WHAT single piece of advice would improve most children’s health quickly and cheaply? “Drink more water,” the Olympic sprinter Darren Cambell told a recent debate on children’s nutrition and activity.

Dr Jonathan Darling, an honorary consultant in paediatrics and child health at St James’s University Hospital in Leeds, says: “Of all the things we could do for children’s general health, drinking water is one of the biggest, and as important as food and exercise.”

Jon Owen Jones, the MP for Cardiff Central, who this week will present a School Vending Machines Bill, is just as unequivocal: “The consumption of manufactured drinks rather than water is at least as harmful to children as high-fat and sugar foods.”

An unusual consensus, perhaps, but chronic dehydration in children is a widespread problem, with sometimes serious health implications. Accor- ding to Darling, conditions linked to underhydration in children include constipation, bedwetting, urinary-tract infections and kidney stones, as well as fatigue and irritability.

As Trevor Brocklebank, a former reader in paediatric nephrology at St James’s, explains: “Children need it to convert food to energy quickly and efficiently. If deprived of it, their physical and mental responses rapidly slow down.”

Dehydration is a particular problem among primary school children. A government survey found that fewer than half of primary children drank water at school, falling to a fifth in secondary. Anne Edwards developed a bad urinary-tract infection when she was five; although it cleared up with antibiotics, the problem recurred. The urologist nurse put this down to lack of fluids, and Anne now drinks most of her prescribed 1.5 litres of water a day at school. “The trouble is that school staff have no idea how much water children need, and don’t think it that important,” says Anne’s mother.

The Department of Health has now set up a pilot scheme under which it will install water coolers into 40 schools across two regions this month. Several studies have shown how dehydration adversely affects mental performance — one of the most recent of these found that children’s cognitive ability deteriorated by 10 per cent when they were thirsty, and that this was most apparent in the afternoon.

Peter Carpenter, the head of Darras Hall school, near Newcastle, introduced water bottles in the classroom a year ago: “Generally, the children are concentrating better. It’s especially noticeable in the afternoon, when there is no break between lessons. The curriculum in the second half of the day is definitely more successful.”

The school has five mainsfed water coolers in the corridors. Each cost £300, paid for by the PTA. The company that supplies the coolers also gives each child a bottle with a stopper-valve top. The children bring the bottles in full from home each morning and keep them on their tables during lessons so that they can drink when they want. At lunchtime, class monitors refill the bottles from the coolers.

Children’s bodies contain a higher percentage of water than those of adults, and lose it through sweat more readily. So it is no surprise that, by drinking more, their physical activity improves. Sam Howells, a senior sports physiologist at Lilleshall National Sports Centre, says children have a higher “cost” per activity than adults “and produce more metabolic heat. This heat must be dissipated through sweat, resulting in excessive loss in body fluid and associated electrolytes, which can cause cramp.”

In other words, dehydrated children will be less physically active. The dehydration need not be extreme either. “At just 2 per cent dehydration there is discomfort; by 4 per cent any activity will require more effort and they will feel lethargic, so they won’t want to exercise,” Howells says.

Despite water’s multiple benefits — and the negligible cost of providing it — schools worry about spillage, and children constantly tripping off to the loo. Nickie Brander, who set up the Water is Cool in School campaign after her daughter developed a urinary-tract infection from dehydration at school, suggests that children don’t get “anything like the amount of water they need” from drinking taps in “smelly toilets” or fountains. “

They need to glug it back from bottles or cups in large quantities. Children like having special water bottles and tend to be very responsible.”

Parents of younger children worry about bedwetting but, ironically, this is often a consequence of dehydration. Typically, children become dehydrated at school, then compensate in the evening by downing pints of fluid, which escape at night. School nurses report an increase in the number of toilet-trained children who start to wet the bed in their first year of full-time school.

So how much should a child drink? Six to eight cups a day, says Brocklebank.

www.waterinschooliscool.co.uk