The International Agency for Research on Cancer (IACR)’s announcement that processed meat is a cause of cancer, and that red meat is a probable cause of cancer, needs to be taken in the context of what these classifications mean.
Crucially, these classifications don’t indicate the size of the risk, how many cases of cancer they cause, or what sort of dose is needed. Group 1, into which processed meat, includes tobacco smoking, alcoholic drinks, air pollution and sunlight. Group 2A includes things which IARC believe are probably carcinogenic, but for which there is insufficient evidence to be more definitive. DDT is an example of a group 2 “probable carcinogen”, but so are burning wood for fuel, high temperature frying, or working as a hairdresser. Again, the classification only relates to the quality of the available evidence, not the magnitude of the risk. The cancer risk from tobacco is much, much higher than the risk from eating processed meat, despite reports in some of the media covering this. Unless you are eating very large amounts on a regular basis, the increase in cancer risk is relatively very small.
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 – Prof. Ian Johnson, Emeritus Fellow, Institute of Food Research
The evidence on which IARC have based their decision comes from over 800 studies, which have tended to show that people who eat more meat have slightly higher incidences of cancer, especially colorectal cancer. However, these epidemiological studies cannot always rule out other confounding factors, as diet and lifestyle are very complicated. We also don’t know how meat and processed meat could increase cancer risk, there are a number of possible mechanisms for this but none have definitely provided a mechanism for how meat, processed meat or compounds contained in it can trigger cancer.
Meat consumption probably is 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 – Prof. Ian Johnson, Emeritus Fellow, Institute of Food Research
The evidence does show a dose response, so that as the consumption of meat goes up, so does the cancer risk. So people who are in the highest category of meat consumption (of which there are not very many) should think about reducing the amount of processed meat or red meat in their diet. In the UK, current guidelines are that an adult should eat no more than 70g of red or processed meat on average each day. The NHS choices website has more information on this recommendation as well as tips on how you can achieve this. Cutting back on meat consumption, or replacing meat with fruit and vegetables, is also likely to have other health benefits, such as cutting down on salt or increasing fibre.