Eating too much fast food can affect pupils' intelligence, seriously undermining their academic ability, according to new research.
Kerri Tobin, of Vanderbilt University in Tennessee, studied the impact of a fast-food diet on the schoolwork of more than 5,500 10 and 11-year-olds. She found that those who ate higher-than-average amounts of junk food scored significantly lower than their classmates in a range of academic tests.
These findings applied even when pupils' backgrounds and poverty levels were taken into account.
Previous studies have shown that hunger affects pupils' achievement. As a result, schools on both sides of the Atlantic often run breakfast clubs for pupils. Such schemes have a positive effect on short-term memory and learning skills as well as on attendance.
Until now, however, no research has shown a conclusive connection between high-fat and sugary foods and low academic results.
Inspired by Jamie Oliver's campaign to expunge the Turkey Twizzler from school lunch menus, most British schools have removed unhealthy snacks from vending machines, tuck shops and dining halls.
But Dr Tobin decided to test whether eating habits out of school also had a significant impact on pupils' achievement. She therefore asked 5,500 primary pupils to record how many times a week they ate at fast-food restaurants such as McDonald's or Wendy's.
A total of 54 per cent had eaten fast food between one and three times during the previous week; 10 per cent had snacked on it between four and six times; and 2 per cent - or more than 110 pupils - said they ate four or more times daily.
Dr Tobin found no correlation between pupils' fast food consumption and their weight, or between their parents' income and the amount of fast food they ate.
But there was a direct correlation between how much junk food they ate and their scores in a series of literacy and numeracy tests.
Once other factors were taken into account, pupils who ate fast food between four and six times in a week scored 6.96 points below average in reading. Those who ate it daily dropped 16.07 points below average. And pupils who indulged three times a day dropped 19.34 points for reading.
A similar trend was noted in maths. Those eating fast food between four and six times a week scored 6.55 points below average. Daily junk-food led to a 14.82-point drop, and a three-a-day habit resulted in an 18.48-point drop.
Overall, higher-than-average consumption of fast food resulted in lower- than-average test scores: 12.79 points less for reading and 12.35 points for numeracy.
Dr Tobin suggests a number of explanations. "It is possible that the types of food served at fast-food restaurants cause cognitive difficulties that result in lower test scores," she said. "Alternatively, it is possible that the propensity to eat fast food is correlated with unobserved characteristics, like parental involvement in homework, which would also affect test scores."
It is also feasible that pupils eat fast food as a means of coping with low test scores, reversing the cause-and-effect pattern.
But, she insists: "Continued investment in school nutrition plans, and curricula designed to make pupils and parents aware of the academic consequences of their food choices, would be one positive step that schools could take."
But a spokeswoman for McDonald's insists that Dr Tobin's findings do not justify depriving pupils of fast food entirely. "The majority of our customers visit us two to three times a month," she said. "Given this, and the choice and variety on our menu, there is no question that McDonald's food can fit into a balanced diet."
Jamie wages US war
Celebrity chef Jamie Oliver is to take his war against Turkey Twizzlers across the Atlantic.
In a new programme, to be shown on the ABC network next year, the 33-year- old will attempt to improve the eating habits of US pupils.
Combining the formula used for his 2005 Channel 4 series Jamie's School Dinners with last year's Ministry of Food, the celebrity chef will encourage communities to make healthy, nutritious meals.
"This is, without question, the most important and challenging thing I'll ever do in my life," he said.