October 26, 2015

expert reaction to IARC classification of processed meat as “carcinogenic to humans” and red meat as “probably carcinogenic to humans”

Following an evaluation the International Agency for Research on Cancer (IARC) of the World Health Organisation (WHO) has classified red meat as probably carcinogenic to humans, placing it in Group 2A, and processed meat as carcinogenic to humans, placing it in Group 1.

Prof. Kevin McConway, Professor of Applied Statistics, The Open University, said:

“The key thing about this classification is that it is based solely on strength of evidence that there is some increase in risk, and not on how much the risk increases. There are well over 100 different agents in IARC’s “Group 1”. These are things where IARC thinks there is sufficient evidence that they really do increase cancer risk. But some may increase by a lot, some by a small amount. Of 100 lifetime non-smokers in the UK, around 1 will get lung cancer. For 100 smokers of a pack a day, more than 20 will get lung cancer. That’s a huge increase in risk. Again in the UK, about 6 people in every 100 will get bowel cancer in their lives. According to the IARC data, if these 100 people in the UK start eating an extra 50g of processed meat a day, then 7 of them will get bowel cancer. OK, an increase, but only one more death and that’s nothing like the effect of cigarette smoking. So why are smoking and processed meat eating both in the same IARC group? Just because in both cases IARC is convinced that the evidence of some kind of effect, big or small, is quite strong. There are plenty of other agents in IARC Group 1 besides smoking and processed meat eating. Many of them are chemicals that most people (including me) won’t have heard of, but others are more familiar, such as burning coal at home, or working as a painter. Some of these have a big effect on risk, others don’t.

“Likewise, IARC have put eating non-processed red meat into their Group 2A, which means that they are considered “Probably carcinogenic to humans”. But all that means is that the evidence they cause cancer is a bit weaker than for the things in Group 1. Again Group 2A contains a huge range of things, for which, if they do actually cause cancer, the increased risk might be big or it might be quite small. The group contains some scary-sounding things like glyphosate weedkiller, but plenty of others, including some kinds of shiftworking, or working as a hairdresser or barber, are familiar. Again you have to remember that the increased risk may be small, and in fact IARC do not consider that there is decisive evidence that there is an increased risk at all (or they’d have put it in Group 1).

“In an important sense there’s really nothing new in this announcement. No new evidence has been collected – instead IARC have collated existing evidence and come to a decision on how strong that evidence is. And they have not yet published the detailed monograph that describes their findings, just a set of summaries. IARC make it explicitly clear anyway that they are not making recommendations on what we should eat. Others have already made such recommendations, based on the evidence that IARC have reviewed. This latest news has not, in itself, changed anything.”

Dr Gunter Kuhnle, a food nutrition scientist at the University of Reading, said:

“The WHO’s decision means that there is sufficient evidence that processed meat consumption causes cancer – it does not mean, as purported by many stories in the media, that eating bacon is as bad as smoking. This is a dangerous oversimplification as processed meat can be part of a healthy lifestyle – smoking can’t.

“It is important to put these results into perspective: three cigarettes per day increases the risk of lung cancer six-fold (600%) – eating 50g of processed meat increases the risk of colorectal cancer by 20%. This is still very relevant from a public health point of view as there are more than 30,000 new cases per year – but it should not be used for scaremongering.

“The evidence is overwhelming: data from a wide range of studies, both observational and experimental, show that processed meat intake increases risk markers and risk of colorectal cancer. The criticism that correlation does not imply causation, and therefore the conclusions by IARC are flawed, overlooks that the classification is not merely based on observational but also experimental data. The evidence is less clear for red meat and therefore it is considered to be only probably carcinogenic to humans.

“So how does processed meat cause cancer? Most processed meat is cured with curing salt, and the nitrite in curing salt can cause the formation of nitrosamines in the intestinal tracts. Many nitrosamines are carcinogenic compounds as they can react with DNA and eventually cause tumours to form. The evidence available shows that this happens: nitrosamines are found in the intestinal tract. Moreover, nitrosamines induce a specific mutation pattern which is found in many colorectal tumours.

“What does this mean? As with most food-related stories, there will be probably be a short term impact on consumption. In the longer term, it is important to address these findings and mitigate the risks, for example by updating dietary advice or developing new meat products. The meat processing industry is already involved in a large EU funded project to develop new meat products.”

