A controversial “dietary guidelines recommendation” published in Annals of Internal Medicine suggests that adults can continue to consume red meat and processed meat at current levels of intake. 
This recommendation runs contradictory to the large body of evidence indicating higher consumption of red meat—especially processed red meat—is associated with higher risk of type 2 diabetes, cardiovascular disease, certain types of cancers, and premature death. However, according to the Annals authors, their guidelines were based on a series of “rigorous” systematic reviews (published simultaneously [2-6]) that would presumably account for all this available evidence.
Confused? We asked our experts to take a closer look at the research behind these guidelines. You can find the in-depth analysis below, but here are their key takeaways:
- The new guidelines are not justified as they contradict the evidence generated from their own meta-analyses. Among the five published systematic reviews, three meta-analyses basically confirmed previous findings on red meat and negative health effects.
- The publication of these studies and the meat guidelines in a major medical journal is unfortunate because following the new guidelines may potentially harm individuals’ health, public health, and planetary health. It may also harm the credibility of nutrition science and erode public trust in scientific research. In addition, it may lead to further misuse of systematic reviews and meta-analyses, which could ultimately result in further confusion among the general public and health professionals.
- This is a prime example where one must look beyond the headlines and abstract conclusions. It is important for journalists, health professionals, and researchers to look beyond the sensational headlines and even the abstracts of the papers to verify the evidence behind the claims. It’s also crucial to understand that nutrition research is a long and evolving process, and therefore critical to look at the totality of the evidence.
- These studies should not change current recommendations on healthy and balanced eating patterns for the prevention of chronic diseases. Existing recommendations are based on solid evidence from randomized controlled studies with cardiovascular risk factors as the outcomes, as well as long-term epidemiologic studies with cardiovascular disease, cancer, type 2 diabetes, and mortality as outcomes. To improve both human health and environmental sustainability, it is important to adopt dietary patterns that are high in healthy plant-based foods and relatively low in red and processed meats.
Q&A: Reviewing the scientific process behind the guidelines
Five reviews were published alongside the panel’s guidelines that informed their recommendations, including three meta-analyses. Can you walk us through the findings and your evaluation of the data?
The three meta-analyses of observational cohort studies looked at the impact of red and processed meat consumption on cardiometabolic and cancer outcomes. [2-4] This included over 6 million participants from over 100 large cohort studies. In the table below, we summarized the main results from the meta-analyses.
Across the board, these studies found a statistically significant association of lower consumption of red meat and processed meats and lower total, CVD, and cancer mortality and incidence of type 2 diabetes. Interestingly, the analyses on dietary meat patterns yielded a stronger protective effect than the analyses of red meat as an individual food because such studies on dietary patterns might have taken into consideration food substitutions (e.g., eating fish or legumes instead of red meat).
However, these findings are not new, as previous meta-analyses have found similar results. For example, in the Dietary Pattern Methods Project, healthy diets (lower in red and processed meats) were associated with 11-28% reduced risk of death from all causes, CVD, and cancer compared with the lowest quintile, independent of known confounders.  For colorectal cancer, the new report found a 7% reduction (relative risk of 0.93 [95% CI 0.89-0.95]) with eating 3 servings/week or less of processed meats, similar to the estimate noted by the International Agency for Research on Cancer of the World Health Organization. 
What’s your take on the other two reviews, one of which considered people's preferences on meat consumption?
The fourth systematic review was intended to summarize results from long-term red meat reduction trials on health outcomes.  However, no such trial has ever been conducted for practical and ethical reasons. Thus, this paper identified two large studies that were randomized and examined long-term health outcomes, but neither were relevant because they were not actually meat reduction trials. The authors discarded one of these studies (Lyon Heart Study) and based their conclusions almost entirely on the other study, the Women’s Health Initiative, but this was a trial of a low fat dietary pattern, not red meat reduction.
The fifth paper is a narrative review of “health-related values and preferences regarding meat consumption.”  Although taste preference is an important consideration when providing personalized dietary advice, it is questionable whether it should be considered as a major factor in developing dietary guidelines. Taste preferences can change, and it is important to keep objective evidence on health effects separate from personal preferences.
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. 
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).”]
