In this week's Scrubbing Up, Dr Gavin Sandercock argues there has been a well-meaning but misguided focus on obesity, while the harm caused by inactivity has been overlooked.
We are used to seeing reports warning of an "obesity time-bomb". But the extent of the problem is often exaggerated.
Figures for 2010-11, from the National Child Measurement Programme, suggest 9% of five- to six-year-olds are obese.
That equates to 2.7 children in each class of 30. In 1990, it was 1.5.
Does a 20-year increase of just over one child per class seem like an epidemic?
Other data suggests childhood obesity levels are starting to plateau or even fall slightly.
Overweight and obesity levels among two- to five-year-olds stayed relatively stable at 25% for boys and 23% for girls between 2003 and 2013.
But public health's continual focus on food and obesity leaves the more harmful effects of inactivity overlooked and ignores the role of physical fitness in health promotion and monitoring.
Despite its absence from the title, "obesity" was mentioned 193 times in the 150-minute parliamentary select committee on The Role of Diet and Physical Activity in Health.
Dietary advice changes almost continually: eat less fat, ban junk food, tax fizzy-pop.
The Local Government Association has proposed a tax on "unhealthy" foods to enable local authorities to help overweight and obese children.
But are we missing the point?
Fitness not fatness
Healthy eating is important, but it is not the answer to obesity. Most interventions, such as breakfast clubs or free school dinners, may actually increase the calories consumed.
Arguably, inactivity is a better predictor of ill health than obesity.
Figures from the Physical Activity Statistics 2015 - British Heart Foundation (BHF) published this January show less than a fifth of children say they move enough - a figure that's still falling.
Shockingly, the BHF's own direct assessment of activity, using accelerometers, showed that none of the 11- to 15-year-old girls and only 7% of boys they measured actually did enough exercise.
All children and young people should engage in moderate-to-vigorous intensity physical activity for at least 60 minutes a day.
But I think a better approach would be to measure fitness.
Changes in activity levels often fail to have a big impact on body mass index (BMI). But it is very rare that anyone who becomes more active, for example by walking to school or work, or training for a fun-run, does not become and feel much physically fitter.
I routinely test the fitness of elite sports people, the public, cardiac patients and entire schools. Our research in schools shows decline in children's fitness.
What is needed is a wide-ranging set of changes to help children be habitually more active, more often.
If we measured fitness, as the chief medical officer recommended in 2009, we would see the problems that physical inactivity is causing and realise just how successfully some of the fantastic initiatives supported by Sport England, the BHF and UK Active really are.
BMI is a symptom of inactivity, but it should not be used as the last word in measuring success of interventions to increase physical activity.
Anybody of any size can benefit from being more active whether they see a change in their weight or not.