Is bacon off the menu? – Reviewing WHO’s Verdict

The media’s huge appetite for publishing reports about newly discovered risk factors for cancer means associations between human diet and carcinogenesis are continuously reaching the headlines. A 2013 ‘systematic cookbook review’ investigated the prolific nature of nutritional epidemiology within the media, with a remarkable 80 per cent of the common ingredients selected at random from cookbooks being reported as a cancer risk. However the review authors claimed the majority of cases found in the primary literature were based on weak statistical evidence, which was escalated by the media to spark an element of debate and public interest. So to what extent should we believe the recent headlines claiming causal links between meat consumption and cancer? Should meat now be considered a guilty pleasure rather than a staple within our balanced diet?

The International Agency for Research on Cancer (IARC) is part of the World Health Organisation (WHO), and this week released a statement evaluating the carcinogenicity of red and processed meat. Meta-analysis from this respected international body concluded that every 50 gram portion of processed meat, if eaten daily, increases the risk of colorectal cancer by 18%. Although this risk could be considered small, Dr Kurt Straif of IARC warned that due to today’s widespread meat consumption, the global impact on cancer incidence is still of public health importance. It wasn’t long before the press began to report that ‘processed meat has the same cancer risk as smoking’. These claims stemmed from IARC’s classification of processed meat as a Group 1 carcinogen, meaning it is amongst tobacco and alcohol as a definitive cause of cancer, with red meat being a probable cause of cancer in Group 2A. However what is hiding behind the headlines is the true definition of IARC’s categorisation. These groups represent the strength of scientific evidence supporting IARC’s theories of certain factors causing cancer, rather than potency of the risk factors – meaning the evaluation is more of a ‘hazard identification’ than ‘risk assessment’. Although consumption of bacon or ham shares the same carcinogenic classification as tobacco; meat consumption is still far off from being as life-threatening as smoking. Cancer Research UK reassured the public with figures that 21 per cent of all cancers are caused by smoking, compared to a minuscule 3 per cent caused by meat consumption.

Although the media sparked the recent frenzy surrounding causal links between meat and cancer, the association has in fact been studied for decades. Some researchers suggest that high meat consumption is often associated with insufficient intake of fruit or vegetables, with alcohol and tobacco abuse also found more often in lower income groups. This points to an underlying problem in growing income and health inequality, with inadequate levels of education surrounding a healthy lifestyle. However a huge study in 2008 adjusted for numerous confounding variables (factors which could interfere with validity of results), from body mass index to educational levels, whilst assessing dietary intake in a cohort of over half a million people. Even after adjusting for lifestyle factors, the results were statistically significant in suggesting that higher intake of red and processed meat leads to elevated risk for cancer and cardiovascular mortality. However experts are still struggling to pin down the exact reasons behind meat’s carcinogenic effects. Theories include a potential role for haem (a component of the blood’s oxygen-carrying pigment haemoglobin) which could be broken down during meat digestion to form N-nitroso compounds in our gut. This family of chemicals damages the lining of the bowel, known as the epithelium, leading to increased proliferation of cells to compensate for injury – which could result in occurrence of cancer-causing DNA mutations. The gut microbiota could also contribute to the pathogenesis (mechanism leading to the diseased state), by breaking down protective mucus layers to allow haem to penetrate, causing similar damage to cells.

Overall substantial evidence does exist to support claims that a highly meaty diet could contribute to tumour development later in life – yet the solution to this is a simple one: consumption in moderation. Meat is a rich source of essential dietary components including protein, iron and zinc, whilst the lack of strong evidence linking fresh white meats (chicken, turkey, fish) to cancer provides further reasons to keep meat on the menu.