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Poor fitness is a bigger threat to child health than obesity

Not as fast as they used to be. Children running via Pan Xunbin/

The least fit ten-year-old English child from a class of 30 in 1998 would be one of the five fittest children in the same class tested today. These are the worrying findings of a new piece of research that has crystallised the need to focus on a sharp decline in fitness levels, not obesity, when it comes to improving children’s health.

Back in 2009, we reported an 8% decline in fitness of ten-year-old children from the borough of Chelmsford, Essex. At the time this was twice the global rate of fitness decline. The story got a lot of media attention and then chief medical officer Liam Donaldson proposed the introduction of fitness assessments to monitor children’s health. But unfortunately he resigned his post soon after and following a change in government, nothing happened and all went quiet. Until now.

This year, the trade body UK Active published its Generation Inactive report which contains five recommendations to improve children’s health through physical activity. One of these is a repeated call to use fitness measurements to follow trends in physical activity, evaluate trials and tell us more about our children’s health.

The next day our follow-up to the 2009 study was published. By testing another 300 ten-year-olds six years later, we confirmed two things. First, there simply is no obesity epidemic – at least not in the schools we visited. Less than 5% of pupils were obese and the average body mass index (BMI) was below 1998 values. This might have been a good news story if BMI was all we had measured; but our fitness test results told a different story.

Thinner, but less puff

A drop in BMI tells us only that the children are “thinner” but tells us nothing about what caused this change. BMI could be lower due to decreased energy intake (food), due to increases in energy expenditure (exercise), or both. One thing we do know is that children with a lower BMI usually do better on the 20m shuttle run used to test their fitness in this experiment because being lighter makes it easier to run and turn. Based on their BMI, we predicted that our 2014 sample would out-perform the relatively heavier children we had measured six years ago.

Yet despite a lower BMI, the 2014 children still couldn’t run as fast as their classmates from 2008. The overall rate of decline was 0.95% per year; faster than the 0.8% per year decline from 1998-2008.

Fitness has been declining even faster over the past six years than in the decade before. Girl’s fitness fell at twice the global average but our data showed boys’ fitness is declining three times faster in England than it is in the rest of the world.

Fitness on the slide for ten-year-olds. Author provided

Analysing pupils’ actual test performance (how many shuttles they run) shows just how big the fall in fitness from 1998 to 2014 is. In 1998, the average boy ran 60 shuttles (1.2 km) before stopping; in 2014, they ran only 33 (660 m). To put this in context, in 1998 the average boy could run a mile in 7 minutes 50 seconds but it would take boys today 9 minutes and 40 seconds. That’s nearly two minutes slower. Girls are also 1 minute 40 seconds slower than in 1998, and it would now take the average girl over ten minutes to cover a mile.

Our fitness data also told us why the BMI had gone down. By process of elimination, it could not be that children were expending more energy by being more active as this would have improved, or at least maintained their cardiovascular fitness. Instead, combining our BMI and fitness findings told us that children are eating less and doing less exercise.

Low activity levels won’t come as a surprise: national surveys repeatedly show an inactivity pandemic; however, the idea that children are eating less might. We purchase around 30% fewer calories today than 20 years ago and there is evidence we’ve been eating less and less since the 1970s. Given the current hysteria over sugar it’s worth mentioning that as well as eating less, the percentage of calories children get from sugar has also declined since the 1990s

BMI isn’t everything

English childhood obesity figures reported in the press mostly originate from the National Child Measurement Programme (NCMP). The way these figures are reported artificially inflates the obesity problem for two reasons. First, the NCMP itself uses a rather out-dated definition of obesity (which is not even allowed in some scientific journals). Second, headline figures usually combine “overweight” and “obese” BMI categories. Overweight is not a health problem, there is growing evidence that adults with a BMI classed as overweight are the most healthy.

Our study has shown that this continued reliance on BMI as the lone measurement of child health is not working. Yet again, we find ourselves calling for a rethink on how we monitor children’s health.

We agree with UK Active that there is an acute need to increase the physical activity levels of young people. Yet activity itself is notoriously difficult to measure. Fitness is the single most important indicator of someone’s health and can be measured safely and objectively in the general population. Perhaps most importantly, and unlike weight or BMI, fitness is very sensitive to changes in physical activity behaviour. You may know (or be) someone who has found it hard to lose weight, but have you ever met anyone who didn’t get any fitter when they started exercising?

The UK spent just nearly £9 billion on hosting the 2012 Olympics hoping to “inspire a generation” but we have no idea if this has had any effect on children’s health or fitness. The government is currently investing £150m annually through the primary school PE and sport premium but again, no one is evaluating whether this is going to have any impact on children’s health and fitness.

Six years since it was first proposed, the need to systematically assess children’s health-related fitness seems greater than ever. The need to drastically increase children’s physical activity levels is even more pressing but it is only through measurement and evaluation that we can see what works. UK Active has put it better than I ever could:

Measurement is the first step that leads to control and eventually to improvement. If you can’t measure something, you can’t understand it. If you can’t understand it, you can’t control it. If you can’t control it, you can’t improve it.

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