these trials would probably have spotted it too – and they haven’t.

So if these antioxidants have a weak effect when consumed in our foods, what happens if we take extra quantities of them in pill form, to try to boost their effects?

Antioxidant supplements

Unlike dietary antioxidants, it’s much easier to study people who take supplements. It’s easier to measure the exact dose they’re taking, and to do controlled studies where people take either a placebo or a supplement.

And spurred on by initially promising results from the laboratory (as we mentioned before), there have been hundreds of such studies – some showing a positive effect, others showing no effect, and some even showing that they increase the risk of cancer.

In February this year, Ed wrote about the lack of evidence that vitamin supplements (many of which are antioxidants) protect against cancer.  The post focused on a large study of over 160,000 women, which concluded that vitamin supplements had no positive benefit in protecting against cancer. Thankfully, it also found that they didn’t do any harm either, as other studies have suggested.

This study essentially confirmed the findings of an even larger, more authoritative ‘study of studies’ conducted by the Cochrane collaboration in April 2008, which, as Ed wrote,

…concluded that supplements of vitamins A, C and E, beta-carotene and selenium do not improve your health. And the very best trials showed that beta-carotene, vitamin A and vitamin E supplements could possibly increase the risk of dying from various diseases.

This is the closest thing we have so far to a definitive answer about antioxidant supplements – their cancer prevention effects are at best, relatively small. And at worst, some can actually do harm.

Case study

The story of antioxidants and cancer prevention is, in fact, an excellent case study about how science works. A few decades ago laboratory studies and small studies in humans had led scientists to feel pretty confident that antioxidants would help prevent cancer.

But when they went to confirm this in larger trials, there was pretty much widespread disappointment when these more rigorous studies failed to show the same effect. This highlights something we come back to time and time again on this blog – it’s perilous to draw general conclusions about a substance’s cancer-preventing ability from small, pilot studies – the only way to be sure is by carrying out long-term, well-designed trials.

So it looks like vitamin supplements aren’t all they’re cracked up to be at helping prevent cancer – and certainly don’t appear to justify the claims made by some suppliers. But what about taking them during cancer treatment itself?

The evidence on antioxidants and cancer treatment is controversial

You might suppose that taking antioxidant supplements during cancer treatment could be a good thing for two reasons. Firstly, they might lessen the side effects. And secondly, they might make the treatment more effective. And there’s some evidence for both of these arguments.

Conversely, it’s also possible that antioxidants might actually interfere with cancer treatments like chemotherapy and radiotherapy. Some chemotherapy drugs work by producing free radicals, and radiotherapy also relies on unleashing their destructive potential inside cancer cells.

So it’s possible that consuming high quantities of antioxidants could reduce or even block these treatments, and there’s evidence for this too.

Overall, once again, the data supporting either of these arguments are inconsistent. For every study that shows one thing, there’s another that shows the opposite, or no effect. It’s hard to be sure whether antioxidants help or harm, or under what circumstances. And it’s likely that people respond differently, just like with treatment itself.

There have also been suggestions that the way antioxidants are physically absorbed determines their effect – as Alison wrote about last August. And there’s an excellent Q&A on the subject on our patient information website, CancerHelp UK (which excitingly is about to be radically revamped).

Some cancer patients take high doses of antioxidants

So given that this is all such a grey area, it was worrying to read about research (mentioned above and nicely deconstructed by the BMJ on the Guardian’s website) that found that 60 per cent of 700 New York-based cancer survivors said they’d taken antioxidant supplements during their treatment for breast cancer.

In fact, about 70 per cent of those who said they’d taken them said they’d taken many different types, in very high doses.

The study has its weaknesses – it only asked people to remember what they did some years previously, and it only looked at relatively well-off women in one area of New York, so it’s not clear that these figures are widely applicable. Nevertheless, it paints a worrying picture.

No evidence of harm?

The authors of this study are clear that they’re not saying antioxidants are necessarily bad. They’re merely pointing out that our society has a tendency to gulp down foods because they contain antioxidants, without considering the lack of evidence that they’re actually good for us.

And although this evidence is far from clear, there’s sufficient reason to think that, in some people, the antioxidants they take may interfere with their treatment. Indeed, going back to green tea, a recent study in animals suggested that cancer drug bortezomib was rendered ineffective by a substance found in green tea.

So until things are clearer, and we get see the results of more high quality trials, we agree with the stance taken by the Royal College of Radiologists – avoid taking untested supplements during cancer treatment, and, most importantly, discuss any that you do take with your doctor.

At best, there are probably better things to spend your money on. At worst, you could be doing yourself more harm than good.



Greenlee, H., Gammon, M., Abrahamson, P., Gaudet, M., Terry, M., Hershman, D., Desai, M., Teitelbaum, S., Neugut, A., & Jacobson, J. (2009). Prevalence and predictors of antioxidant supplement use during breast cancer treatment Cancer DOI: 10.1002/cncr.24378