Well, I think today’s lesson is, many people, including many scientists and science reporters, just do not get that there is a difference between correlation and causation. I know – this is like beating a dead horse since many write about this issue. But it just needs to be called out every time until it stops. And today’s fun comes from stories and the original research articles about how exercise supposedly alters the gut microbiome.
I was pointed to this just a few minutes ago on Twitter:
— Bernat Olle (@bernatolle) June 10, 2014
In this Tweet Bernat Olle points to a “news” story in Medpage Today: Exercise Boosts Gut Microbiome Diversity by Kristina Fiore. Well, so of course I started digging around. And, not surprisingly, the study that this is based on shows absolutely no causal connection between exercise and the gut microbiome. The study is in the journal “Gut”: Exercise and associated dietary extremes impact on gut microbial diversity. And here is what they did:
- They selected subjects – 40 “elite” rugby players.
- They identified healthy male “controls” with similar age and size and from similar place.
- Then they collected faecal and blood samples from participants and did surveys about their nutrition and clinical data.
- Among many measurements, they did 16S sequencing from the fecal samples
- Then they did some bioinformatics and found differences between the rugby players and the controls in many features including microbiomes.
Cargo Cult Science strikes again. Feynman was such a genius.
Beat this into your head – correlation is not causation. All they showed was a difference in microbiomes between rugby players and controls.
But why this is, what is the cause? Well they blithely say it is due to exercise. And the journal let them do this.
In fact the journal had a write-up where it says the same thing. All they showed was there existed a difference but not that the difference in physical condition was the cause.
Maybe it was due to the diet of beer after each game? Or some other correlated fact of the rugby players rather than their exercise.
Confirmation bias is what makes inappropriate linking of causation and correlation so wickedly hard to remove. Everyone should read Feynman’s speech before they write or review any paper.