Colorectal cancer

There are now sufficient studies of the relationship between red meat consumption and cancer of the colon and rectum, using the preferred cohort design, to make meta-analysis possible. Few of these cohort studies have reported individually significant associations of high red meat consumption with increased colorectal cancer, but summary estimates of risks obtained from three meta-analytic studies have all found a modest, but significant elevation in risk among the highest consumers of red, or red + processed, meat. Two of three analyses of data pooled across cohort studies have found no association between red meat consumption and colorectal cancer; a third reported a relative risk of 1.22 per 100 g of red and processed meat per day. While the combined relative risk estimates are modest (ranging from 1.0 to 1.3 for highest vs lowest consumption categories, or per 100 g of meat/day), the very high proportion of omnivores in the population means that even a modest association, if proven to be causal, could have a significant impact on public health. In the absence of randomised controlled trials, it is important to establish credible mechanisms by which red meat might cause colorectal cancer. Polyaromatic hydrocarbons (PAHs) and heterocyclic amines (HCAs) are carcinogens in the food supply, but as food items not implicated in the aetiology of colorectal cancer may actually be greater contributors to the total dietary intake of these compounds than red meat, they do not provide a plausible basis for implicating red meat in carcinogenesis. N-nitroso compounds (NOCs) found in some processed meat products and produced endogenously in the gut, are another class of carcinogens. Although the epidemiological evidence implicating NOCs in colorectal cancer is weak, the propensity of haem iron in meat to facilitate the endogenous production of NOCs is currently under investigation. Despite the lack of consistent evidence that red meat is carcinogenic, and despite some obvious epidemiological inconsistencies associated with possible mechanisms currently under scrutiny, it may be prudent to minimise the use of cooking and preparation techniques that are responsible for introducing all three classes of carcinogens into meat dishes.