"New research that claims red and processed meat is probably not harmful to our health has caused controversy among experts who maintain people should cut down," The Guardian reports.
The World Health Organization currently classifies processed meats, such as bacon and salami, as being cancer-causing (carcinogenic) to humans – putting it in the same category as tobacco – while red meat is classed as "probably carcinogenic". The strongest link is with bowel cancer. World Cancer Research Fund recommends adults eat no more than about 3 portions of red meat per week, about 350g to 500g cooked weight in total. It also advises adults to consume little, if any, processed meat.
However, an international group of researchers who reviewed many previous trials and observational studies found weak evidence for such recommendations. Researchers found that reducing red or processed meat intake by 3 servings a week resulted in only a small reduction in risk of cardiovascular disease, diabetes or cancer outcomes. They give an – arguably unhelpful – "weak" recommendation that adults "continue with their current consumption" of red or processed meat.
It's difficult to know how much confidence the conclusions of this review warrants. All of the evidence was assessed to be "low" or "very low certainty" by the researchers themselves, meaning the results could be unreliable.
Also, even a small reduction in risk of common health outcomes such as cardiovascular disease and cancer could make a big difference at a population level.
Arguably it would be prudent to stick to World Cancer Research Fund recommendations, at least until higher-quality evidence becomes available.
Who produced the new guidelines?
The clinical guidelines were issued by an international team of researchers, including some from the UK, who developed the Nutritional Recommendations (NutriRECS) international consortium. It involved teams of researchers who designed the project, conducted the literature reviews, analysed the findings and drafted the document. A separate guideline development panel of 14 members, including researchers, healthcare experts and members of the general public from 7 countries, reviewed the findings and voted on the final recommendations.
The guidelines themselves are not reported to have received funding, but individual researchers report funding grants and affiliations with numerous organisations and academic institutions. The recommendations were published in Annals of Internal Medicine, which is freely available to access online.
The UK's media coverage of the guidelines was generally accurate and balanced with most sources including quotes from independent experts expressing doubts about the reported findings.
What forms of research were the guidelines based on?
This research involved a series of systematic reviews looking into the effect of red and processed meat upon health. In particular it asked 2 questions:
- among adults, what is the effect of a diet lower in red or processed meat versus a diet higher in red or processed meat on the risk of health outcomes that are important to the community?
- what are their health-related values and preferences for red and processed meat consumption?
Systematic reviews are the best way of gathering the available literature to see whether a particular exposure or intervention is linked with a health outcome. However, the conclusions are only as good as the quality of the underlying studies. A lot of dietary studies tend to be observational. As people choose their own diet, it's hard to prove cause and effect because you cannot exclude the possible influence of other health and lifestyle factors. The researchers here looked at a mix of observational and trial evidence, which should balance out such differences.
They then used recognised methods to assess the quality and risk of bias for the studies they reviewed. They graded the certainty of evidence for each outcome in order to produce a recommendation.
What did the research involve?
The team searched literature databases to identify large observational studies (more than 1,000 adults) or randomised controlled trials that had assessed the effect of different intakes of red or processed meat upon health outcomes. In addition, they identified cross-sectional surveys and other "qualitative" research (such as interviews or discussion groups) to look at quality of life effects and views.
The panel considered the following outcomes as "critically important" for developing recommendations:
- all-cause mortality
- major cardiovascular or metabolic outcomes (such as heart attack, stroke or diabetes)
- cancer incidence and mortality
- quality of life
- willingness to change unprocessed red or processed meat consumption
The panel considered 3 servings a week to be a realistic reduction for the public to achieve. They studied the available evidence to look at the change in risk linked with a decrease in meat consumption by 3 servings a week (for example, reducing intake from 7 to 4 servings per week).
What were the basic results?
Harms of red meat
The findings from 12 randomised controlled trials involving 54,000 people suggested that reducing intake of red meat had little or no effect on cardiovascular outcomes, diabetes or cancer.
The pooled results from 23 cohort studies (1.4 million people) suggested that reducing red meat consumption by 3 servings a week may result in a very small reduction in cardiovascular outcomes and diabetes – about 1 to 6 fewer events per 1,000 people. They found no effect on cardiovascular deaths or all-cause deaths.
