The World Health Organization (WHO) recently published a monograph summarizing the results of a systematic review of the evidence for the association of red meat and processed meat with cancer. Their results were red meat to journalists, or at least their headline writers, who love sensational headlines like, “Red meat causes cancer.” Whenever you can say something common causes cancer, that’s a good day for journalists.
Let’s put their report into perspective. For red meat they found that there was only limited evidence for any risk for colorectal cancer, but there was strong mechanistic evidence, meaning that a causal connection was plausible. Red meat is essentially any meat from a mammal (beef, pork, lamb).
For processed meat there was sufficient evidence for an association, and they didn’t comment on the evidence for a mechanism. Processed meat includes any meat that is smoked, cured, or salted (bacon, sausage, jerky, ham).
For processed meat they concluded that eating 50 grams per day (a little less than two slices of bacon) increases risk of colorectal cancer by 18%. That is a relative risk increase, the absolute risk increases by 1% (from 6 to 7% lifetime risk).
Red meat also has nutritional value, and so risks always have to be balanced against benefits to determine net health effect.
Part of the limitation of the studies reviewed is that they are observational, not experimental. Therefore there is the potential for confounding factors. A 2009 study, for example, found that:
“Subjects who consumed more red meat tended to be married, more likely to be of non-Hispanic white ethnicity, more likely to be a current smoker, have a higher body mass index, and a higher daily intake of energy, total fat and saturated fat; whereas they tended to have a lower education level, were less physically active and consumed less fruits, vegetables, fiber and vitamin supplements.”
Red meat consumption, therefore, is associated with a host of other factors known to be risk factors for cancer, heart disease, and diabetes. It is therefore difficult to single out the red meat as a causative factor.