Obesity and Cancer

Science, you could say, comes in two sorts. There’s the stuff we more or less understand – and there’s the rest. We’re pretty secure with the earth being round and orbiting the sun, the heart being a pump connected to a network of tubes that keeps us alive, DNA carrying the genetic code – and a few other things. But human beings are curious souls and we tend to be fascinated by what we don’t know and can’t see – why the Dance of the Seven Veils caught on, I guess.

Scientists are, of course, the extreme example – they spend their lives pursuing the unknown (and, as Fred Hoyle gloomily remarked, they’re always wrong and yet they always go on). But in this media era they pay a public price for their doggedness because they get asked the pressing questions of the moment. Is global warning going to finish us off soon, why is British sport generally so poor and – today’s teaser – does being fat make you more likely to get cancer?

A few facts go a long way

The major cancers have become familiar because the numbers afflicted are so staggering – but the one good thing is that the epidemiology can tell us something about the disease. Thus for cancers of the bowel, endometrium, kidney, oesophagus and pancreas and also for postmenopausal breast cancer there is clear evidence that being overweight or obese makes you more susceptible. In other words, if you compare large groups with those cancers to equally large numbers without, the disease groups contain significantly more people who are fat. We should add that the above list is conservative. A number of other cancers are almost certainly more common in those who are overweight (brain, thyroid, liver, ovary, prostate and stomach tumours as well as multiple myeloma, leukaemia, non-Hodgkin lymphoma and malignant melanoma in men).

Sizing up the problem

The usual measure is Body Mass Index (BMI) – your weight (in kilograms) divided by the square of your height (in metres). A BMI of 25 to 29.9 and you’re overweight; over 30 is obese. In England in 2009 just over 61% of adults and 28% of children (aged 2-10) were overweight or obese and of these, 23% of adults and 14% of children were obese. And every year these figures get bigger.

How big is the risk?

Impossible to say exactly – for one thing we don’t know how long you need to be exposed to the risk (i.e. being overweight) for cancer to develop but in 2010 just over 5% of the total of new cancer cases in the UK was due to excess weight. That’s another conservative estimate, but it means at least 17,000 out of 309,000 cases, with bowel and breast cancers being the major sites.

What’s going on?

Showing an association is a good start but the important thing is to find out which molecules make that link. For obesity and cancer detail remains obscure but broad outlines are emerging, summarised in the sketch. In obesity fat (adipose) cells increase in both number and size (so it’s a double problem: more cells – and the fat cells themselves are fatter). As this happens other cells are recruited to adipose tissue and, from this cellular cooperative, signalling proteins are released that have the potential to drive tumours. This picture is similar to that of the microenvironment of tumours themselves, where many types of cell infiltrate the new growth. Initially this inflammatory and immune response aims to kill the tumour but if it fails the balance of signalling shifts so that it actually helps the tumour grow. In addition to signals from fat cells themselves, obesity is usually associated with increased levels of circulating growth hormones (e.g., insulin) and of lipids, both of which may also promote tumour development.

Thus many signals with cancerous potential arise in obese individuals. In principle these could initiate tumour growth or they could accelerate it in cancers that have started to develop independently of obesity. So it is complicated – but at least as new signalling strands emerge they offer new targets for drug therapy.

In obesity abnormal signals from fatty tissue can combine with others arising from perturbed metabolism to help cancers develop

Reference

World Cancer Research Fund (WCRF) Panel on Food, Nutrition, Physical Activity, and the Prevention of Cancer (WCRF, 2007).

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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.