The PE kit is washed and bagged, ready for a new term. But new research is about to reveal how important it is that your children get to use it. An important new study of UK schools indicates that pupils who do more physical education at school have significantly smaller waistlines. This is a significant contribution to the debate about whether it’s fitness or food we need to concentrate on if we’re seriously going to address the nation’s “obesity epidemic”.

It’s a subject we really do need to address: about 16 per cent of 6 to 15-year-olds are now obese, and doctors have warned that 20 per cent of boys and 33 per cent of girls could be clinically obese by 2020.

The study, supported by Cancer Research UK and to be published in the International Journal of Obesity next month, will show that boys in secondary schools that provide three PE sessions a week have waists about 3cm (1.2in) slimmer than boys at schools that provide one or two sessions. The study also showed that, while waist size for girls in high-PE schools was smaller, the difference was less pronounced.

The extent of this difference surprised the lead researcher, Professor Jane Wardle, the director of the Cancer Research UK Health Behaviour Unit at University College London, who says: “We were quite struck by the results.”

The study examined the progress, over five years, of nearly 3,000 schoolchildren from the age of 11 at 34 secondary schools in London. It found that over that period there were consistently lower gains in waist circumference in boys and girls from schools that had more PE. The difference in waist size between the children getting the most PE and the least PE was 4.5cm after just four years.

The research comes at an important time because the debate about whether children are getting enough sport at school has gone quiet recently. This is partly because schools appear to be on course for the government target that by 2010 all children should receive at least two hours of sport and recreation a week as part of the curriculum. A 2005-06 school sport survey found that 80 per cent of pupils in schools within “school sport partnerships” participate in at least two hours of PE and sport a week.

Critics, however, point out that the partnerships (government-fostered links between schools and sports colleges) cover only about half of schools, and half the sport recorded in the survey was outside curriculum time.

Margaret Talbot, the chief executive of the Association for Physical Education, says: “What we don’t know at the moment is exactly how many schools are providing good quality and active physical education within schooltime.”

The food-or-fitness debate has also become more subdued because recent research about childhood obesity has suggested that the amount and type of food that children consume, and the amount of activity they get outside school, may be more important than “formal” physical education. A government-backed project in Plymouth, called the Early-Bird study, appears to suggest that no matter how much exercise children are offered, they tend to find their own level of activity; if they don’t get much at school, they’re likely to make it up at home. The leader of this study, Professor Terence Wilkin, from Peninsula Medical School, Plymouth, believes it’s not our environment that determines activity but an in-built activity regulator, and in some children it is set differently from others.

But now the balance of evidence may be shifting. In May, research involving 5,500 children from the Avon Longitudinal Study of Parents and Children suggested that an extra 15 minutes a day of moderate or vigorous exercise halved the risk of obesity in 12-year-old children. And now the new Cancer Research UK study makes a convincing case that school exercise can make that crucial difference.

“It’s an interesting study that raises a few questions,” says Dr Ian Campbell, a GP specialising in obesity issues and chairman of the National Obesity Forum. “It would suggest that physical activity at school can be a useful way of tackling at least some aspects of childhood obesity, and would support the calls for greater priority to be given to PE lessons at school.

“Obesity is such a complex issue that it can’t be explained simply in terms of PE at school or any other isolated feature. But I do believe that schools hold one of the keys to tackling childhood obesity in that they present a fantastic opportunity for kids to learn about the importance of health, diet and activity, and can lead by example in the canteen and the PE class.”

One of the key reasons that Professor Wardle’s study has picked up different patterns from other studies into childhood obesity and exercise is that it looked at the children’s waist size and not their body mass index (BMI). Most studies use changes in BMI (a figure based on the ratio between height and weight) as an indicator of how children are putting on weight over time. But Professor Wardle was concerned that BMI measurements weren’t sensitive to the fact that exercise, while reducing fat, builds up muscles. Waist measurements are sensitive to this.

“Waist measurement is a much purer indication of fatness,” she says. It’s also important because studies have shown waist measurement to be a better indicator of the risk of coronary artery disease in adolescents than BMI.

Another reason to take it seriously is that it picks out the long-observed difference to exercise at school between boys and girls. Though the girls’ increase in waist size over the five years was less with more school exercise, the change was not large enough to be statistically significant, unlike the pronounced pattern in the boys’ waist sizes.

“We suspect that this is because girls tend to go off games much earlier,” says Professor Wardle. A big study by the Institute of Youth Sport at Loughborough University in 2005 found that girls hated school PE because they disliked the kit (30 per cent) and were self-conscious about their bodies (40 per cent). One girl in five said that she took part in PE only because she had to.

The new study doesn’t provide all the answers on how we prevent young people becoming overweight. But it does tell us that we shouldn’t let the Government off the hook on school exercise. And if that gym kit does get lost, dirty or worn out during the term, it’s better than it not being used at all.

How much PE do children need?

The Department of Health recommends that children between 5 and 18 participate in at least one hour of moderate-intensity exercise (eg, swimming, cycling) every day. Norwegian scientists in The Lancet last year said that children engaging in 90 minutes of moderate exercise a day are less likely to have heart disease, high cholesterol or high blood pressure. A 2006 study at Glasgow University found that the recommended level of adult exercise – 30 minutes, three times a week – isn’t enough to have any effect on weight control in kids.

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