Prof. Sir David Spiegelhalter, Winton Professor of the Public Understanding of Risk, University of Cambridge, said:

“There may be good evidence for there being an increased risk, but the magnitude needs to be put into perspective.  In the normal run of things, around 6 in every 100 people would be expected to get bowel cancer in their lifetime.  If all these 100 people ate a three-rasher (around 50g) bacon sandwich every single day of their lives, then according to this report we would expect that 18% more would get bowel cancer – which is a rise from 6 cases to 7 cases. So that’s one extra case of bowel cancer in all those 100 lifetime bacon-eaters.”

Prof. Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London, said:

“The IARC classification seems to be falling in line with the WHO and World Cancer Research Fund on strength of evidence – that is, convincing and probable – as those on which to take action. However, these do not take into account effect size of the risk. Generally for colorectal cancer, where the data is more consistent than other sites of cancer, the relative risks are small – 1.1 to 1.3. This is in contrast to the large variation in colorectal cancer between different countries.

“The comparison on which evidence is based is for high versus low categories of consumption. Studies of vegetarians who eat no meat do not show consistently lower incidence rates of colorectal cancer, and in the case of the UK vegetarians, a higher rate was noted in the Oxford cohort compared to meat-eaters.

“The definition of red meat generally includes beef, lamb and pork. Processed meats include burgers, sausages, salami and ham. Some of these processed meats are treated with nitrites and may contain nitrosamines, which are known carcinogens – however, nitrate/nitrite curing of meat prevents Clostridium botulinum bacterial growth in these products.

“There is no evidence to show saturated fat is linked to cancer risk. The evidence regarding salt is not so much about sodium chloride as nitrates/nutrites.

“Red meat is a good source of heme iron, which is better absorbed than plant sources.  It is also a good source of zinc and vitamin B12. Iron and zinc deficiency are major nutrient deficiencies of concern worldwide but you do not need to eat large amounts of meat to get what you need – say 30g/d would be sufficient.

“Generally dietary advice has been to reduce the intake of red meat to 1-2 times a week and to avoid fatty meat products as much as possible. Ham is not fatty but is salty.

“The problem with this issue is that food is not like tobacco – we have to eat something.”

Prof. Tim Key, Cancer Research UK’s epidemiologist at the University of Oxford, said:

“Cancer Research UK supports IARC’s decision that there’s strong enough evidence to classify processed meat as a cause of cancer, and red meat as a probable cause of cancer.

“We’ve known for some time about the probable link between red and processed meat and bowel cancer, which is backed by substantial evidence.

“This decision doesn’t mean you need to stop eating any red and processed meat. But if you eat lots of it you may want to think about cutting down. You could try having fish for your dinner rather than sausages, or choosing to have a bean salad for lunch over a BLT.

“Eating a bacon bap every once in a while isn’t going to do much harm – having a healthy diet is all about moderation. Overall red and processed meat cause fewer cases of cancer in the UK than some other lifestyle factors. And by far the biggest risk to your health is smoking – causing over a quarter of cancer deaths in the UK and nearly one in five cancer cases.”

Dr Ian Johnson, nutrition researcher and Emeritus Fellow, Institute of Food Research, said:

“IARC has concluded that the evidence in favour of an association between processed meat consumption and cancer, probably because of the ingredients used in processing, meets their criteria for placing this class of foods on the list of recognised human carcinogens. It is important to emphasise however that this classification reflects the strength of the evidence for an effect, not the actual size of the risk. Meat consumption is probably one of many factors contributing to the high rates of bowel cancer seen in America, Western Europe and Australia, but the mechanism is poorly understood, and the effect is much smaller than, for example, that of cigarette smoking on the risk of lung cancer. It is also worth noting that there is little or no evidence that vegetarians in the UK have a lower risk of bowel cancer than meat-eaters.”

http://www.iarc.fr/en/media-centre/pr/2015/pdfs/pr240_E.pdf

‘Carcinogenicity of consumption of red and processed meat’ by Véronique Bouvard et al. was published in The Lancet Oncology on Monday 26 October 2015. 

Declared interests

Prof. Kevin McConway: “I have no relevant interests to declare.”

Dr Gunter Kuhnle: IARC used Gunter’s research and he is a Member of Council, Nutrition Society.

Prof. Sir David Spiegelhalter declares that he has no conflicts of interest.

Prof. Tom Sanders: “I was a member of the FAO/WHO Joint Expert Committee that recommended that trans fatty acids be removed from the human food chain. I have previously acted as a member of the Global Dairy Platform Scientific Advisory Panel and I am a member of the Programme Advisory Committee of the Malaysian Palm Oil Board. In the past I have acted as a consultant to Archer Daniel Midland Company and received honoraria for meetings sponsored by Unilever PLC. My research on fats was funded by Public Health England/Food Standards Agency.”

Dr Ian Johnson: “I have no conflicts of interest.”

None others received.