BEWARE the banana louts. Healthy school lunches, rich in fruit and vegetables, may make children rowdy in the afternoons if they are not carefully supervised, government school food advisers have found.

The study by the School Food Trust, chaired by Prue Leith, the restaurateur, found children who had eaten a healthy lunch were more than three times as likely as those who ate less healthily to become boisterous.

It challenges many assumptions about the differences in the way that healthy and junk food affect children’s attentiveness. The increased boisterousness was seen only when the children were left to study among themselves in small groups in after-lunch classes. If the teacher was directly controlling the class, by contrast, the healthily fed children were far more alert and attentive than the other group.

Judy More, a child nutritionist based in Chiswick, west London, said: “A good meal will make their blood sugar levels steady, meaning they concentrate better when working with a teacher. But with group work they are being stimulated by each other and are simply acting as children do.”

The healthy alertness of well fed children differs from hyper-activity that can be brought on by chemicals contained in fizzy drinks, sweets and cakes. While mental alertness can be channelled into concentration, hyperactive children are often incapable of finishing tasks.

The School Food Trust findings add a new dimension to research showing that healthy school lunches, rather than large amounts of chips and soft drinks, make children’s minds work better as well as improving their long-term health.

Despite the benefits of eating fruit and vegetables, the new study suggests the effect on behaviour is more complex than simply calming children down.

The researchers studied 146 children at six primary schools in Sheffield, observing them over 12 weeks in the lesson after lunch to measure levels of concentration or disruptive behaviour.

The study found that children who had eaten the improved menu “were 5.4 times as likely to be on-task in the teacher-pupil setting compared with the control schools. They were also 3.6 times as likely to be off-task in the pupil-pupil setting”. The researchers say that despite this apparently contrary result, the overall findings demonstrate the benefits of healthier school meals.

The research was consistent with the anecdotal evidence from teachers that pupils are more on-task following a healthier lunch: “Generally increased arousal in pupils who have eaten a healthier lunch may help to explain the increased off-task behaviours when pupils were being asked to work together.”

Leith, a food writer as well as one of Britain’s best-known cooks, is a prominent campaigner for improved school food, along with Jamie Oliver, the celebrity chef.

Last month Leith warned that some of the government’s most expensive new schools were being built without proper kitchens, making it impossible to provide pupils with a healthy lunch. Too much prominence, she said, was being given to deep-fat friers.

More said that whatever effect food may have on children’s paying attention in class, this was not the main reason to encourage them to eat healthily: “It will be fantastic for their long-term wellbeing and, compared with that, the effect on their behaviour does not really matter a fig.”

The government is trying to encourage children to eat more healthily to help to slow the increase in obesity.

It has introduced tighter nutritional guidelines for school meals. Cooking lessons will become compulsory and schools will be expected to introduce guidance on the nutritional content of packed lunches.

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