Eat bacon and die

The Herald, under the arguably-overstated headline Eating processed meats could cut your life short, have the reasonable lead

A diet packed with sausages, ham, bacon and other processed meats appears to be linked to an increased risk of dying young, a study of half a million people across Europe suggests.

The main problem with the summaries of risk that reported in the story is that they are for the people who eat the highest amount of processed meat.  It’s notable that nowhere in the Herald story do they tell you how high this consumption level was, either as a fraction of the participants or as a weight or number of servings. (3News did better)

It’s probably true that you would have lower risk if you ate less processed meat than this highest-consumption group, but you probably already do — they were the top half a percent of the 450000 participants, and they averaged more than 160g per day, or 1.1kg per week.

There are also problems with how the statistics get translated into deaths.  The study estimated hazard ratios, which compare the rates of death for high and low processed meat consumption, and then try to turn these into proportions. The Herald quotes a study researcher as saying

“Overall, we estimate that 3 per cent of premature deaths each year could be prevented if people ate less than 20 grams of processed meat per day.”

This should get the response “define ‘premature'”, but it’s actually more carefully phrased than in the research paper, which says

We estimated that 3.3% (95% CI 1.5% to 5.0%) of deaths could be prevented if all participants had a processed meat consumption of less than 20 g/day.

suggesting that  3.3% of vegetarians would be immortal.

Turning hazard ratios into information about life expectancy or premature death is tricky.  David Spiegelhalter’s microlives are useful here. The study estimates a hazard ratio of 1.18 for 50g extra per day of processed meat.  If that really is due to the meat, not to other differences in health risk,  and if it really is approximately constant across all types of processed meat, it corresponds to about 2 microlives per 50g — about an hour of life per serving, or about the same as four cigarettes.

There are reasons to be a bit skeptical about the magnitude of the results: the study didn’t find any evidence of higher risk in people who eat a lot of red meat, contradicting previous studies.  Also, the analysis used statistical techniques to correct for measurement error in meat consumption, but not in any of the other risk factors they analysed.  If people with high processed meat consumption are also at higher risk in other ways (which they are), this analysis will tend to shift the apparent risk towards processed meat.

Still, I shouldn’t think anyone is really surprised that bacon’s not a health food.