Rasher Than I Thought?

A recent report concluding that if you eat processed meat (bacon, sausages and suchlike) you’re more likely to get cancer in your pancreas has attracted predictably wide media coverage. More surprisingly, the reports I noticed (BBC News, Sky News and Guardian) were fairly reasonable accounts, quoting the main figures, the source of the information (British Journal of Cancer) and one or two ‘expert’ comments thereon. Usually science reporting in the ‘media’ is more feel than fact and appears to be motivated by coming up with eye-catching headlines rather than precise explanations (being precise, there is a Bacon Eaters Warned Of Deadly Cancer Risk in the above – but let’s not be too critical).

What the papers didn’t say

What such reports almost always fail to mention – and these were no exception – is how devilishly difficult it is to do surveys linking what we eat to what happens to our bodies. One method is to get a group of people with a given disease and ask them what they’ve eaten over the last umpty months/years/decades. You don’t need to be a stats wizard to see the major problem with this! Alternatively, so-called ‘prospective studies’ start with healthy individuals who are followed for exposure to potential factors and subsequent development of disease. Exposed and unexposed sub-groups are compared for disease rates. There are huge problems with these studies too, not the least being that you have no real idea how well the punters stick to the rules – in this case, what they eat.

The predictable upshot over many years has been that, apart from fruit and veg (good anti-cancer stuff, as we all know), for pretty well every survey showing something we eat gives us cancer there’s another that says it either has no effect or it’s actually protective.

Much easier than actually doing either type of survey is to do what these processed meaters did: put together all the sensible studies you can find (in this case eleven prospective surveys between 1966 and 2011) and see if a clear message emerges. Though not perhaps evident at first sight, this is actually quite a useful thing to do because by lumping all the data together you get a large number of patients and controls and the hope is that, out of the confusion of multiple smaller surveys, clarity will come forth.

And, up to a point, it did. The relative risk of pancreatic cancer emerged as 1.19 if you eat 50 g of processed meat every day (it would be 1.00 if you take The World Cancer Research Fund’s advice and avoid the stuff altogether). And, of course, the risk goes up the more of it you eat.

How scary is that?

So where does that leave us and how scared should we be by the scary headline? Have I been unwittingly irresponsible indulging a life-long taste for bacon, sausages and such like? Mmm…bacon…Mmm…sausages. (Sorry – Homeric moment there). Well, something like a 20% risk increase may be significant but it isn’t huge. Then 50 g is a fair wodge of bacon or whatever to eat every day. What’s more, the authors admitted that they’d had to make a few assumptions about just how much processed meat people actually had eaten in the various studies they collated, because some only listed ‘servings’ or ‘times’. Then there’s the question of how is the deed done if processed meat does drive cancer? The study authors noted that the most likely culprit is preservatives commonly added to such food – because these can indirectly cause DNA mutations. Having just salivated round the wondrous display of meats, hams, bacons, sausages etc. in my local Farm Shop (Gog Magog Hills: don’t miss it if you’re anywhere near Cambridge) I note than none of their stuff contains additives or preservatives. Whew!!

And the bottom line…

So my advice to me is: don’t panic, don’t pig out – but do keep an eye on where piggy bits come from. All of which is not to minimise the threat of pancreatic cancer. It’s the eighth biggest cancer killer worldwide, nearly 8,000 Brits died from it in 2008 and there’s no effective treatment. What’s the best thing to do – or not to do? Well, as we’ve said, take it easy on the bacon butties. But two things are strongly associated with pancreatic cancer: smoking (contributes to 20% of cases) and obesity. Not smoking’s easy, of course. Now, how to avoid getting fat…

Reference

Larsson, S.C. and Wolk, A. (2012). Red and processed meat consumption and risk of pancreatic cancer: meta-analysis of prospective studies. British Journal of Cancer advance online publication 12 January 2012; doi: 10.1038/bjc.2011.585.

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4 comments on “Rasher Than I Thought?

  1. Good old common sense, and not something that sells papers eh? I always think avoiding preservatives and buying the most expensive fresh meet you can afford is good advice… But I am sure there will be a survey out shortly that the recession is causing increase in cancers as organic and fresh food is sidelined for cheaper produce! What’s the story though on cases like Steve jobs who I can’t imagine smoked, ate meat or was ever overweight? And myself with bowel cancer, again don’t smoke, drink or eat red or processed meat and was fit and healthy with regular exercise! Are some of us just unlucky or is this genetics? I must read your book betrayed by nature!

    • You comment that the recession might drive a (further) shift to junk food – I think this a really serious threat: people are generally lazy (I certainly am when it comes to food). Despite all the evidence (just walk down the street) and all the publicity, Britain and the US is getting fatter by the day and the prospects look grim.
      Your other point about you and Steve Jobs is another very good one. You ask ‘are some of us just unlucky or is this genetics?’ I think the answer is ‘unlucky genetics’ or, in a phrase I use in undergraduate lectures, life is genetic roulette and cancer is part of life – you can do a simple sum to show that if we live to 140 essentially everyone would get a cancer of some sort. One of the best ways of looking at this is to consider lung cancer, 90% of which is caused by tobacco. How do we know that? Because the stats are overwhelming – in essence it was unknown before people started smoking, rose to huge levels in the UK and the USA at the peak of smoking and is now declining in the wake of (slowly) falling cigarette sales. Women took up smoking after men and their figures reflect this precisely. What’s more, chemicals have been isolated from nicotine that are highly mutagenic – i.e. they react directly with DNA. In particular some react with bases that are part of the coding sequence for specific proteins known to be important for DNA repair – just about the strongest evidence you could have. But what about the 10% or so who never smoked? Of course there’s likely to be a few in that figure that were exposed to tobacco or some other lung carcinogen but there’s clearly a small number who were unlucky: the genetic hand they were dealt predisposed them to developing a disease that is very rare unless driven by smoking. Their cancers are probably started not by mutation but by being born with a group of variants in their DNA – the things that makes us different from each other but that can also make us a bit more likely to get certain diseases.

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