Coffee news
Q: Did you see that two cups of coffee a day will prevent dementia?
A: In Stuff? That’s not what it says.
Q: “Two cups of coffee a day can keep dementia at bay – research”
A: Read a bit further.
Q: Ok, so it’s just in women over 65, and two to three cups, and it’s a 36% reduction, not as good as the headline says, but still pretty good, surely?
A: There’s a lot of uncertainty in that number
Q: So what’s the margin of error or whatever the medical folks call it?
A: According to the research paper a 95% confidence interval for the reduction goes from 1% to 44%. And it’s a reduction in rate, not in risk — it could easily be postponing rather than preventing dementia, even if it works.
Q: Was there a link to the paper?
A: No, but there was a link to the press release, and it linked to the paper.
Q: That interval. Why isn’t 36% in the middle of the interval?
A: I don’t know. The results in the abstract and tables of the paper give a hazard ratio of 0.74. I can think of two possibilities. One is that the 36% isn’t based on the primary findings in the abstract but on a less well-described secondary analysis. The other is that someone subtracted 74% from 100% and got it wrong.
Q: Why is it just women over 65?
A: Because that’s who was in the study.
Q: So the coffee-drinking didn’t necessarily start at 65?
A: No, and it wasn’t necessarily coffee. It could have been tea or soda.
Q: Could they look at whether the coffee drinkers were different at the start of the study?
A: Yes — and they were. The difference in their cognitive test scores stayed pretty much constant during the study, and the correlation with caffeine mostly goes away if you compare people starting out with the same test scores.
Q: So it might be that caffeine matters at an earlier age, not over 65?
A: And it might not matter — perhaps the people who drink a lot of caffeine were at lower risk for some other reason.
Q: Could it still be true?
A: It could. It is in some lab-animal models of Alzheimer’s, but no-one really knows how relevant they are to human dementia
Q: Rats.
A: Yes, and mice.
Q: No, that was a colloquial exclamation expressing frustration, disappointment, or annoyance.
Thomas Lumley (@tslumley) is Professor of Biostatistics at the University of Auckland. His research interests include semiparametric models, survey sampling, statistical computing, foundations of statistics, and whatever methodological problems his medical collaborators come up with. He also blogs at Biased and Inefficient See all posts by Thomas Lumley »