Monday, 16 September 2019

Evidence? We don't need no stinking evidence!

The Royal Society of Public Health produced a report last week calling for a ban on fast food shops opening within 400 metres of a school. In effect, this means a near-total ban in most cities.

They also want some form of regulation to clamp down on the shops that already exist...

We are calling for local authorities to introduce planning restrictions on fast food in their local planning schemes; however, it must be noted that this approach is necessarily limited as it can only be used to stop outlets opening, and not to close existing ones. While it is important that comprehensive planning policies are in place so that fast food prevalence near schools cannot increase, it is also vital that councils are able to reduce the current number of outlets near schools (or at least their sales of unhealthy food to school children) – which is already unacceptably high in many areas.

To this end, they suggest banning the sale of food to children and banning shops from opening 'during the post-school hours of the day'.

What could possibly justify such state interference in the out-of-home food market? There must be masses of evidence showing a strong link between fast food outlets near schools and child obesity, right?

Wrong. When I conducted a systematic review last year, I found 74 relevant studies. Of these...

… only fifteen (20%) found a positive association between the proximity and/or density of fast food outlets and obesity/body mass index. Forty-four (60%) found no positive association, of which eleven (15%) found evidence that living near a fast food outlet reduced the risk of putting on weight. Fifteen (20%) produced a mix of positive, negative and (mostly) null results, which, taken together, point to no particular conclusion.

And the evidence that fast food availability causes obesity among children is even weaker... 

Of the 39 studies that looked specifically at children, only six (15%) found a positive association while twenty-six (67%) found no effect. Seven (18%) produced mixed results. Of the studies that found no association, five (13%) found an inverse relationship between fast food outlets and childhood obesity. Two-thirds of the studies found no evidence for the hypothesis that living near fast food outlets increases the risk of childhood obesity and there are nearly as many studies suggesting that it reduces childhood obesity as there are suggesting the opposite.

So how does the RSPH justify its proposals? It says...

A number of studies both at home and abroad have indicated a significant link between repeated exposure to fast food outlets along daily commutes, fast food consumption, and obesity.

'A number' is doing a lot of heavy lifting in that sentence. Does the RSPH summarise the literature? Does it cite any of the seven systematic reviews that have been conducted, all of which concluded that the link between fast food outlet proximity and obesity is extremely weak, at best?

It does not. The RSPH cites just two studies. One of them is a recent study from America which found an association between the body mass index of adult employees (not children) and the number of supermarkets, grocery stores and fast food outlets on their commute route (but not near their workplace).

The other is a study from the UK which showed that adults (not children) who live near lots of takeaway outlets consume fewer calories and had a slightly lower body weight than those who didn't. As the statistician Jeremy Franklin pointed out, the authors had to make some dramatic adjustments to the data to turn those findings on their head, and even this required supermarkets to be defined as 'takeaway food outlets'. If you exclude supermarkets, there is no association under any model.

Not the most compelling evidence then, but even if the studies were stronger it would still be wrong to cherry-pick two studies out of 70-odd.

'Public health' is not an evidence-based enterprise. The RSPH have found a policy they like and they want to do it because they want to do it.

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