Has Sadiq Khan’s junk food ad ban really stopped London getting fatter?

London Mayor Sadiq Khan made a bold claim this week: ‘As a result of our junk food advertising ban on Transport for London, nearly 100,000 cases of obesity have been prevented since 2019.’ Hailing the ‘incredible result’, Khan said ‘it’s expected to save the NHS over £200 million’. Is it true though?

To ascertain whether the ban worked, researchers from Sheffield university and the London School of Hygiene and Tropical Medicine produced a graph. They found that there were some 4.8 per cent fewer obese people than expected and 1.8 per cent fewer overweight people.

What they didn’t do was actually count the obesity rate in London’s population. Instead, their estimates were based on a mathematical model. And as Christopher Snowdon has pointed out the modelling is questionable. The model concludes that after the ban came into effect, Londoners purchased 1,000 fewer calories of high fat and sugary foods. Rather than comparing frequent tube travellers to Londoners who don’t use the underground, it relied on people in the North East as a control group. It did so on the grounds that there was no ‘contamination through regular commuting to London’.

NEW: As a result of our junk food advertising ban on @TfL, nearly 100,000 cases of obesity have been prevented since 2019.

It’s expected to save the NHS over £200 million. An incredible result. https://t.co/SIUi25RG95

— Mayor of London, Sadiq Khan (@MayorofLondon) August 2, 2022

The study’s authors note the limitations themselves: ‘there is considerable uncertainty within the effectiveness data’, they say. Specifically they draw no link themselves between calorie purchases and calorie consumption, something fundamental to the whole idea.

It’s also worth noting that they don’t even claim obesity has decreased. Their model found rising purchases of junk food, both in London and the North East. But the growth was slower in London, which is the basis for the 100,000 figure mentioned by Khan. 

‘This suggests that whilst advertising restrictions may have some role in slowing the growth in obesity at the population level, other strategies will be required if obesity levels are to be actively reduced,’ the authors say.

But what about real world data? Adult obesity measures for last year haven’t been published yet, but we do have a wealth of data on childhood obesity. This makes for grim reading. Figures for ten and 11-year-olds in England show a jump during the pandemic. In 2019/20, just over a fifth of kids in this age group were obese; now it’s over a quarter. That’s a 24 per cent rise and represents nearly half-a-million kids. As the below graph shows, things are even worse in London:

In the capital, some 30 per cent of Year 6 pupils are now obese, 25 per cent higher than before lockdowns. The figures for the North East are striking similar to London, with the obesity rate of 29 per cent raising questions for the findings from the study. It turns out London has more obese 10 and 11-year-olds per head than any other part of England. Increases can be seen in younger children, too. In England, obesity among reception children has increased some 45 per cent; and in London, it has risen 53 per cent since 2019.

So when we look at hard data, rather than modelling, there’s no signs of London getting any thinner. The truth is, measuring the success of public health campaigns such as Khan’s ban on images of burgers is very difficult. But relying on opaque modelling and highly uncertain assumptions probably isn’t the way to do it.