But research has now found that if children are given bad diet as young as three the damage has already been done by the time they go to school.
Studies showed that pupils who had been fed processed grub as toddlers were the worst behaved in class and performed the worst in tests.
The findings emerged from a major study by the University of London's Institute for Education.
The probe, part of the Bristol Children of the 90s medical research project, looked at data from 14,000 children.
It found that those on a junk food diet aged three were less likely to achieve the expected levels of improvement between six and ten.
"We are confident that this is a robust association. It indicates that early eating patterns have effects that persist over time, regardless of later changes in diet," The Sun quoted Dr Pauline Emmett, a nutritionist from the University of Bristol, as saying.
"So it is very important for children to eat a well-balanced diet from an early age if they are to get the best out of their education," Emmett added.
The study showed that a child's diet at a later age has less impact on their school performance.