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Red and Processed Meat – It’s All About Interpretation

Posted: 2019-10-02
The International Agency for Research on Cancer (IARC) reported in 2015 that high processed meat or red meat diets are associated with higher relative risks of cancer, with stronger negative evidence for processed meats than red meat (1). 

 

Johnston et al recently published a series of five systematic reviews in Annals of Internal Medicine that recommended that adults continue to consume red meat and processed meat at current levels of intake (2). These researchers considered three servings per week as a realistic reduction in meat consumption. Using meta-analyses to summarize the findings, they interpreted the data in two ways:  they quantified how many per 1000 people would likely benefit from reducing red and processed meat consumption and then used Grading of Recommendations Assessment, Development and Evaluation (GRADE) tools (3,4) to translate these findings into recommendations. 

 

Who’s Right?

Both the IARC and Johnston et al recognized limitations of the strengths of the evidence about red and processed meats (1,2).  It was when Johnston et al interpreted their findings and made their recommendations that they differed from IARC’s conclusions. Johnston et al (1) used GRADE tools (4) to translate the findings into absolute risk differences and then make recommendations. GRADE instructs researchers when making recommendations to transparently consider the evidence concerning effects on health, as well as to consider values and preferences, costs, acceptability and feasibility of a recommendation. The GRADE methods have been endorsed by over 100 organizations including the World Health Organization and the Cochrane Collaboration (5). 

 

Both groups looked at the body of evidence, which is observational study data. Regarding cancer, the IARC reported relative risks and statistical significance.

“Positive associations of colorectal cancer with consumption of processed meat were reported in 12 of the 18 cohort studies that provided relevant data, including studies in Europe, Japan, and the USA. Supporting evidence came from six of nine informative case-control studies. A meta-analysis of colorectal cancer in ten cohort studies reported a statistically significant dose–response relationship, with a 17% increased risk (95% CI 1•05–1•31) per 100 g per day of red meat and an 18% increase (95% CI 1•10–1•28) per 50 g per day of processed meat” (1). 

 

Johnston et al translated their summary of the evidence into absolute risks of having a cardiometabolic or cancer outcome event:

“Although statistically significant, low- to very low-certainty evidence indicates that adherence to dietary patterns lower in red or processed meat is associated with a very small absolute risk reduction in 9 major cardiometabolic and cancer outcomes (range, 1 fewer to 18 fewer events per 1000 persons), with no statistically significant differences for 21 additional outcomes observed” (2).

 

The Johnston et al (2) group were divided when they voted about their recommendations to “continue current levels of red meat and processed meat consumption” (11 voted for the recommendation, three voted against it). They considered the following data for their recommendation:

  • the low to very low certainty of evidence for the potential adverse health outcomes 
  • the very small absolute risk reduction based on three fewer servings of red or processed meat per week
  • the small risk reductions combined with “peoples' attachment to their meat-based diet”, “is not likely to provide sufficient motivation to reduce consumption of red meat or processed meat” 
  • the large variability in peoples' values and preferences related to meat, and
  • the panel focused exclusively on health outcomes associated with meat and did not consider animal welfare and environmental issues.  

There is strong criticism of Johnson et al's paper. As an example, a press release from the Harvard T.H. Chan School of Public Health described the Johnston et al publication as “irresponsible and unethical to issue dietary guidelines that are tantamount to promoting meat consumption” (6).

 

Interpreting the Evidence

GRADE recommends looking beyond statistical significance to examine several quality of research indicators and the actual numerical strength of the effects. (See PEN® eNews article - Why Using GRADE (to grade the evidence in PEN®) is Important to Practicing Dietitians). The GRADE tools are intended to be used to evaluate the body of evidence about any health interventions for its strength, consistency, directness and precision for improving health and the individual studies for the risks of bias. (Refer to PEN® eNews article, How PEN is ‘GRADE’ing the Evidence for You for reasons why PEN is using GRADE methods). Essentially, both those who recommend continued meat consumption (2) and those who recommend decreased meat consumption (1,5) have made subjective assessments to judge the evidence and make their recommendations. Being transparent based on the GRADE methods, the new analysis (2) may be more substantiated, but as before, the evidence is not strong and decisions about eating or not eating meat comes down to individual preferences.

