There is a narrative that runs through health discourse that has been repeated so many times that no one really thinks about it. In the US and UK, obesity prevalence was fairly constant until about 1980, then started to skyrocket. The burning question is: what changed around that time to cause such a seismic shift?
Spurious correlations abound. Was it increased seed oil consumption? The push towards low-fat diets? Breakfast cereals replacing bacon and eggs? The rise of ultra-processed foods? High-fructose corn syrup? Blondie releasing their seminal album Parallel Lines?
All are used to sell a particular health narrative. Everything was fine before [insert reason of your choice], so if we cut that out, we’ll go back to being fine again. We know correlation is not causation, and that obesity is a fiendishly complex problem, but we are still suckers for a convincing story.
The full obesity story
Many of these stories have been used to sell fad diets over the years (although my new wave synth pop exclusion programme never really caught on). Recently, they have started to significantly influence food policy, as Make America Healthy Again attempts to crack down on corn syrup and seed oils. But at the heart of all these stories is a misunderstanding of what the obesity statistics actually mean.
When you think about it, looking at changes to the population prevalence of obesity is a strange way to measure things. It tells you the number of people who cross an arbitrary threshold of BMI. That’s a bit like trying to understand the education system by measuring how many geniuses there are. It gives you some information, but not enough to make a sound judgement.
If we look at how the average population BMI changes over time, that might provide a more complete picture. And curiously, when we do this, we find that the average BMI steadily increased from about 1900 until 2000. There was no big change in 1980, just a slow, century-long rise as food availability improved and extreme poverty fell.
Don’t blame Blondie
So why the early 1980s jump in obesity prevalence? Well, the BMI distribution in a population roughly follows a standard bell curve. As average BMI increases, that bell curve shifts along to the right. The obesity cut-off is set at a BMI of 30, and up until 1980 that cut-off was sat on the thin tail of the curve, not changing very much.
But just as ‘Hanging on the Telephone’ was reaching the top of the UK charts, the curve reached a point where the steep right-hand side of the bell was starting to break through the obesity cut-off line, meaning rates rose rapidly.
It is worth noting the cut-off for obesity was set at 30 by the WHO simply because it is a nice round number. If it had been set at 28, the rapid rise of obesity would have happened in 1973 and we’d have been blaming roller discos and glam rock. If it had been 32, we’d be blaming mobile phones and acid house.
Obesity is still an issue that needs addressing, but unless we understand what the numbers mean, we will continue to be drawn into false narratives. When these false narratives start to influence policy, we probably need to act.
And putting to bed the idea that the world before 1980 held some magical health secret would be a good place to start.
Anthony Warner, development chef at New Food Innovation
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