Wednesday, April 01, 2009
Photo by procsilas.
Last week I wrote about a study in the Archives of Internal Medicine which strongly suggests that a high consumption of red meat and processed meat is associated with increased risk of mortality from multiple causes in both men and women. Later in the same week, a lively discussion developed on the Weighty Matters blog after Dr Yoni Freedhoff posted on the same topic. Apparently another paper had been published ahead of print on the American Journal of Clinical Nutrition website which one reader felt showed the "opposite" of the Archives paper. Out of curiosity I saught out the paper, and wanted to share my thoughts on it today.
In their new paper, Dominik Alexander and colleagues at Exponent Health Sciences (a private consulting firm) performed a meta-analysis which pooled the results of six prospective studies which have examined the association of animal fat and animal protein with colorectal cancer. The interesting finding, which seemingly opposed the findings of the recent Archives paper, was that neither animal fat nor animal protein intake was associated with increased risk of colorectal cancer in any of these studies individually or when their data were combined. Subgroup analyses examining the influence of gender, tumor site, and study design similarly showed no link between animal fat or protein intake and colorectal cancer risk.
At first glance, these findings may seem a bit surprising. However, there is a very critical difference between this paper and the one published in the Archives of Internal Medicine last week (no, it's not the fact that this paper was supported by the National Cattleman's Beef Association or the or the National Pork board, although that is certianly true). The Archives paper examined the association between the consumption of types of meat (red, white, and processed) with mortality from various causes. In contrast, the current study by Alexander and colleagues examined the association between all types of animal fat and protein with cancer risk. In the Alexander study, animal fat and protein from all sources - red meat, white meat, fish and dairy - were all lumped together. You may remember that the Archives paper found that white meat (including fish) consumption was actually associated with less risk of mortality, so it's not that surprising that when we combine both white and red meat in the same analysis that we might not see a tremendous association with cancer risk.
Further, the Alexander paper lumps all fats - saturated, unsaturated, polyunsaturated, etc - together as well, even though evidence suggests that these different subgroups may have opposing influences on health. And finally, by reducing meat to fat and protein, this study assumes that they are the only links between animal products and cancer risk. Other possible carcinogens such as nitrites were completely ignored in this analysis. To their credit, I should point out that Alexander and colleagues discuss many of these same caveats in the discussion section of their paper (although they may be convenient for some to overlook).
Now all this is not to say that the new study by Alexander and colleagues is not interesting, or that it is not informative. It is good to know that animal fat and protein per se is not associated with increased risk of colorectal cancer. Scientists can now focus their energies on the other potential mechanisms linking meat consumption with mortality from multiple causes (not just colorectal cancer). But to say that this study disproves the association between certain types of meat and mortality risk is completely wrong, and I am glad to see that the authors of the study did not interpret their data in this way. I hope that others will take a similarly nuanced view of their findings.
The article described above can be found by here.
Alexander, D., Cushing, C., Lowe, K., Sceurman, B., & Roberts, M. (2009). Meta-analysis of animal fat or animal protein intake and colorectal cancer American Journal of Clinical Nutrition DOI: 10.3945/ajcn.2008.26838
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