A series of reviews, published today, found little evidence of health benefits from reducing red or processed meat consumption, and already they’re proving highly controversial. NZ experts assess the findings.
Existing guidelines tend almost invariably to advise people to curb the red meat component of diets. Following five reviews of “the relationship between meat consumption and health”, however, published today, a panel of experts recommend that most people can continue to consume red meat and processed meat at current levels.
Four systematic reviews were conducted focusing on randomised controlled trials and observational studies assessing the impact on cardiometabolic and cancer outcomes. A fifth systematic review “addressed people’s health-related values and preferences on meat consumption”. The panel, comprising 14 members from seven countries voted to back guidelines that most adults should continue to eat their current levels of red and processed meat intake
The authors of the guidelines, published published in Annals of Internal Medicine, say “while the new recommendations are bound to be controversial, they are based on the most comprehensive reviews of the evidence to date”.
What do New Zealand scientists have to say about the recommendations? Below a selection of responses, compiled by the Science Media Centre.
Jim Mann: Potentially unhelpful and misleading
In my opinion the “weak recommendations” based on “low certainty” evidence that adults “continue current consumption of unprocessed red meat and processed meat” are potentially unhelpful and could be misleading.
The recommendations were based on a majority decision of a 14-member panel appointed by a core leadership team. While I do not question the integrity, expertise and good intentions of the leadership team and panel, I have some reservations about the “guidelines” which they have issued.
The recommendations for red meat intake by individuals is not appreciably different from that made by other organisations (eg World Cancer Research Fund) given that current consumption in many countries is around three to four portions per week. It should be noted that recommendations to limit intake of red meat (in terms of effects on human health) are principally based on the relationship between red meat and colorectal cancer.
The panel opted to consider personal preferences along with cancer and cardiovascular outcomes but not to take into account environmental and animal welfare issues when making their recommendations. In my opinion it is irresponsible not to consider sustainability and planetary health (a key, if not the major, determinant of the health of future generations) when developing nutrient and food-based dietary guidelines.
The panel appears to have relied exclusively on the GRADE approach for assessing quality of evidence and guideline development. While this approach is appropriate and widely used (including by the WHO) there are some situations where additional methods are needed. GRADE quality assessment is based on a hierarchy of study types which are used to examine the relationship between an exposure – in this situation a particular food – and disease outcome or mortality. For evidence to be classified as being of high quality or high certainty, it must generally be derived from high quality randomised controlled trials (RCTs). In some situations it is not possible to undertake RCTs, and other approaches – such as experimental studies – which are not sanctioned by GRADE, are essential to complement the epidemiological studies included in the GRADE hierarchy. The conclusion of the International Agency for Research on Cancer that processed meat is carcinogenic to humans is based on experimental (ie not considered by the Panel) as well as other types of evidence.
I have not been able to review all the papers which underpin the guidelines but the article “Effect of lower versus higher intake of red meat intake on cardiometabolic and cancer outcomes” (Zeraatkar and colleagues) provides an illustration of my concern. The paper is a systematic review of randomised trials which have examined the effects of lower versus higher intakes of red meat on relevant cardiometabolic and cancer endpoints.
It concludes: “Low- to very-low-certainty evidence suggests that diets restricted in red meat may have little or no effect on major cardiometabolic outcomes and cancer mortality and incidence.” The 12 trials included in the review were required to involve a comparison of groups which differed by a gradient of at least one serving of red meat per week for six months or more. The potential difference between the two groups – which could be as little as one serving of meat per week – may be too small and the duration of the trial potentially so short that there would be little or no chance of detecting any adverse effect of red meat, should this exist.
Any risk (especially cancer) associated with a dietary attribute is likely to develop over a period of years, if not a life time, and a short term, partial reduction in exposure will almost certainly also not show benefit. While the authors acknowledge the limitations of their approach, the low quality of the evidence and their conclusions are tentative, I am not persuaded that it is even appropriate to suggest on the basis of these data that red meat may have little or no effect on disease incidence and mortality.
It should be noted that the group does not represent any national or international organisation or government. Guidelines are generally issued by authoritative bodies rather than self selected groups. While the bona fides and expertise of the leadership team and panel are not questioned, I would dispute the criticisms which appear to be levelled at all other groups which have suggested guidelines in the past.
Professor Jim Mann is Professor of Medicine and Human Nutrition and co-director, Edgar Diabetes and Obesity Research Centre, University of Otago; Director, Healthier Lives National Science Challenge. He is a member of WHO Nutrition Guidance Advisory Group, former member of the World Cancer Research Fund panel which developed the Second Report on Diet, Physical Activity and Cancer.
Rod Jackson: Why I think these studies are flawed
I’ve reviewed much of the evidence covered in these papers over the years and have come to the conclusion that it is impossible to undertake a useful medium- to long-term (more than about six months) randomised controlled trial or cohort study to assess the effects of common foods like meat, vegetables or dairy products, or common nutrients like fats, proteins, or carbohydrates, on “hard” outcomes like coronary disease, cancer or death.
