Toddlers brought up on a diet of junk food are more likely to make slow progress at school, say researchers.
They found diet in the preschool years has more influence on a child's ability to learn than the quality of school dinners.
The research team from the Institute of Education at London University and the Children of the 90s study at Bristol University noted that the government increased funding for school meals in England after a high-profile campaign in 2005 by celebrity chef Jamie Oliver.
Staff in schools that switched to a healthier menu reported improvements in children's behaviour, but the research team found the quality of school dinners made no difference to performance in Sats tests.
The team said: "Children who do poorly at school are more likely to have been affected by the food they ate many years earlier, rather than the chicken nuggets they had at lunchtime."
The study looked into the progress made by children in the last four years at primary school. It found the 25% of children who ate the most junk food at three were 10% less likely to achieve the expected improvement between tests at ages six to seven and tests at 10 to 11.
The research was based on data from the Children of the 90s study, which followed the development of 14,000 children since their birth in 1991 or 1992.
Parents were asked to record each child's consumption of food and drink at the ages of three, four and seven. Eating lots of junk food at three was associated with slower progress in primary school, but a poor diet at four and seven made little educational difference.
The study found the correlation for three-year-olds persisted even after adjusting for the social, economic and ethnic characteristics of the family.
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