June 18, 2017

Unbiased anecdote is still anecdote

RadioNZ has a new “Healthy or Hoax” series looking at popular health claims. The first one, on coconut oil, is a good example both of what it does well, and of the difficulties in matching the claims and science.

The serious questions about coconut oil are about changes in blood fats and in insulin resistance when saturated fats replace various other components of the diet.  Replacing sugar and starch by  saturated fat is probably good; replacing, say,  monounsaturated fat by saturated fat probably isn’t. But in both cases the effects are small and are primarily on things you don’t notice, like your cholesterol level. That’s why there’s disagreement, because it’s actually hard to tell, given all the individual variability between people.

The superfood questions about coconut oil are about whether eating loads of it makes dramatic improvements in your health over a period of a few weeks.  There’s no reason to think it does, and the story quotes various people including Grant Schofield — who is at one end of the spectrum of respectable views on this subject — as saying so.

That’s all fine, but a big part of the story is about Kate Pereyra Garcia trying it for herself.  If the scientists — any subset of them — are right, a study on one person isn’t going to say anything helpful.  A one-person experience might disprove some of the extreme superfoodie claims, but no-one who believes those claims is likely to pay attention.

So, on one hand, the series looks like a great way to bring up the relatively boring evidence on a range of health topics. On the other hand, it’s reinforcing the concept of individual testimonials as a way of evaluating health effects.  If it was that easy to tell, we wouldn’t still be arguing about it.


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 »