The implications of this conclusion are three-fold:
- We have spent hundreds of millions of dollar on studies incapable of giving us useful information because of inherent biases in medium- to long-term nutrition studies that are almost impossible to deal with. This is not a criticism of the researchers” ability to design studies, although they should have realised by now that the studies can’t be done well.
- The misinformation given to the public, based on the result of these two types of seriously flawed study designs (for examining the medium- to long-term effects of common foods and nutrients), have led to huge confusion and likely harm. This includes the conclusions of these latest articles.
- We need to re-educate many of the researchers who write these papers and the “experts” who write so-called evidence based guidelines that they have to radically rethink what is considered acceptable evidence. These study designs are usually considered the gold standard study designs and many researchers and most guideline writers have yet to appreciate that they are next to useless.
The key bias in randomised controlled trials is crossover between intervention and control groups. Not surprisingly, it has proven impossible to keep different groups on the diets they are randomised to for more than a month or two. The reason this is not surprising is that in randomised trials of a once a day drug versus an identical placebo for a couple of years we are very lucky if we get a 60 to 70% adherence rate. So how anyone can assume one could achieve anything like even a 50% adherence rate is beyond me.
Also, you can’t measure adherence because most people cannot accurately remember what they have been eating and tend to report what the researchers want to hear so we overestimate adherence.
As a result most randomised trials of diet on hard outcomes show minimal or no effects on outcomes, because the comparison groups gradually converge to have quite similar diets. That’s why the current studies report small or negligible effects.
The cohort studies are even worse. Firstly, questionnaires used to divide people into different baseline groups based on their diets are notoriously inaccurate so the studies start off with a major bias – the groups” diets are probably not as different as the researchers think they are. Secondly the different baseline groups always differ in other ways over and above their diets and these other factors commonly cloud or exaggerate any real effects of diet. This is called confounding. Thirdly, long term cohort studies also suffer from the same cross-over problem that randomised trials suffer from. As they are generally longer than randomised trials, cross-over can be even worse.
I hope this explains why I don’t think these new studies reported in this journal are meaningful.
What we need to do instead is to bring together all the other evidence, including: from short-term randomised studies that are short enough to limit cross-over but can also only measure proxy outcomes like blood lipid levels or blood pressure levels; from ecological studies of whole populations; from biochemistry; from pathology and from long-term drug trials (e.g. with statins) etc. This is messy and requires people with serious expertise and experience, but evidence is messy. It doesn’t matter how many meta-analyses of randomised trials and cohort studies are done and how many millions of people are included. They are still seriously flawed.
Professor Rod Jackson is Professor of Epidemiology, University of Auckland
Nick Wilson: Lacking in wider context
These new review findings lack a critical wider context in that there is an urgent need for a global shift to a more plant-based diets for planetary health reasons. The current patterns of meat consumption are completely unsustainable and are damaging the climate, polluting waterways, and depleting water supplies. Intensive livestock farming is also breeding microbes that are resistant to antibiotics and pandemic influenza can even arise from such dense collections of pigs and poultry. Such critical issues around sustainability are why the major EAT-Lancet Report recommends a shift to more plant-based diets – so we can feed a future population of 10 billion people within planetary boundaries.
From the direct human health risk perspective these new review findings are also out of sync with other major reviews – so they should also be seen in that context – for example, the very major review by the International Agency for Research on Cancer (involving over 800 studies) on processed meat and red meat and increased risk of cancer. Furthermore, other lines of evidence indicate hazards of processed meat – it is invariably high in salt, which is a proven risk factor for raised blood pressure and cardiovascular disease. Many forms of meat are also high in saturated fat – which is also a risk factor for cardiovascular disease.
Overall the evidence for predominantly plant-based diets being healthy is overwhelming. In particular, the Mediterranean diet (which is low in meat) has been consistently found to be associated with a lower risk of chronic diseases. But we need both healthy diets and a liveable planet – which is why shifting to sustainable food systems (from “paddock to plate” or from “seed to sewage”) should be the dominant consideration around the world.
Professor Nick Wilson is based in the Department of Public Health, University of Otago, Wellington
Andrea Braakhui: We need more research
The group of systematic reviews are interesting, but not in contrast to the recommendations by leading nutrition guidelines. The guidelines generally suggest we consume small to moderate quantities of red meat as part of a balanced diet. The evidence that highly processed meat should be limited is a little more convincing than red meat as a whole.
The nutrition guideline recommending the least amount of red meat is the EAT Lancet planetary diet, which recommends around 7g a day of red meat (or 35g a day when combined with alternative meat sources). The EAT Lancet recommendation is based on climate change, not health, and is modelled off high input farming. Even so, red meat is still suggested a part of a healthy diet.
The authors of the systematic reviews conclude there is no significant benefit to reducing red meat consumption; however, the strength of that conclusion is weak. It would be fair to say more research is warranted.
Dr Andrea Braakhuis is an academic and registered dietician at The University of Auckland