MEDICUS FEBRUARY 2016

R E S E A R C H

Terry Slevin Director, Education and Research Cancer Council WA

A mate of mine in the UK described a recent report¹ on processed meat and risk of bowel cancer releases by the International Agency for Research on Cancer (IARC) as “confusogenic to humans.” During the week of the report being released, a regional public health physician expressed concern about the ham she’d put on her primary school-aged kids’ lunches that day. I understand the frustration. The IARC report found that a) processed meat is now considered a Class One – known to cause cancer in humans – carcinogen and that b) each additional 50 gram portion of processed meat eaten daily increases the risk of colorectal cancer by 18 per cent. We can explain risks of cancer and other diseases in several ways; some are easier to understand than others. The IARC statement is based on a summary of many epidemiological studies assessing the relationship between meat consumption and, for the most part, colorectal cancer, including the work of a former UWA (now University of Melbourne) Epidemiologist, Dallas English.

In this case, IARC was comparing the risk of bowel cancer for people who eat 50 grams of processed meat per day with the risk for those who eat no processed meat. The 18 per cent increase means the risk of developing bowel cancer is 1.18 times higher (or a relative risk of 1.18) for those who eat 50 grams of processed meat per day, every day, compared to abstainers. In reality, the increase is quite small. By contrast, men who smoke cigarettes have about 20 times the risk of developing lung cancer as men who do not smoke.² Using the “% increase risk” framing, the increase in risk due to smoking is 1,900 per cent. The problem is that many people will incorrectly conclude that if they ate processed meat, they had an 18 per cent (almost one in five) chance of getting bowel cancer. Not so. Presenting relative risks to the public in any format is not very informative. Perhaps a better way to communicate the effect of specific risk factors is to use the absolute risk.

Those fortunate enough to live to the age of 85 have an 8.2 per cent chance of being diagnosed with bowel cancer over their lifetime; this is the “lifetime risk”. In the absence of good data to allow a good estimate, assuming a quarter of the Australian population eats 50 grams per day of processed meat, then the lifetime risk for the three-quarters who eat no processed meat would be 7.9 per cent (or about one in 13). For those who eat 50 grams per day, the lifetime risk would be 9.3 per cent (or about one in 11). Changing this assumed consumption estimate from a quarter to an eighth or less results in little change to the two absolute risk figures – and ultimately not an enormous variance on colorectal cancer incidence or mortality. who eat processed meat differ in no other ways that affect risk of bowel cancer from those who do not. In reality we know many factors contribute to risk of bowel cancer – being overweight, alcohol consumption, being physically inactive This rudimentary calculation assumes everything else is equal; that people

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