Cancer: processed meat and colorectal cancer

Evidence quality

The evidence supporting a causal relationship between processed-meat consumption and colorectal cancer is high at the level of expert hazard evaluation (IARC, 2015). The International Agency for Research on Cancer classified processed meat as carcinogenic to humans (Group 1) on the basis of sufficient evidence in humans and classified red meat as probably carcinogenic (Group 2A) on the basis of limited evidence in humans and strong mechanistic evidence (IARC, 2015). The evaluation addressed hazard—whether an exposure is capable of causing cancer—not the absolute risk faced by a particular person at a particular intake (IARC, 2015).

Epidemiologic findings and dose

A meta-analysis of prospective studies reported a relative risk of 1.18 (95% CI 1.10-1.28) for colorectal cancer per 50 g/day higher intake of processed meat (Chan et al., 2011). For fresh red meat, the estimate was 1.17 (95% CI 1.05-1.31) per 100 g/day higher intake (Chan et al., 2011). These are relative-risk estimates: the corresponding absolute increase depends on baseline colorectal-cancer risk, age, follow-up, and other characteristics of the population (Chan et al., 2011).

The dose-response result supports the existing stub's claim that regular daily amounts raise risk relative to lower intake (Chan et al., 2011). It does not identify a threshold below which risk is zero, and it should not be converted into an individual prediction without baseline-risk information (Chan et al., 2011). IARC's public explanation similarly summarized an analysis of 10 studies as indicating about an 18% increase in colorectal-cancer risk for each 50 g portion of processed meat consumed daily (IARC, 2015).

Mechanistic evidence

Processing and high-temperature cooking can generate or introduce several classes of compounds relevant to carcinogenesis (Demeyer et al., 2015). A mechanistic review identified heterocyclic aromatic amines and polycyclic aromatic hydrocarbons, along with heme-related promotion of N-nitroso compound formation and lipid peroxidation, as the most studied hypotheses (Demeyer et al., 2015). The same review concluded that none of these hypotheses completely explains the epidemiologic association, so mechanistic plausibility should not be presented as a fully resolved pathway (Demeyer et al., 2015).

Meaning of the IARC groups

Group 1 means that the working group judged the evidence sufficient to conclude that the exposure causes cancer in humans (IARC, 2015). It does not rank the size of the effect, and IARC explicitly contrasted the estimated cancer burden from diets high in processed meat with the much larger burden attributed to tobacco (IARC, 2015). Group 2A for red meat reflects a lower level of certainty than Group 1, not a quantitative estimate of how much red meat increases risk (IARC, 2015).

The strongest human evidence evaluated for processed meat concerned colorectal cancer, although positive associations were also considered for stomach cancer; for red meat, the strongest but still limited evidence concerned colorectal cancer (IARC, 2015). Accordingly, the classifications should not be generalized into a claim that every meat product causes every type of cancer (IARC, 2015).