How much difference does a child's diet make to how well they perform at school? A great deal, most teachers would instinctively reply.
The evidence is in front of them every day. There's little Hannah, yawning and listless, with no breakfast inside her. And young Steve, off his head on the bottle of fizz, salty MSG crisps and sugary cakes that constituted his midday meal. It won't be long before he's plunged into lethargy, too.
Neither of them can concentrate; and children who can't concentrate, can't learn.
Never mind obesity, rotten teeth, malformed bones and stunted growth. It may be that the inadequate fare consumed by many British pupils does most damage to their brains and thus their behaviour. Yet nutritional guidelines rarely mention behaviour, let alone academic performance and much of the evidence on the effect of food on the brain relies either on animal experiments or on small trials involving children or young people who have special needs.
Now, however, the first large-scale evidence is emerging that diet does affect behaviour and performance. Results from the first two years of a ground-breaking experiment in Hull suggest that offering free, nutritionally-balanced lunches to all young children in the city's primary and special schools has not only had a calming effect but also kept children alert all day.
Since April 2004, pupils have been able to choose whether to have healthy meals or to continue with packed lunches. (Free breakfasts and healthy afternoon snacks are also available.) In the second year, of the 24,000 pupils eligible, nearly two-thirds have opted for the healthy meals, at a time when national school meal take-up is plummeting.
And researchers have found that the greater alertness and staying-power of the children eating school meals is reflected in their academic performance.
In letter recognition tests carried out on a sample of 300 children at four points in a school day, the performance of children taking packed lunches deteriorated between lunch and home-time as their concentration flagged, while the performance of children taking healthy meals improved.
"The ethos, attendance and behaviour have been transformed in this school over the past three years," says Claire Pattern, head of Maybury Primary, where more than half of the 234 pupils are eligible for free school meals and take-up of meals has risen from 60 per cent to 95 per cent.
She singles out improved behaviour at lunchtime - children not only have much healthier food but also eat more formally, with staff, at tables - and a much improved attitude to school in the afternoon. "We have 20 per cent of children with chronic social and emotional problems," she says. "But if you came into the dining hall today you wouldn't realise we had any. The school is a calmer and more pleasant place," she adds. "I'm no longer going round in jackboots, firefighting all the time."
At Paisley Primary, Robin Petch, the headteacher, is a fellow enthusiast of the scheme. At first, take-up of school meals dropped dramatically as children rebelled at the unfamiliar food. Then parents decided to take advantage of the bargain and stand up to them.
Now, it has risen again to more than half of the pupils and teachers find that children are more alert and less likely to complain of hunger in the afternoon. Robin Petch stresses the importance of the free breakfast club in changing the habits of children in families with chronically poor attendance.
"They used to come in bleary-eyed and late, having been dragged straight out of bed," he says. "Now they're bright-eyed and involved from the start of the day."
Lisa Gatenby, nutritional adviser to the scheme, has been testing the contents of a sample of packed lunches and checking how much of the healthy school meals children eat. She found that the food consumed by the children had improved over the first two years, although some nutrients, such as iron, still remained below the minimum recommended level. (Beef stew is going back on school menus to try to put that right.) Children on packed lunches were actually eating more food than the others (and thus more iron) but far too much sugar, fat and salt. The calorie content of their lunches averaged 125 per cent of the recommended guidelines for lunch - and saturated fat levels nearly twice the recommended maximum.
Hull decided to introduce its three-year Eat Well, Do Well scheme both to raise educational attainment and to reduce obesity in this deprived city of 250,000 people.
The council has languished at or near the bottom of league tables ever since the unitary authority was formed in 1996, its affluent suburbs having been hived off to neighbouring East Yorkshire. It is too early to say if the scheme is having an impact on key stage 2 test results.
Professor Derek Colquhoun and his team at Hull University ignored them when assessing the impact of the scheme because he considers them unreliable.
But it seems clear that pupils' greater concentration throughout the day will work its way through to better test and exam results - if the scheme continues.
It was started by the city in 2004 when it was under Labour control. Now, the Lib Dem-controlled administration says it cannot afford to pay the extra Pounds 3 million cost of free meals for all when the initial three-year phase ends next month.
Although the quality of meals will remain the same, heads fear that take-up will fall once charging starts again, jeopardising the progress that has been made.
"I cannot understand why we are even asking the question why we give children - especially in deprived areas - free, healthy meals," says Robin Petch.
"Just as the evidence shows it's beginning to work, it seems such a shame to lose it."
SOUR SIDE OF SWEET FOODS
Huge changes in children's diets over the past 50 years have not been systematically assessed for their effects on the brain, say researchers.
The Government pumps millions into schemes to improve school behaviour, for instance, but does not invest in research to investigate the role of junk food in poor conduct - and in the dramatic increase in conditions such as dyslexia, hyperactivity and autism.
"Obviously there are many factors behind these problems," says Dr Alex Richardson, senior researcher at Oxford University's department of physiology, anatomy and genetics and co-director of the Scottish-based Food and Behaviour research group (FAB). "But to deny the role of nutrition completely is indefensible." She and the other experts at FAB are concerned by the effects of too much sugar in the diet, which leads to a blood-sugar yo-yo that leaves the brain in a fog.
Perhaps most worrying of all for brain health is the change in the fats people eat. We get too little omega-3 oil, contained in fatty fish and fish-oil supplements, (essential to brain function and "tragically lacking"
in most modern diets says Dr Richardson). And we consume too many toxic fats - hydrogenated and trans fats - in food such as cakes, pastries, popcorn, burgers and chips. These make the membranes of the brain less flexible and affect neurotransmitters.
More randomised trials with a control group taking a placebo are needed, says Dr Richardson. Her Oxford-Durham study with dyspraxic children (2005) showed that pupils given omega-3 made faster reading and spelling progress, had better attention and memory and less disruptive behaviour than the control group over a three-month period.
They are what you feed them - how food can improve your child's behaviour, mood and learning by Dr Alex Richardson. HarperCollins, pound;12.99.
BEST OF THE BRAINFOODS
What Hull primary children can get for free - a typical school day's menu.
This is the fare that keeps pupils calm, alert and learning all through the day.
* Breakfast: Porridge, Weetabix or Rice Krispies - low in sugar and salt, high in starch for long-lasting fuel.
Semi-skimmed milk - low in fat but high in calcium and protein.
Fibre-rich white bread with low-sugar jam.
Orange or apple juice, high in vitamin C.
* Mid-morning: Piece of fresh fruit or carrot.
* Lunch: Salmon fishcakes (brain food - protein and omega-3 oils) or pork loin with apple stuffing (protein and vitamins) with creamed potatoes and mixed vegetables (starch, vitamins and folic acid).
Or a jacket potato and baked beans (starch, vitamin C, protein, and iron).
Full salad bar always available.
Semolina pudding (starch, protein, calcium) with orange sauce. Home-made, wholemeal bread.
* Afternoon snack (if staying for after-school club): Two digestive biscuits, fruit, milk or fruit juice.
Cost to pupils: nil.