Many nutrition science recommendations are based on observational studies of what people eat and their subsequent health outcomes. A weakness of this approach is that people who eat healthy diets usually have higher incomes, do not smoke and practice other healthy lifestyles including more physical activity and a moderate alcohol intake. It is challenging, if not impossible, to disentangle these effects to be able to say that a healthy diet is the reason for better health outcomes. 


Bottom Line
There is no strong evidence to definitively support either the recommendations for continued red and processed meat consumption (2) or decreased meat consumption (1). The new analysis by Johnston et al (2) is based on the latest and endorsed methods (5) to summarize essentially the same evidence and to make recommendations. By using recommended methods, the new report (2) may be more substantiated. However, individual preferences and choices about eating meat are key to how we as dietitians work with clients. 

The weaker the evidence, the more dietitians’ knowledge and professional training matter to interpret and to provide nutrition guidance. We need to continue to respect people’s values and preferences as well as to encourage a variety of foods in moderation and to make wise and informed choices to meet nutrition needs. Those who avoid or decrease their meat consumption for health, environmental and/or animal welfare reasons may need assistance obtaining sufficient protein, iron, zinc and vitamin B12. Those who avoid dairy products, may need assistance obtaining sufficient protein and calcium, especially children (7). People who eat generous quantities of meat may need assistance to ensure their diet contains sufficient vegetables, fruit, calcium and fibre.


Most diets would likely be improved with the inclusion of some plant-based proteins including some beans, lentils and nuts for their fibre, low glycemic index and nutrient contributions. Recommended nutrient intakes for protein (8) and current country dietary guidelines should continue to be followed. See Additional Content: International Healthy Eating Guideline Collection.

 

References

  1. Bouvard V, Loomis D, Guyton KZ, Grosse Y, Chissassi FE, Benbrahim-Tallaa L, et al. Carcinogenicity of consumption of red and processed meat. Lancet Oncol. 2015  Dec;16(16):1599-600. Abstract available from:  https://www.ncbi.nlm.nih.gov/pubmed/26514947
  2.  Johnston BC, Zeraatkar D, Han MA, Vernooij RWN, Dib EL, Marshall C, et al. Unprocessed red meat and processed meat consumption: dietary guideline recommendations (NutriRECS) Consortium. Ann Intern Med. 2019 Oct. doi:10 .7326/M19-1621. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/31569235
  3.  Guyatt GH, Oxman AD, Akl EA, Kunz R, Vist G, Brozek J, et al. GRADE guideline: 1. Introduction – GRADE evidence profiles and summary of findings tables. J Clin Epidemiol. 2011 Apr;64(4):383-94. Abstract available from: https://www.ncbi.nlm.nih.gov/pubmed/21195583
  4.  Schünemann H, Brożek J, Guyatt G and Andrew Oxman A, Eds. Handbook for grading the quality of evidence and the strength of recommendations using the GRADE approach. 2013. Available from: https://gdt.gradepro.org/app/handbook/handbook.html
  5.  GRADE. GRADE working group. [cited 2019 Oct 2]. Available from: http://www.gradeworkinggroup.org/
  6.  Harvard T.H. Chan School of Public Health. New “guidelines” say continue red meat consumption habits, but recommendations contradict evidence. The Nutrition Source. 2019 Sep 30. Available from:  https://www.hsph.harvard.edu/nutritionsource/2019/09/30/flawed-guidelines-red-processed-meat/

  7. Dietitians of Canada. Plant-based beverages – are they really healthier for young children? Trending Topic. In: Practice-based Evidence in Nutrition [PEN]. 2017 Aug. Available from: https://www.pennutrition.com/resourcestools.aspx?trcatid=496&trid=26285&sr=plant-based Access only by subscription. Click Sign Up on PEN login page.

  8. The National Academies Press. Dietary Reference Intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids. 2005. Available from: https://www.nap.edu/read/10490/chapter/12
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