Meat and Colorectal Cancer

The media has concluded that there is a link between meat and colorectal cancer (CRC).  The basis for this conclusion is various statistical studies relating colorectal cancer rates to consumption of red and/or processed meats.  None of the studies are randomized controlled studies, which are the type that are most reliable.

No doubt, there will be much hype about the need for reducing meat consumption.  The hype is likely to ignore that even a large increase in relative risk does not imply a large increase in risk if the original risk is small enough.  For example, if the lifetime risk of dying from disease X is increased from 1% to 2%, a relative risk of 2, by giving up an enjoyable activity, does that make it sensible to give up the activity?  Not necessarily (drinking, driving, sports, etc.).

Here is a link to one such article that summarizes the results of various studies.  Here is an excerpt from the article.

 A cohort of 30,000 men and women in Japan was studied by Oba et al. (2006), with 231 CRC cases.

To sum up these recent prospective studies, they bring some support to the conclusions of Larsson’s metaanalysis that processed meat intake is associated with increased risk, and the RR is in the range 1.5–2. However, the link was not found in all sub-groups (male/female, colon/rectum), and the risk associated with dietary patterns cannot be attributed to processed meat alone.

The fact that processed meat intake increases colorectal cancer risk seems established from the published meta-analyses of epidemiologic studies. The evidence is weak, however, since the RRs were all less than 2, and observational studies never fully avoid biases and confounders. The excess risk in the highest category of processed meat-eaters is comprised between 20 and 50% compared with non-eaters, which is modest compared with established risk factors like cigarette smoking for lung cancer (RR=20). However, the excess risk per gram of intake is clearly higher than that of fresh red meat.

Assume that the studies' relative risks are correct.  Then eating red meat or processed meat increases the risk of getting colorectal cancer by about 20% and 50%, respectively.  This sounds really bad, but should be put in perspective.  For example, the Japanese study quoted in the article notes 231 CRC cases in 30,000 people over the period from 1992 to 2000.  That suggests a probability of about 231/30,000=0.77% of developing CRC over eight years, or a probability of about 7.4% of developing CRC over 80 years. Relative risks of 20% and 50% increase these probabilities to about 8.9% and 11.0%, respectively.  Thus, eating red meat or processed meat increases the probability of lifetime CRC by about 1.4  and 2.6 percentage points, respectively.  Considering the likely improvement in the production processes for both meats and the likely improvement in CRC detection and treatment, risk increases of this magnitude may be acceptable vs. giving up meat.

Finally, what if meat reduces the death rate from some causes?  Shouldn't the decision to eat meat or not be based on the relationship between meat and the death rate from all causes, not just colorectal cancer?