This morning, Today Programme allowed the World Cancer Research Fund (WCRF) to peddle the link between processed meat, sausages, bacon, salami and ham, and colorectal cancer. You can listen to the interview here.
The link was made in the 2007 report of the World Cancer Research Fund and American Institute for Cancer Research, with the title ‘Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective‘. It is a 537 page report with over 4,400 references. Its panel was chaired by Professor Sir Michael Marmot, UCL’s professor of Epidemiology and Public Health.
Over a year ago, the Daily Telegraph reported it with this a scary headline " sausage a day can increase bowel cancer risk"
This morning, Lisa Cooney, the head of education at the WCRF, asserted " there is convincing scientific evidence linking the consumption of processed meat with an increased risk of bowel cancer". The hapless Evan Davis accepted this assertion and moved on.
There is a brilliant piece on precisely this topic on Professor David Colquhoun's blog, DC's Improbable Science
Ok, there's a lot of statistics in there, but my O level maths didn't let me down.
Colquhoun's piece starts with this challenge to the WCRF report.
..... there has never been a randomised trial to test the carcinogenicity of bacon, so it seems reasonable to ask how strong is the evidence that you shouldn’t eat it? It turns out to be surprisingly flimsy.
He then asks the relevant question. Is there a causal link between the consumption of processed meat and colorectal cancer? If there is no such link, then why are the WCRF telling us not to eat bacon and sausages?
In the case of sausages and bacon, suppose that there is a correlation between eating them and developing colorectal cancer. How do we know that it was eating the bacon that caused the cancer – that the relationship is causal? The answer is that there is no way to be sure if we have simply observed the association. It could always be that the sort of people who eat bacon are also the sort of people who get colorectal cancer. But the question of causality is absolutely crucial, because if it is not causal, then stopping eating bacon won’t reduce your risk of cancer. The recommendation to avoid all processed meat in the WCRF report (2007) is sensible only if the relationship is causal. Barker Bausell said
[Page39] “But why should non scientists care one iota about something as esoteric as causal inference? I believe that the answer to this question is because the making of causal inferences is part of our job description as Homo Sapiens.”
That should be the mantra of every health journalist, and every newspaper reader.
Colquhoun then goes on to examine in detail the studies relied on by the WCRF paper and concludes.
After all this, we can return to the original question. Do sausages or bacon give you colorectal cancer? The answer, sadly, is that nobody really knows. I do know that, on the basis of the evidence, it seems to me to be an exaggeration to assert that “The evidence is convincing that processed meat is a cause of bowel cancer”.
Sometimes I think we should say “I don’t know” rather more often.
Hear, hear to that.