Why would a prestigious medical journal publish dietary guideline recommendations by a self-appointed panel?
The publication of a cluster of five systematic reviews in the same issue of the journal gives the impression of a major scientific breakthrough, but this is clearly not the case. It is puzzling that the journal would publish dietary guidelines developed by a self-appointed panel that are tantamount to promoting meat consumption, despite their own findings that high consumption is harmful to health. Of note, these recommendations are not based on consensus of the panel because three panel members actually voted against their own recommendations. Furthermore, among the 14 panel members, only two were listed as “nutritional scientists” while most others were listed as “methodologists.”
It should also be noted that the journal may have exacerbated the situation by circulating a press release entitled “New guidelines: No need to reduce red or processed meat consumption for good health.” Such sensational headlines can cause enormous confusion among health professionals, journalists, and the general public.
In summary, what are the major problems with these new guidelines concerning red and processed meat?
- These guidelines are inconsistent with the principle of “first do no harm.” In clinical practice, it would be irresponsible if a patient who reports eating two servings of red/processed meat daily is told by their doctor not to worry and continue the habit.
- These guidelines are inconsistent with the precautionary principle in public health. From a public health point of view, it is irresponsible and unethical to issue dietary guidelines that are tantamount to promoting meat consumption, even if there is still some uncertainty about the strength of the evidence.
- The panel declared “considerations of environmental impact” out of the scope of their recommendations. This is a missed opportunity because climate change and environmental degradation have serious effects on human health, and thus is important to consider when making recommendations on diet, even if this is addressed separately from direct effects on individual health.
- Johnston BC, Zeraatkar D, Han MA, et al. Unprocessed red meat and processed meat consumption: dietary guideline recommendations. Ann Intern Med. 1 October 2019 [Epub ahead of print]. doi:10 .7326/M19-1621
- Vernooij RWM, Zeraatkar D, Han MA, et al. Patterns of red and processed meat consumption and risk for cardiometabolic and cancer outcomes. A systematic review and meta-analysis of cohort studies. Ann Intern Med. 1 October 2019 [Epub ahead of print]. doi:10 .7326/M19-1583
- Han MA, Zeraatkar D, Guyatt G, et al. Reduction of red and processed meat intake and cancer mortality and incidence. A systematic review and meta-analysis of cohort studies. Ann Intern Med. 1 Octo- ber 2019 [Epub ahead of print]. doi:10.7326/M19-0699
- Zeraatkar D, Han HA, Guyatt GH, et al. Red and processed meat consumption and risk for all-cause mortality and cardiometabolic outcomes. A systematic review and meta-analysis of cohort studies. Ann Intern Med. 1 October 2019 [Epub ahead of print]. doi:10.7326 /M19-0655
- Zeraatkar D, Johnston BC, Bartoszko J, et al. Effect of lower versus higher red meat intake on cardiometabolic and cancer outcomes. A systematic review of randomized trials. Ann Intern Med. 1 October 2019 [Epub ahead of print]. doi:10.7326/M19-0622
- Valli C, Rabassa M, Johnston BC, et al. Health-related values and preferences regarding meat consumption. A mixed-methods sys- tematic review. Ann Intern Med. 1 October 2019 [Epub ahead of print]. doi:10.7326/M19-1326
- Liese AD, Krebs-Smith SM, Subar AF, George SM, Harmon BE, Neuhouser ML, Boushey CJ, Schap TE, Reedy J. The Dietary Patterns Methods Project: synthesis of findings across cohorts and relevance to dietary guidance. The Journal of nutrition. 2015 Jan 21;145(3):393-402.
- Bouvard V, Loomis D, Guyton KZ, Grosse Y, El Ghissassi F, Benbrahim-Tallaa L, Guha N, Mattock H, Straif K. Carcinogenicity of consumption of red and processed meat. Lancet Oncology. 2015 Dec 1;16(16):1599.
- Guasch-Ferré M, Satija A, Blondin SA, Janiszewski M, Emlen E, O’Connor LE, Campbell WW, Hu FB, Willett WC, Stampfer MJ. Meta-analysis of randomized controlled trials of red meat consumption in comparison with various comparison diets on cardiovascular risk factors. Circulation. 2019 Apr 9;139(15):1828-45.