It’s interesting that the results from the meta-analyses actually confirm existing findings. So why did the panel issue a recommendation that adults continue their habits, effectively contradicting their own findings and existing guidelines on red meat consumption?

It seems the panel’s recommendation was based on four factors: (1) The observed effects are very small; (2) The quality of evidence is very low; (3) Meat eaters enjoy meat and won’t change their behaviors; and (4), Environmental impact was considered “outside of the scope” of their guideline.

All four of these considerations are problematic. First, the effect estimates may seem small because the unit of exposure (3 servings/week) is small. However, the potential health benefits of reducing consumption would be much larger for individuals consuming 1 serving/day of red meat or more (among approximately 1/3 of US adults).* Based on their meta-analyses of large cohorts, dietary patterns with a moderate reduction in red and processed meat consumption were associated with lower total mortality by 13% (95% Confidence interval 8% to 18%), CVD mortality by 14% (6% to 21%), cancer mortality by 11% (4% to 17%), and type 2 diabetes risk by 24% (14% to 32%). These risk reductions are substantial at both individual and population levels. We currently spend tens of billion dollars per year on screening and treating risk factors for cardiovascular disease and diabetes that have benefits of this magnitude.

The second rationale used by the panel is that although red meat and processed meat consumption is associated with adverse health outcomes, the quality of evidence is too low. The authors applied a GRADE (Grading of Recommendations Assessment, Development and Evaluation) criteria, which resulted in all observational studies receiving “low- or very-low” scores for “certainty of evidence,”[2-4] due to potential for confounding. This should come as no surprise, since GRADE criteria were mainly developed for evaluating evidence from drug trials. Unlike drugs, dietary, lifestyle, and environmental factors are typically not amenable to large, long-term randomized clinical trials. For this reason, modified criteria have been developed. For example, HEALM (Hierarchies of evidence applied to lifestyle Medicine), or the criteria developed by the U.S. Department of Agriculture or the Word Cancer Research Fund would have been more appropriate.

Previous meta-analyses have rated the strength of evidence from large cohort studies as “moderate” if the studies meet several criteria: consistent finding across multiple cohorts, large number of participants and long-duration of follow-up, low dropout rates, and a dose-response relationship. This is clearly the case for most of the effect estimates. Many reviews also upgrade the rating of evidence if data from randomized trials show effects on risk factors for the diseases being studied. An example of this is when evidence indicates that red meat increases blood levels of LDL cholesterol when compared to plant sources of protein. [9]

Relatedly, Dr. John Sievenpiper, professor in the Department of Nutritional Sciences at the University of Toronto and co-author on one of the meta-analyses, strongly disagreed with the panel’s conclusions and recommendations:

Unfortunately, the leadership of the paper chose to play up the low certainty of evidence by GRADE as opposed to the protective associations that directly support current recommendations to lower meat intake…Very few nutritional exposures are able to show associated benefits on the big three of all-cause, cardiovascular, and cancer mortality as well as type 2 diabetes. The signals would be even stronger if one considered substitution analyses with plant protein sources or investigated dose-response gradients which are used to upgrade data by GRADE, both of which I had requested. Unfortunately, I never saw the galley proofs to ensure that these changes had been made.

These statements raise serious concerns about the methodology of the study, and suggest that the “very low” evidence grade given to high-quality cohort studies is inappropriate. If the same procedure were used to evaluate the evidence for other dietary (such as low consumption of fruits and vegetables, high consumption of sugary beverages), lifestyle (such as physical inactivity and inadequate sleep), and environmental (such as passive smoking and air pollution) factors, none of the current recommendations on these factors would be supported by high- or even moderate-quality evidence. Basically, the foregone conclusion would echo that of this new report: that people should ‘eat whatever they want and do whatever they want;’ no need to bother with the systematic reviews and meta-analyses.

[*Updated October 2, 2019. Due to an editing error, the previous version incorrectly stated: “However, the health benefits or risk would be much larger for individuals consuming less than 1 serving/day of red meat (among approximately 1/3 of US adults).”]