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Another rigorous analysis of the science on processed meat shows evidence is weak and uncertain

A new re-analysis of the science concerning links between processed meat and chronic disease indicates that studies showing a relationship between the two are very low quality and suffer from, as the authors put it, "serious risk of bias and imprecision."

This conclusion is unsurprising, as it follows a recent set of analyses that rocked the nutrition world. That earlier set of studies, published in Annals of Internal Medicine earlier this month, concluded that guidelines warning us to consume less red and processed meat are based on evidence with very low certainty. The researchers who performed those analyses asserted there is no way to determine, for any given individual, what the risks or benefits of eating meat might be.

In that regard, this new study, published October 17 in PLOS ONE, is similar not only in its contradiction of decades-old dietary guidance, but in its indictment of the science behind nutritional guidance in general.

Unlike the studies from Annals of Internal Medicine, the new study was not a systematic review or meta-analysis of available studies, but a review of those kinds of reviews. The eight authors, all from the University of Copenhagen, used two methods to assess the quality of all the studies considered.

The first method, called AMSTAR, is a widely-used tool designed specifically for determining the quality of a systematic review or meta-analysis. Using this tool, the researchers in this new study found that previous systematic reviews or meta-analyses relating processed meat to chronic disease were of low quality. They also found that the better the study design, the less likely it was that any association was found.

One of the most important AMSTAR criteria asks whether researchers provided a public version of their research plan before beginning data collection. Such a plan limits opportunities for researchers to "massage" protocols or models in ways that can change what outcomes are found. Only one of the 22 reviews in this study did that. For the other 21 reviews, the researchers could not tell whether the original authors followed where the data led or made sure the data went where they wanted it to go. That so few of these studies used any sort of predetermined research plan points to a critical flaw in how nutrition research is currently done.

The other method used by the researchers in the processed meat study is the GRADE system. The highly regarded, transparent framework of GRADE is the most widely adopted tool for assessing the quality of evidence used for making recommendations. It was also the method used by the studies in Annals of Internal Medicine that caused such an uproar. The GRADE system is notable for the fact that results from observational studies (case-control and cohort studies) are by default considered to be of low quality, a perspective that Diet Doctor agrees with. The quality of observational studies may be upgraded if the studies are of the highest quality โ€• no confounding issues, large effect sizes, and a consistent dose-response relationship โ€• but this is rare in nutrition research.

In the processed meat analysis, the researchers evaluating reviews on this topic indicated that none of the studies involved met these standards. In fact, as the researchers explained, "The certainty of the effect estimate was downgraded to very low, due to serious risk of bias and/or serious imprecision."

As with the studies in Annals of Internal Medicine, this study does not say that processed meat is "good for you" or that you should eat more of it. It simply says that guidance to reduce your consumption of processed meat is based on studies of very low quality and such conclusions are highly uncertain.

At Diet Doctor, you can count on us to follow the science where it leads โ€” such as our recent re-evaluation of soy โ€” even if some of our readers find the conclusions unsettling. After all, advice not based on strong scientific evidence is unlikely to produce positive health outcomes, as we have found after 40 years of lower-fat, higher-carb dietary guidance. At Diet Doctor, we think the public deserves better.