It seems like only yesterday I was writing about the idea that scoffing mounds of chocolicious contraband might actually be — bear with me — bad for your physical health. That’s right. Call me Doctor Bigbrains and give me my Nobel Prize. I’ll even say it again: gorging on chocolate is NOT ACTUALLY GOOD FOR YOU.
One hint that this chocolate-is-actually-bad-for-you position is likely to have merit is the very frequency with which chocolate-might-actually-be-GOOD-for-you stories appear in the media. You see, if chocolate really was good for you then research studies drawing this conclusion wouldn’t actually be newsworthy.
The latest example is a study from yesterday’s issue of the medical journal Archives of Internal Medicine, as reported by (among others) BBC News. In summary, the study — conducted in California and involving nearly a thousand participants — found that people who eat chocolate more frequently have a lower body mass index (or BMI) than people who eat chocolate less frequently. In other words, they’re less fat. Or as BBC News puts it, regular chocolate chomping ‘may help keep people slim‘. Well, I’ve got news for BBC News. It ain’t so.
The problem is that the research being quoted was entirely cross-sectional. This means that the researchers simply asked people on one occasion to fill out a health questionnaire (which included questions about chocolate consumption), while on the same occasion gathered data regarding their BMIs. And the problem with cross-sectional data is that it is impossible to know what’s causing what. X could be causing Y, but then again, Y could be causing X. You know. That stuff.
So when the data showed that thin people eat chocolate more frequently than fat people, it is impossible to establish causality. Yes, it could mean that ‘eating chocolate more frequently leads people to be thin‘. However, it could also mean that ‘being thin empowers people to eat chocolate more frequently‘. And pardon me from pointing this out, but doesn’t this latter interpretation make more sense? Isn’t it consistent with the simple idea that thin people feel less guilty about their chocolate consumption?
In an effort to get to the bottom of their key finding, the researchers did go to trouble to incorporate a load of other data into their statistical analyses. One outcome was the finding that is that it was frequency of chocolate consumption — and not amount of chocolate consumed — that was statistically associated with thinness. Again, this is consistent with the view that thin people are simply less guilty about eating chocolate. They indulge more often but remain sensible; permitting themselves regular titbits only “as part of a healthy balanced diet“ (to coin a phrase).
Interestingly, neither the researchers nor BBC News draw such a conclusion. Instead, without quite declaring it a mystery, they stick to the interpretation that ‘attitudes to chocolate determine bodyweight’ rather than the other way round. In other words: There is a chicken. There is an egg. The egg came first. End of.
To be fair, the BBC News writer does make clear that chocolate has a downside too, and recommends that people stick to fruit and vegetables if they want to eat healthily. The writer also identifies some other research to add context to the latest Archives of Internal Medicine paper. Both features — the presentation of competing views and the identification of the previous research context — are welcome, as they are often missing from science journalism.
However, the piece doesn’t actually criticise the Archives study for being cross-sectional, nor question the plausibility of the interpretation that is offered. This is not only a shortcoming in how this particular study was approached, it also reflects a tad poorly on the standard of reporting in more general terms. After all, the whole chicken-and-egg thing is extremely elementary stuff for a science journo. Isn’t it?
Anyway, it seems as though this chocolate-is-actually-good-for-you meme is not going to go away any time soon. But no harm done. It’s not as though we have to face a worldwide obesity epidemic or anything…