How to understand the risk of a bacon sandwich giving you bowel cancer

By | April 17, 2019

What’s the relative risk of eating that bacon sandwich?

Mint Photography/Stockimo/Alamy

This article was originally published in February 2011 and was updated with a new headline and image on 17 April 2019.

Is there anything that has not been claimed to cause cancer? In the past few years, we have learned, among other things, that drinking very hot cups of tea leads to an eightfold increase in the risk of developing oesophageal cancer; that a quarter of a grapefruit a day increases breast cancer risk by 30 per cent in post-menopausal women; and, most deliciously, that a daily bacon sandwich raises the likelihood of bowel cancer by 20 per cent. This last finding was encapsulated by the British tabloid The Sun in the headline “Careless pork costs lives“.

These assertions may or may not be valid, but hidden within them is a more important and insidious source of confusion. The figures quoted measure relative risks: how much more likely you are to get ill when indulging in the supposedly dangerous substance or activity compared with not indulging. But they tell you nothing about what that increase in risk amounts to in absolute terms, so there is no way of telling whether it is something worth being concerned about.

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Scary or not?

“For an average person, the chance of getting bowel cancer at some point in their life is around 5 per cent,” says Spiegelhalter. So a 20 per cent relative increase in bowel cancer risk translates to an absolute increase in risk from 5 per cent to 6 per cent – just 1 per cent. That’s big enough not to ignore, but less of a deterrent to those who like their daily bacon sandwich.

Journalists are by no means the only ones who exploit the greater headline-grabbing potential of relative risk; health professionals do it too. “One of the most misleading, but rather common, tricks is to use relative risks when talking about the benefits of a treatment, while potential harms are given in absolute risks,” says Spiegelhalter.

This technique is known as mismatched framing. In his book Reckoning with Risk, psychologist Gerd Gigerenzer of the Max Planck Institute for Human Development in Berlin, Germany, quotes the example of a patient information leaflet concerning hormone replacement therapy. It claimed that HRT cuts the risk of bowel cancer by 50 per cent (a relative risk), but leads to 6 extra cases of breast cancer per 1000 women (an absolute risk). At first glance, the benefit here seems to hugely outweigh the additional breast cancer risk of just 0.6 per cent.

But until we know the absolute rates of bowel cancer in the target population, we are none the wiser. Assuming that rate is 5 per cent, as it is in the general population, the reduction in risk is 2.5 per cent, putting the benefit to harm ratio in a very different light.

Once you are aware of this trick, it’s relatively easy to spot, but this doesn’t eradicate it even from peer-reviewed medical journals. According to a study published in 2007, one-third of papers reporting on the benefits and harms of medical interventions in the BMJ, The Lancet and The Journal of the American Medical Association presented them using a mixture of different measures (Medical Care, vol 45, p S23).

Read more:Spin doctors: The truth behind health scare headlines

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