The Sunshine Roof

Obesity in London? 100,000 Cases Prevented!

According to a press release from the University of Sheffield, it sounds like the city of London pretty much has obesity prevention figured out. All they had to do is ban adverts for junk food from public transport. Voilà! With that simple act, says the press release, London has prevented nearly 100,000 cases of obesity, nearly 3,000 cases of diabetes, and nearly 2,000 cases of heart disease. Savings for the NHS will add up to £200 million!

There’s just one little problem with this wonderful news. The research behind it doesn’t actually involve any case counts. Nobody has actually observed these excellent results in the real world. Instead, these numbers come from a model that assumes the ad ban works to keep people from gaining weight.

Not a Bad Model

The source of these extravagant claims is a paper published late last week in the International Journal of Behavioral Nutrition and Physical Activity. Chloe Thomas and colleagues modified a diabetes microsimulation model to account for the London transport ad ban and estimate the effects on obesity in London. As with any model, this one can be a useful tool for exploring a question. In this case, the question is how might the London transport ad ban affect the prevalence of obesity.

It cannot, however, provide a definitive answer to that question. Because a definitive answer requires data and data for the effect of the ad ban on obesity rates simply does not exist. All they have is a set of assumptions and unfortunately, you will not find a discrete inventory of those assumptions in this paper.

A Big Leap of Faith

Instead of a careful discussion of assumptions that drive the conclusions, this paper requires a big leap of faith. The authors start with data on reductions in rates of growth in purchases of high salt, sugar, and fat products banned from advertising on London transport. These data come from a study published earlier this year.

Then they assume that people won’t buy other products instead. And they assume that these changes in purchasing behavior – all by themselves – will translate directly into changes in eating behavior, which in turn will bring weight loss. For the weight loss, they apply Kevin Hall’s modeling for energy balance and body weight.

Flawed Assumptions

Unfortunately, these are flawed assumptions. A careful reading of the research these modelers are using will tell you so:

“Single interventions cannot be expected to work on their own and should be seen as one part of a wider strategy to improve population health, with multiple interventions needed at multiple points within the food system to improve diet.”

So no. It’s not likely that simply banning ads from London transport has, all by itself, prevented 100,000 cases of obesity. Pretending this is true, though, does have an effect. It has the effect of leading us to rely upon ineffective strategies for preventing obesity and then waking up decades later to find they’re not working.

It’s a sad case of self-delusion. We desperately need to look for more complete and effective answers with genuine curiosity and disciplined objectivity.

Click here for the modeling study, here for the press release, here for the earlier study of the London transport ad ban, and here for further perspective.

The Sunshine Roof, linocut by Cyril Power / WikiArt

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August 3, 2022