Feeding hungry children at school may stop their tummies rumbling in the classroom, but a study has found free breakfasts do not make much difference to learning.

Associate Professor Cliona Ni Mhurchu, of the University of Auckland, says about 55,000 children leave for school without having breakfast each day.

It is more common among Maori and Pacific Islanders living in deprived areas, and more common for girls than boys.

While the final paper in a study on the effects of two free school-breakfast programmes is still several months away, Professor Ni Mhurchu said the research indicated there was a significant decrease in children's hunger.

"Our study showed there was a significant difference when children were having the school breakfast in terms of how they rated their satiety."

But there were no other measurable results such as an improvement in school attendance, learning or behaviour.

It also found that rather than increasing breakfast consumption in those who needed it, many children stopped eating at home and started eating at school.

Professor Ni Mhurchu said similar results had been found in a large study in Wales which found an improvement in general nutrition but no improvement in learning.

"Children who have school breakfasts clearly have healthier breakfasts than children at home."

But, despite not finding any hard evidence, Professor Ni Mhurchu acknowledged that many teachers had said they did see educational improvements in children who ate breakfast.

"It's fairly complex to evaluate and while we didn't see an effect overall, I don't think we can dismiss them out of hand.

"I think there is potential for them to have an effect if children attend regularly and if you get the right children attending. I think the really tricky thing is to really identify those at-risk children and getting them attending."

That view was backed by a 2008 Red Cross evaluation of its Breakfast in Schools programme that found the free breakfasts were making a difference to the lives of students throughout the country.

Behaviour, concentration and learning ability all improved.

Three-quarters of the teachers in that evaluation felt the free breakfast had had a high to very high influence on students' learning capacity.

"Because I'm also a teacher's aid I have seen that the children's reading ability has very much improved," said one teacher.

Other benefits of the free breakfast included an improvement in children's general behaviour. Students became more friendly, helpful and polite.

"A child used to come with a pie and drink. He was angry and took a long time to settle in the morning. Now the breakfast and social time around the table has made or supported him to be a better boy."

A 1998 study by Harvard University researchers in the US also found that students who ate breakfast were "significantly more attentive in the classroom, earned higher grades in math, and had significantly fewer behavioral and emotional problems".

Professor Ni Mhurchu said a possible reason for the difference between her study's findings and anecdotal evidence was people's perceptions. As well, the study also had some limitations, as it included some children attending breakfast for only part of the week.

"When we did restrict our analysis to children who attended five days a week we did see an effect on attendance ... so I guess there's a possibility that if children dip in and out of the problem you can't really expect it to have a major effect."

Child Poverty Action Group researcher Donna Wynd, who published a favourable report on breakfast programmes yesterday, said the clinical trial was "just one study" in a vast body of research.

"The weight of the evidence in general from overseas is that these things do improve school attendance and academic performance," she said.

"Certainly, principals and teachers noticed the difference in the classroom, even if it's just behavioural improvement. Maybe that takes a bit of time to show through in attendance and academic performance."