The Irish Times quoted Dr Mary Flynn, chief specialist in public health nutrition with the Food Safety Authority of Ireland, saying that that results were no surprise but that
What people tend to do when studies like this come out is panic,” she said.
“The healthy eating guidelines advise you not to choose red meat every time. Choose fish once or twice a week. Choose poultry one or two times a week.”
People would be well advised not to panic even though the figures sound alarming. The report says that each serving of red meat raised the risk of death by 13%, and processed meat raised it by 20%. But these figures are not given any context.
If we look at the abstract we see that the study looked at 23 926 deaths in 2.96 million person-years of follow-up. This equates to about a single person having a chance of 0.8% of dying in a given year of the study. If the chance of dying is increased by 20% that would raise the chance of dying to 0.96% – still less than 1% in a year. If we look just at the men (average age 53), that individual man has a chance of dying in a single year of 1.2% and an increase of 20% brings it to 1.44%. So it is not a negligible difference, but you are not going to drop dead immediately*.
David Robert Grimes writing in the Journal.ie outlined the core problem:
Imagine the odds of getting a certain disease were two in a million or 1/500,000. Now let’s say a study finds that eating lots of red meat brings this up to three in a million. The absolute risk increase is 1/million, or 0.000001% – a minor, almost negligible risk.
But this is more likely to be reported as a relative risk increase. Imagine the screaming headlines – “Eating red meat makes you 50 per cent more likely to get this disease!” They both convey the same information, yet one is far more likely to shift copy. The numbers themselves do not lie – it is how they are read that causes problems.
There are other criticisms of this report. It is clear from the baseline data that the highest eaters of meat are also the most unhealthy (they eat more unhealthily, are more likely to be over-weight and are more likely to smoke), though the study has attempted to adjust for these. It is also clear that, especially in the lower quartiles (headed Q1, Q2), the calorie intakes are remarkably low. For example the average calorie intake for all male quartiles is below the average calorie intake in the US in 1971 let alone the higher intake now. Since the food consumption is estimated by the individuals in the study and not measured directly, this group may be unusual or the diet information may be skewed by wishful thinking. (Or, indeed, both).
Finally, this study can only show a correlation between red meat eating and higher mortality. The people in the study are being observed in their daily lives, rather than assigned randomly to groups and told how much red meat to eat. Without random assignment, we cannot be sure the red meat is increasing mortality – it could be another factor causing both.
In their analysis of the report NHS Choices concluded:
This study had several strengths including its size, long follow-up period and detailed and repeated assessments of people’s meat intake. It also adjusted the results for other factors that might affect risk of mortality. However, relying on participants to self-report factors, such as their meat intake through questionnaires, introduces the possibility of error, although the questionnaires were validated. Furthermore, participants were mainly white health professionals, so the results may not be relatable to other populations.
This study cannot prove that regular consumption of red meat ‘kills’, as The Sun put it. Red meat is a good source of protein and certain nutrients such as iron, some vitamins and zinc, but it is already recognised that it is likely to raise the risk of cancer especially bowel cancer. The Department of Health advises adults who eat high levels of red and processed meat should reduce their intake to no more than 70 gram a day to reduce this risk.
*Addendum: The BBC’s More or Less have spoken to David Spiegelhalter, a Cambridge University biostatistician, and the Winton Professor of the Public Understanding of Risk. He has calculated in real terms what extra risk eating one extra portion of red meat every day would involve for a 40 year old man:
- the difference between living to age 79 (extra potion a day) versus age 80.
- the loss of a half hour off his life per extra portion.
Compared with other health risks, eating an extra portion of red meat per day is equivalent to being 5kg over-weight, or smoking two cigarettes every day.