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Colorectal cancer.

Article Excerpt
KEY POINTS

* 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.

INTRODUCTION

An early ecological study (1) found strong associations between the consumption of animal protein and the incidence of several cancers, and ignited a huge research effort to confirm and refine these findings. Given the apparent strength of these correlations, it is perhaps surprising that some 30 odd years later, the picture is still both confused and confusing. The evidence for an association of high meat consumption with an elevated risk of cancer, especially colorectal cancer, is neither strong nor consistent. While most observers would agree that the association between high consumption of white meat (fish and poultry) and colorectal cancer is either null or inverse (in causal terms, 'protective'), they would be divided on what these studies say with respect to a high consumption of red meat (beef, lamb and pork).

The reasons for this confusion are manifold. The question cannot be readily addressed using randomised controlled trials, and the observational studies published so far have varied markedly with respect to:

* study design (case-control vs cohort studies)

* the definitions of the various types of meat (despite compelling reasons to consider processed meats separately from fresh meat, several studies have failed to make that distinction)

* the instruments used to assess red meat intake

* the measurement and adjustment for potential confounding variables

* the outcomes measured (mortality vs incidence, colon vs rectum and other subsites)

* the range of meat consumption in the study population

* the number of intake groupings chosen

* the cultural and geographical context

But despite this obvious lack of comparability, three groups have applied meta-analytic techniques to those studies which have used the more robust cohort study design. (2-4) This paper summarises the literature on relevant studies to address the question: Does eating red meat increase the risk of colorectal cancer?

POPULATION-BASED RESEARCH

Figure 1 summarises a meta-analysis of cohort studies undertaken by Sandhu et al., (2) who reported their findings as odds ratios of contracting colorectal cancer per 100 g of meat consumed. The only two studies that individually reported significant associations with increasing red meat intake were the Health Professionals' Study (5)...