Friday, 18 February 2022

Calories and the Transport for London food advertising ban

From the Guardian...

A ban on junk food advertising by Transport for London has contributed to a 1,000 calorie decrease in unhealthy purchases in people’s weekly shopping, a study has estimated.

... Dr Amy Yau, from LSHTM and the study’s lead author, said: “Many governments and local authorities are considering advertising restrictions to reduce consumption of HFSS products as part of obesity prevention strategies.

“However, evidence of the effectiveness of such policies, especially away from broadcast media, is scarce.

“Our study helps to plug that knowledge gap, showing TfL’s policy is a potential destination for decision-makers aiming to reduce diet-related disease more widely.”

As you can probably tell from Dr Yau's quote, the study in question was created a number of activist-academics who are keen supporters of food advertising bans, including Emma Boyland and Martin White
The claim they are making is a big one. 1,000 calories a week is quite a lot. Even divided by the 2.1 people in the average household, it amounts to 476 calories a week or 68 calories a day. This is much, much more than the Department of Health - and several of the authors of this study - expect to achieve from its pre-watershed broadcast advertising ban and its total online ban combined. Could an advertising ban on 'high in fat, sugar and salt' food (HFSS) on buses and trains in one city really reduce calorie consumption by this much? It seems fishy from the outset.
The study uses Kantar survey data to see how food purchases changed in the ten months after the ban started. The Kantar survey involves enrolling households to scan the barcode of all food brought into the home. Importantly, it does not include restaurant meals, fast food, snacks bought outside the home, etc. This is significant not only because a large amount of HFSS food is bought from the 'out-of-home' sector, but because this is the kind of food that is (or was) most often advertised on TfL. I use the Tube fairly regularly and in my experience it was companies like Pizza Hut and McDonalds that were advertising. It would be interesting to see the breakdown, but I don't remember seeing adverts for biscuits or breakfast cereal very much.
Data from nearly a thousand households in London and a similar number in the North of England were used in the study. The North of England was chosen as the control because the people there are unlikely to take the tube (or, as the authors put it, suffer from 'contamination ... through regular commuting to London'!). 

If you look at the data, both sets of households reduced their overall calorie purchases and they reduced the amount of HFSS food purchased. HFSS food sales fell by 1.47% in London and by 0.51% in the North. Click to enlarge...
Both groups increased the amount of chocolate and confectionery they bought. Both groups reduced the amount of sugary drinks and sugary cereals they bought. The Londoners slightly decreased their purchases of puddings and biscuits while the Northerners slightly increased them. And the Northerners slightly reduced their purchases of savoury snacks while the Londoners slightly increased them. 

The differences here are tiny and seem indistinguishable from random noise. The biggest difference is in the 'puddings and biscuits' category, purchases of which went up in the North by 35 calories a week and down by 31 calories in London. It seems unlikely that this was caused by a ban on advertising 'junk food' on trains and buses in London and it should be put in the context of overall calorie purchases which amounted to 26,819 a week in London and 29,133 a week in the North before the ban was introduced (note the large difference between the two areas).

There isn't much to see here and certainly no evidence of the TfL food ban reducing household calorie consumption by 1,000 calories a week. To achieve this finding, the authors turn to their old friend, the opaque counterfactual model. In classic 'public health' style, details about the variables in this model are thin on the ground, but once the authors start imaging what might have been had Sadiq Khan never banned Burger King adverts, they come up with a much more impressive conclusion:

The implementation of HFSS advertising restrictions was associated with a relative reduction in average weekly household energy purchased from HFSS products of 1,001.0 kcal (95% CI 456.0 to 1,546.0), or 6.7% (95% CI 3.2% to 10.1%), in the intervention group compared to the counterfactual.

Er, OK. If you say so. For what it's worth, here's the model...

Even with a model of their own design, they fail to find a statistically significant difference between London and the North for most categories. The main exception is chocolate and confectionery...
Chocolate and confectionery. Using the whole study period, average weekly household purchases of energy from chocolate and confectionery fell by 317.9 kcal (95% CI 200.0 to 435.8) in the intervention group relative to the counterfactual, equivalent to an observed decrease of 19.4% (95% CI 13.4% to 25.4%).

This is slightly astonishing because the actual data show that the amount of chocolate and confectionery rose in both areas, by 3.3% in London and by 4.9% in the North. Somehow, the authors have turned a 42 calorie increase in chocolate and confectionery purchases in London after the TfL ban into a 318 calorie reduction! 

A look at the model shows how this trick was pulled off. They created a counterfactual in which purchases of these products in London (blue line), which had consistently been well below the North in the recent past (orange line), suddenly leapt up in late 2019 to Northern levels (dotted blue line).

Why would demand for chocolate and confectionery rise so far above the secular trend in London out of the blue like this? Alas, the authors do not explain, but we don't need to worry about it because HFSS food ads were banned on the Tube before it could happen. Phew, what a close shave!

If you believe this obvious nonsense, you will believe anything.

London mayor Sadiq Khan said: “I am pleased to see the positive impact these groundbreaking measures have had, leading to a real reduction in the amount of junk food being purchased.”

We're being taken for fools.

Still, a reduction of 1,000 calories a week is enough to make a real impact on obesity rates so I'm sure we can expect them to start falling any day now, right?

No comments: