Is being overweight good for you?

There has recently been a dispute about obesity and mortality, briefly as follows. A paper reporting a systematic review was published in the Journal of the American Medical Association (JAMA) earlier this year by Katherine Flegal of the US National Center for Health Statistics, in which Flegal and colleagues suggested that being overweight was associated with lower all-cause mortality than being normal weight. They confirmed that being obese, as opposed to merely overweight, is associated with higher mortality. This provoked a public response from Walter Willett, Chair of Epidemiology and Nutrition at Harvard, who called the paper “a pile of rubbish” and suggested that “no one should waste their time reading it”. The reasons that have received widest circulation are that such results undermine public health efforts, which require simple messages.

There is a thoughtful editorial in Nature here, siding with Flegal against Willett:

The argument developed by the editors of Nature is that by simplifying public messages about science, we open the door to easy refutation. They draw an interesting comparison with messages about climate change. The line that climate change is settled science makes the life of opposing lobbyists easier, they suggest, because all those lobbyists need to do is point to any kind of scientific controversy about any detail of climate change science, and they will have demonstrated that the public message is false, and perhaps even known to be false by the very people pushing it. Likewise, the editors argue, over-simple messages on obesity and mortality will be easy to falsify. In both cases, this leads to disputes about details and obscures the big picture, which is clear, even if the exact point at which bodyweight becomes a problem (or the exact rate of rise in sea levels) is still a topic of active research.

A couple of thoughts struck me as I read this editorial, and the coverage thereof. First, most obviously, none of the parties seem that interested in truth. The question seems to be: what is the most effective communication strategy for improving public health? – and not: for communicating the truth of the matter in question? The editors of Nature claim that communicating the truth of the matter in question is the most effective strategy for improving public health, but they do not argue (here) for the intrinsic value of promoting scientific knowledge.

Perhaps that shouldn’t surprise me, but it does. Science derives its claim to a special voice in social and political matters from its impartiality and transparency, and not merely from the fact that it gets the right answers (when indeed it does).

My second thought is that this is, perhaps, a point at which medical and scientific traditions conflict. In science, there is no “patient”, and scientific authority is based on reason, at least in the Enlightenment ideal. (HPS colleagues, you will please forgive my simplification here, but perhaps you will enjoy the associated irony.) There is no corollary of the doctor-patient relationship in science, or between science and society more generally. I wonder if disputes of this kind about public health messages illustrate a conflict between two ways of thinking about how technical information ought to be conveyed: one broadly educational, as a physicist might seek to communicate her findings to a wider audience; the other broadly manipulative, in the (intended-as-neutral) sense of seeking to influence the behaviour of the public, as if it were a patient being treated.

There are of course a number of other things that might be said about this particular study, and which Willett may well have been referring to in his comments. Body-mass index is an outdated and crude measure of adiposity, devised when people were on average slighter, and it puts even mildly athletic persons with little body fact but a solid build in the “overweight” category. Then there are various criticisms of systematic reviews. Finally, there is the fact that a causal inference on the basis an association like this would be excessively hasty. Of course, the conclusion of the study is not that there is a causal link between being overweight and living longer; but being coy about causal talk is a poor substitute for being clear that the evidence is compatible with a range of very different causal stories.

It remains surprising that such high profile studies remain open to such basic objections. If public health messages are to be more honest and more complicated, then perhaps objections could also be more widely communicated, and in particular causal euphemisms and hints could be replaced by a franker admission of ignorance. Willet is not doing anything special by advocating a simplified message; he is just advocating a more explicit, less euphemistic kind of simplification, and a different emphasis – less kindly, spin – from that adopted by the authors. To this extent, the rebuke in Nature is, I suggest, not entirely fair.

1 thought on “Is being overweight good for you?

  1. Pingback: Is breast best? | philosepi

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