Physician-blogger Marya Zilberberg, a professor of epidemiology at U-Mass, Amherst, writes,”Unpacking the meat data.” She says a study from Harvard – and news coverage of it – about how red meat is bad for you “deserves some unpacking.” Excerpt:
The investigators examined two large observational cohort studies totaling over 100,000 subjects and tried to estimate the risk of death associated with red meat consumption. Now, first, it has been widely acknowledged that dietary habit surveys are a difficult beast, and that is how these two studies got at the food history. Next, let us look at some of the numerators and denominators. The paper reports 23,926 deaths among these >100,000 subjects over 22 to 28 years of observation. The denominator for this type of a study is person-years, where you simply multiply the number of persons observed by the corresponding number of years of observation. In this instance, this value is 2,960,000 person-years. So, the roughly 24,000 deaths occurred over 2.96 million years of observation, simplifying to 24,000/2,960,000 = 8 deaths per 1,000 years overall. If we were to translate this to an individual’s risk for death over 1 year, it would be 0.008, or under 1%.
The study further reports that at its worst, meat increases this risk by 20% (95% confidence interval 15-24%, for processed meat). If we use this 0.8% risk per year as the baseline, and raise it by 20%, it brings us to 0.96% risk of death per year. Still, below 1%. Need a magnifying glass? Me too. Well, what if it’s closer to the upper limit of the 95% confidence interval, or 24%? The risk still does not quite get up to 1%, but almost. And what if it is closer to the lower limit, 15%? Then we go from 0.8% to 0.92%.
Does this effect size matter, even if statistically significant? What if this were a randomized controlled trial for a statin? What would we say to this result? Even if this is a real signal, which is questionable given the observational design (yes, despite holding a special affection for observational studies, I don’t think that this cause-effect is completely unconfounded; and this matters greatly in view of this minuscule magnitude), I am far more likely to die next time I get into my car than from eating burgers, even if I do indulge in one a couple of times per week.
On Twitter, she highlights this New York Times story as being “way out.” But an online search suggests they may not be alone: