During my schooldays, you'd hear people talking about "brain food" as if it were common knowledge that certain types of food made you clever. It was usually fish - although fish fingers didn't qualify - but sometimes it extended to "innards", things like heart, kidneys and yes, probably brain too. Of course, the talk about increasing brain power was a sauce to disguise food that no self-respecting youngster wanted to eat. It was a selling job like being told that carrots helped you see in the dark and greens made you grow tall.
So are there any proven links between what children eat and their ability to learn?
The Schools Health Education Unit, based in Exeter, is looking for such evidence. Angela Balding, who manages the unit's surveys of children's eating habits, believes that a definite link between diet and pupils'
performance will eventually be established.
But it's far from simple. Because how a child eats is part of the wider social picture of how a child lives. Isolating the influence of food is difficult. If a high-achieving, middle-class child, with lots of parental support, eats muesli for breakfast every morning, does this mean that eating muesli makes you clever? And if a child who struggles with their reading has a fizzy drink for breakfast, does that mean that fizzy drinks inhibit your learning capability? Or are the parents who can't organise breakfast the same people who aren't going to help their child to read?
It's not easy to detach food habits from all the other factors that mark out social advantage. Angela Balding says that the clearest objective information so far has come from a continuing survey of what children eat for breakfast, which has been matched against SATs results. And this seems to show a connection between eating a healthy breakfast and achieving a higher result.
These findings, which also apply to less well-off families, show that pupils who eat cereals for breakfast are more likely to do well; specifically, it is the muesli-style cereals, rather than the sugary variety, that are linked to the best results.
"You couldn't put your hand on your heart and say that the high results are definitely because of eating cereal," says Ms Balding. But she believes there is evidence of a connection. The boys' test results, in particular, suggest a link with eating habits.
Such a connection lies behind the Welsh Assembly's recent decision to offer free breakfasts to all primary pupils. The theory is that healthier cereals provide the most useful and long-lasting energy, unlike the sudden and short-lived rush from sugar-packed food, which is usually followed by sluggishness.
Supporting this, the impact of a chocolate and sugary drink breakfast on the mental alertness of teenagers has been tested by researchers at the University of Reading, who concluded that such a junk breakfast left 16-year-olds with the mental reaction times of a 70-year-old.
Their findings were backed up late last year at Oxford Brookes University.
Researchers looking into breakfast and obesity measured meals eaten by nine to 12-year-olds using the glycaemic index (GI), which charts the rise in blood sugar levels after eating different foods.
They found that the children who had a low-GI or less sugary breakfast such as bran, muesli, porridge or soya had a"significantly lower lunch intake", compared with those on a high-GI meal such as cornflakes, chocolate-flavoured cereal or white bread. The high-GI group were also more likely to feel hungry between meals and they ate more for lunch.
The Schools Health Education Unit has been asking children questions about their breakfast habits for the past 20 years and the findings cast an interesting sidelight on the wider question of diet and achievement.
About one in 20 primary schoolchildren leaves home without any breakfast so for them it's not a question of good or bad diet, but none at all. And there are others who only have chocolate for breakfast, who are among the most likely to report feeling too tired to learn.
More recently, says Angela Balding, there has been a big increase in the number of secondary schoolgirls missing breakfast - with the latest figures suggesting that 40 per cent of older schoolgirls do not have any breakfast, usually because of worries about putting on weight. If only the smell of grilled kippers were enough to persuade them to change their habits. For then they would be eating some of the healthiest food available for a hungry mind.
Top of the list of food for learning is oily fish, such as mackerel, herring, salmon or sardines, which provide the fatty acids that are meant to help children's brain development. The Food Standards Agency, the official government watchdog, says that the importance of fatty acids in brain development is now the "prevailing opinion". Research appears to back this up.
Scientists at the Institute of Brain Chemistry and Human Nutrition, at London Metropolitan University, have highlighted the lack of these types of oily fish in the diets of many young people - and linked this deficiency to problems with depression and mental illness.
Perhaps the Government should organise to give every pupil a piece of oily fish once a week in a follow-up to its school fruit scheme, which goes national this year. Kath Dalmeny of the Food Commission, which campaigns for healthier eating, commends the scheme which gives four to six-year-old pupils a daily piece of fruit. It could have "an enormous impact on public health in the longer term", she says. And, in the short term, it sets a pattern for healthier eating which will lead to healthier children who are more receptive to learning.
There are also researchers who want to take the link between "intelligence" and diet back even further - to before birth. A large-scale, long-term study in the United States established that babies with a heavy birth weight were more likely to have "higher" IQs when they grew up than low birth weight babies.
These bigger babies, with a head start even before they are born, owe their advantages to the diet and nutritional intake of their mothers, and once again the question of "brain food" is tangled up with lifestyles and social class. In an attempt to level the playing field, the Department of Health is proposing to launch Healthy Start this year. The initiative, an update of the 60-year-old Welfare Food scheme, aims to give pregnant women, mothers and young children in low-income groups better access to healthy food. Maybe we should all be changing to a diet of muesli, mackerel and water, so that we can feed our minds as well as our bodies. But is it still really the case that fish fingers don't count?
For information about the work of the Schools Health Education Unit, with advice for schools and young people, see www.sheu.org.uk