17 cohorts (2.2 million people) suggested that decreasing red meat by 3 servings per week may give a very small reduction in lifetime cancer deaths – about 7 fewer deaths per 1,000 people.
Harms of processed meat
The researchers found no randomised controlled trials that assessed the effect of a change in weekly servings on their target outcomes.
Of the cohorts they reviewed, 10 studies (778,000 adults) suggested that decreasing processed meat by 3 servings a week gave a very small reduction in risk of all-cause mortality, cardiovascular outcomes and diabetes of about 1 to 12 fewer events per 1,000 people.
31 cohort studies (3.5 million people) found a decreased intake of processed meat was linked with a small reduction in lifetime cancer deaths and the development of bowel, breast and oesophageal (food pipe) cancer – about 1 to 8 fewer events per 1,000.
All evidence for both red and processed meat was graded as low to very low certainty.
Values and preferences
54 studies from the US, Canada, Australia and Europe found that: "Omnivores reported enjoying eating meat, considered meat an essential component of a healthy diet, and often felt they had limited culinary skills to prepare satisfactory meals without meat. Participants tended to be unwilling to change their meat consumption."
What did the researchers recommend?
The researchers made 2 identical recommendations for red and processed meat consumption, which have been combined here into 1 for the sake of simplicity. Essentially their recommendations seem to suggest that adults do not need to alter their current consumption levels (whatever these may be).
Recommendation for unprocessed red meat and processed meat
"For adults 18 years of age or older, we suggest continuing current unprocessed red meat [and processed meat] consumption (weak recommendation, low-certainty evidence). 11 of 14 panellists voted for continuation of current consumption, whereas 3 voted for a weak recommendation to reduce red meat [and processed meat] consumption."
These recommendations appear to go against current understanding around red and processed meat consumption. So, is it now safe to eat as much as we want?
All the evidence was assessed to be low or very low certainty. This means the pooled studies give an unreliable estimate of the effect of red or processed meat consumption. The true effect may be very different to these results. You need to use considerable caution when basing recommendations on such evidence, which is why the recommendation is "weak".
The low-certainty evidence likely reflects that most of these were observational studies, with highly variable study population, design, method, outcome assessment and risk of bias. Many confounding health and lifestyle factors can influence the relationship, making it difficult to establish the direct effect that meat consumption is having upon health.
The review does not assess a particular level of meat consumption but has looked at the effect of reducing intake by 3 servings per week. There was weak evidence that reducing intake by this amount may give a small reduction in cardiovascular, diabetes or cancer outcomes. But it's hard to know what this means as we do not know what amount people were consuming to start with.
Their recommendation suggests adults continue with their current consumption level. If we assume people are already consuming the recommended limit of 3 servings per week, on average, then this is exactly the same as current recommendations.
What it does not mean is that people can increase their intake or eat as much red and processed meat as they want without ill effect. The recognised links with cancer and general cardiovascular health from red and processed meat consumption were still seen, even if reducing intake was considered to make only a "small" difference to these outcomes.
Many experts have commented on this point, highlighting that even though a reduction of up to 1 event per 100 may seem small, they're not trivial when you consider that conditions such as cardiovascular disease and cancer are common. This potentially amounts to many people and a large health impact.
With the overall uncertainty of these findings, it is difficult to predict whether these new guidelines will change the current expert consensus on eating red and processed meat.
A Professor of Medicine and Human Nutrition at University of Otago, New Zealand, sums up a fairly common feeling: "In my opinion, the weak recommendation based on low-certainty evidence that adults continue current consumption of unprocessed red meat and processed meat is potentially unhelpful and could be misleading."
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Guardian, 30 September 2019
BBC News, 30 September 2019
The Independent, 1 October 2019
Daily Mirror, 30 September 2019
The Sun, 1 October 2019
The Telegraph, 30 September 2019
Mail Online, 30 September 2019
The Times (subscription required), 1 October 2019
Links to the science
Annals of Internal Medicine. Published online 30 September 2019