Its damage to health is well known but obesity could take a heavy toll on a child’s grades.

At least six studies have found that boys and girls who are dangerously overweight do worse at school than others – with girls particularly vulnerable.

Researcher Anne Martin said the effect is big enough to make a difference to grades and urged teachers to think about the effects of obesity on their pupils. It is unclear why being overweight harms academic success but possible reasons include fat children being written off by teachers and obesity slowing the development of the brain. Bullying may also take a heavy toll, especially on girls.

Miss Martin, who is close to completing a PhD, found and analysed 14 pieces of research into the topic. Around half of the studies showed that children who became obese or who stayed obese did worse in maths over time. The studies also suggested that the effects aren’t instant – with obesity in primary school taking until secondary school to impact on achievement.

Girls may be particularly vulnerable, with a British study showing that those who were obese at 11 did less well in maths, science and English at 16. The effect was big enough to make the girls drop down from an average grade C, to a D, the UK Congress on Obesity heard.

Miss Martin, of Edinburgh University, said it is possible that girls are more psychologically scarred by the bullying that fat children often suffer.

Other reasons for the phenomenon include teachers writing off overweight children as lazy or naughty, poor diet and lack of exercise.

It is possible that obesity affects brain development – and also that the diabetes and sleep problems that can accompany obesity lead to youngsters missing more school than usual.

Miss Martin said the finding is important because a child’s performance in school affects their chances of going to university and the kind of job they get.

She says that headteachers need to realise that educating children on obesity could improve academic success, rather than simply waste valuable time.

However, it is not all bad news.  One of the studies Miss Martin analysed showed that obese youngsters who slim down end up doing better in maths than children who have never had a weight problem

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