Can You Walk off the Risk of Breast Cancer?

One of this week’s health stories is typical of how rather unexciting research can reach the headlines by virtue of its association with a condition like breast cancer, but it also serves as a good example of two of the most common sources of sloppy reporting that plague health stories – which makes me think it a subject worthy of a blog.

The research relates to the possible effect of exercise on the risk of developing breast cancer, and the headline is Walking ‘cuts breast cancer risk’. If true, this is hardly an earth-shattering discovery. Perhaps it will add in some small way to our understanding of the mechanisms involved in the development of cancer, but this is for the journals to worry about. When it appears in mainstream media, the point is surely whether it means anything to an individual concerned about her breast cancer risk – in other words, if you want to reduce your risk of developing breast cancer, should you take up walking? Unfortunately, the way the results are reported makes it very difficult to answer this question.

 

Problem 1: associations are not the same as cause and effect

The first problem is that the study has made an observation, which has been presented as a cause. The researchers did quite a simple thing: they arranged for a group of over 73 000 post-menopausal women to complete a questionnaire at intervals over a 17 year period from 1992 to 2009, asking questions about how many hours walking the women did, and any diagnosis of breast cancer. They found that those who walked for 7 or more hours per week were less likely to have been diagnosed with breast cancer than those who walked for 3 hours or less. This does not mean that the walking caused the reduction in risk, however. It may well have done, but it could have been some other factor. There could have been a different cause that was linked to both breast cancer risk and the amount women walk. For instance, walking less could be linked to obesity, which could explain the extra breast cancer risk.

The researchers were aware of this problem, and tried to exclude some factors – for instance, it was not due to those who developed breast cancer being more overweight than those who did not – but they can never exclude all of the possible confounding influences. For instance, it may be that those who walked less were more likely to have other health problems, and the increased risk of breast cancer was in some way linked to this.

In my experience, observational health studies are very frequently reported as cause and effect. I can understand why – Walking ‘cuts breast cancer risk’ Has more of a ring to it than Walking is associated with a reduced risk of breast cancer. The problem is that the more catchy headline is misleading, and it is left to the reader to spot the error.

Problem 2: what do we mean by a reduction in risk?

The second pitfall when it comes to knowing what to make of a study like this is more serious – and more troubling, because the fault lies not with mainstream journalists trying to enhance their stories, but researchers and journal editors being guilty of the same. The problem is this: as is so often the case, the results have been presented in terms of a reduction in relative rather than absolute risk.

The trial demonstrated a 14% Relative Risk Reduction (RRR) – but is that a 14% reduction of a big number or a small number? If the Dragons in Dragons’ Den are offered a 14% share in company profit, they are very quick to ask how big that profit will be before they part with their money. The same should apply to us before we invest our energies in a health intervention. If the Dragons want to know the absolute amount of money they can expect to receive then we should expect to know the Absolute Risk Reduction (ARR) of any intervention.

The problem is that ARRs are always a lot smaller than RRRs, and so they make research look far less impressive, and researchers are reluctant to give them the attention they deserve. From the BBC article it is impossible to find the ARR, and so you have to go to the original research – and even here only the abstract is available without paying a fee and so you have to work the numbers out for yourself. It turns out that the risk of developing breast cancer over the 17 years of the study was 6.4 percent, making a 14% RRR equate to a 0.9% ARR.

Let us assume for the moment that the reduction in risk really is due to walking. Then if you are a woman after the menopause, and you walk for 7 hours a week rather than 3, then over a 17 year period you would reduce your risk of getting breast cancer by 0.9%. Put another way, if 1000 women walked the extra 4 hours a week for 17 years that would be 3 560 000 hours of walking to save 9 cases of breast cancer, or 393 000 hours of walking per case. At 3 miles per hour, it’s the equivalent of walking more than 47 times round the world! Now I do know that this statistic is probably as meaningless as being given a 14% relative risk reduction – but it was fun to work out!

That’s not to say that walking is a bad idea – there are clearly very good reasons for walking more. However, whatever the associated health benefits might be, the two most compelling reasons to walk will always be these: it’s a very useful way of getting from A to B, and most people find they rather enjoy it!

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