How certain can we be about the association of meat consumption and cancer?

Abstract Text:

The recent evaluation from the World Health Organization’s International Agency for Research on Cancer (IARC) Working Group 114 concluded that processed meat consumption is a definite carcinogen in humans and red meat is probably carcinogenic. These were expert opinions of the majority of members of the group who evaluated the evidence they deemed most convincing. Most of the data that drove these two decisions were epidemiological studies, which are limited in their ability to distinguish causation from association. Animal studies do not support the conclusions and mechanistic studies are not convincing. Importantly, there are two large intervention studies in humans published that reduced meat intake significantly, as part of an overall dietary pattern, but had no effect at all on development of either colon polyps or cancer which were not considered. Schemes for evaluating the totality of evidence to establish causality have been in the literature for 50 years but were not used by IARC. In addition, no systematic review or meta-analysis was done, both of which have become the standard for such comparisons. Weaknesses inherent in evaluating long-term intake of any foods in observational studies should preclude using such data as the sole or primary determinant of causality in relation to health. It has been demonstrated conclusively that the food frequency questionnaire used to estimate meat intake in those studies is not able to quantify the amount of protein consumed. Statistical adjustment for confounders and covariates differed from study to study making combined analysis problematic. Finally, lack of registration of most observational studies without clear designation of the hypotheses to be studied and the numerous associations reported, force the conclusion that any findings from them need to be accorded the higher statistical threshold that should be required of secondary results. A convincing argument can be easily defended that we do not have enough valid data to classify the carcinogenicity of red or processed meat for humans. 

Keywords: red meat, processed meat, cancer, health, human nutrition