The science behind the conflicting headlines

In Step 2.4 you looked at two conflicting headlines about the effects of red and processed meat on our health. We then reflected on the way our own biases as well as journalistic techniques influence the conclusions we draw. Let’s look at the original scientific papers now to discover whether they shed light on the question.

The first study produced controversial results and generated numerous newspaper articles which were listed in Step 2.5.

The guidance was produced by an international team of researchers making up the Nutritional Recommendations (NutriRECS) Consortium. The team conducted a series of systematic literature searches to identify clinical trials and observational studies that had assessed whether eating less red or processed meat had an effect on a set of health outcomes including all-cause death, cardiovascular disease and cancer. A reduction of 3 servings per week was considered to be a ‘realistic’ reduction for the public to make, so they looked for evidence in relation to this amount. They also identified surveys and interviews that had looked at people’s preferences and willingness to change.

For red meat, the researchers identified 12 clinical trials (54,000 adults) which found that reducing intake had minimal effect on cardiovascular outcomes, diabetes or cancer. They identified 23 cohort studies which found that reducing red meat intake (by 3 servings a week) would give ‘a very small’ reduction in the risk of cardiovascular outcomes and diabetes – about 1-6 fewer events per 1000 people. 17 cohorts (2.2 million adults) suggested that reducing red meat consumption would give ‘a very small’ reduction in lifetime cancer deaths, about 7 fewer deaths per 1000 people.

For processed meat, no clinical trials met their inclusion criteria. 10 cohorts (778,000 adults) suggested that reducing processed meat intake (by 3 servings a week) gave ‘a very small’ reduction in risk of all-cause mortality, cardiovascular outcomes and diabetes, about 1-12 fewer events per 1000 people. 31 cohorts (3.5 million adults) found that reduced processed meat was linked with ‘a very small’ reduction in lifetime cancer deaths, prostate cancer deaths, and the development of colorectal, breast and oesophageal cancer – about 1-8 fewer events per 1000.

Meanwhile the researchers concluded from 54 survey and interview studies that people would be overall unwilling to change their meat consumption.

A guideline development panel of 14 people, including research and healthcare experts and members of the general public, reviewed the findings and voted on the final recommendations. The overall graded recommendation was identical for both red 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). Eleven of 14 panelists voted for continuation of current consumption, whereas 3 voted for a weak recommendation to reduce [unprocessed red meat and processed meat consumption].’

We think the following are important points to note:

• The evidence was all assessed to be low or very low certainty. Most of this evidence was observational with highly variable study populations, design, method, risk of bias and findings. The pooled studies may give an unreliable estimate of the effect of red or processed meat consumption. The true effect may be very different from these results. Considerable caution needs to be taken when drawing recommendations from such evidence, which is why the recommendation is only ‘weak’.
• The recommendation does not relate to a particular level of meat consumption. The researchers have looked at evidence on the effect of reducing intake of red or processed meat by 3 servings per week. The resulting recommendation is to ‘continue current consumption.’ However, there is no baseline level of intake to compare this against. If it is assumed that people are, on average, consuming the recommended limit of 3 servings per week, then this is no change from current recommendations. What the results do not mean is that there is now good evidence that it is safe to start increasing intake above existing guideline levels.
• Even ‘a very small’ reduction in disease risk could make a big difference at the population level. The researchers report that reducing red or processed meat intake gives only very small reductions in risk of heart attack, stroke, diabetes and cancer at up to 1 fewer event per 100 people. However, this not negligible when considering that these diseases are very common. Reducing consumption could make a large difference at the population level.

• The study cannot prove direct cause and effect. The researchers have adjusted for various health and lifestyle factors that may be influencing the link. However, as with all observational research it is difficult to ensure that you have fully removed their effect and isolate the direct effect from a single dietary factor.
• The risk increase is small. The relative risk increases only just crossed the threshold of statistical significance, particularly for red meat and poultry. Accordingly the difference that this level of intake would make to an individual’s baseline risk of cardiovascular disease was small – less than 2% over 30 years. The specific effect of meat may be small alongside other factors that may contribute to your overall cardiovascular risk such as age, genetics, obesity, smoking or alcohol.
• Estimations of portion size may be inaccurate. Food frequency questionnaires are a valid way of obtaining dietary information but can introduce inaccuracy, particularly when estimating portion sizes. For example, ‘2 small links of sausage’ may mean different things to different people. Similarly this assessment doesn’t account for the quality of meat: a portion of red meat could mean a lean cut or a fatty one, or poultry could mean plain meat or deep-fried.
• Representation is uncertain. The dietary assessments were taken only once at study recruitment, which in all of these 6 studies was between 20 and 30 years ago. One-off assessments may not reflect the consistent lifetime eating patterns of these participants, but neither may they be representative of dietary intakes today. Also being US-based we don’t know whether the findings can be generalised to other countries.

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Now that you’ve taken into account your own biases, the journalists’ agendas and the conflicting scientific findings, what conclusions do YOU draw about whether or not eating red and processed meat affects your health?