I felt a sense of excitement when I huddled in the school hall with my fellow “new parents”. What would the headteacher of my local primary school have to say on this auspicious occasion? The messages of comfort came thick and fast, but one of them surprised me: “Don’t worry, we know how important water is for learning, we’ll make sure your child drinks enough.”

Should I have been worrying about that? Apparently I should, at least according to the trainee teachers I surveyed later.

More than a third (35 per cent) thought their brains would shrink if they drank less than six to eight glasses of water a day. That’s a scary idea but one, like every neuromyth, that began with scientific fact.

Serious dehydration increases the concentration of sodium in the blood and can produce a reversible shrinkage of the brain. In recent years, this fact was graphically illustrated when a man in Japan tried to commit suicide by overdosing on soy sauce. As with more common cases of serious dehydration, unusual pressure gradients can develop between the brain and the rest of the body. That means sodium levels need to be restored slowly and carefully to avoid water entering the brain itself. The medical team thus had plenty of time to image the patient’s brain and note the striking extent of brain shrinkage. Three weeks later, after appropriate treatment, the man’s brain was shown to have returned (mostly) to its original dimensions.

While such cases of extreme dehydration are unusual, it has been shown that even moderate dehydration can reduce our ability to think. Thankfully, like other animals, we have a sophisticated system that should tell us when we need water - we get thirsty.

To consistently resist or ignore such an impulse can produce voluntary dehydration. But is there any evidence that pupils are ever at risk from this? Yes, but you have to go a long way to find it: at the lowest point of the Earth’s surface, in one of the hottest places on our planet.

A study of children in the Dead Sea region showed evidence of voluntary dehydration and reduced cognitive function. Beyond that, there is no evidence of it in schools, except after exercise or in some instances of unusually hot weather.

So where has this six to eight glasses of water come from? Who is to blame for the interrogations after school about fluid intake and the numbers of children presently clutching at their water bottles like lifebelts?

A possible source is Fredrick J. Stare, the nutritionist, whose obituary stated he was “an early champion of drinking at least six glasses of water a day”.

The six to eight figure features explicitly in a text co-authored by Dr Stare, in which the authors appear unworried by the dehydrating effects of alcohol: “How much water each day? This is usually well regulated by various physiological mechanisms, but for the average adult, somewhere around six to eight glasses per 24 hours and this can be in the form of coffee, tea, milk, soft drinks, beer, etc ... ”.

The same review that identified the source of the six to eight myth also concludes: “such large amounts are not needed” in the case of healthy individuals in a temperate climate leading a generally sedentary existence.

Despite headlines such as: “Water improves test results”, it is (sadly) untrue that the more you drink the more you learn. And, in a study of adults, drinking water when not thirsty was shown to decrease the ability to think. Indeed, in the extreme, drinking too much water can be as dangerous as drinking too little.

Thank goodness then that adults and children are usually aware of when they need to drink, unless a medical condition or developmental disorder is involved. Rather than constantly monitoring how much children are drinking, it seems more prudent to ensure access to good quality water, and to encourage children to drink when thirsty, unless it’s unusually hot or they have been exercising.

Raising achievement by encouraging children to drink water is such a wonderfully inexpensive, relatively effortless and attractive idea. Perhaps it’s no wonder that the thirst for knowledge is sometimes confused with the thirst for water. But, unless the current makeover of school dinners includes massive amounts of soy sauce, it seems unlikely that schools will ever be guilty of shrinking pupils’ brains

Paul Howard-Jones is a senior lecturer in education and co-ordinator of the Centre for Psychology and Learning in Context at the Graduate School of Education, University of Bristol.

REFERENCES

Machino T. and Yoshizawa T., Brain Shrinkage Due to Acute Hypernatremia, Neurology (2006) 67, 880.

Rogers P. J., Kainth A. and Smit, H. J., A Drink of Water Can Improve or Impair Mental Performance Depending on Small Differences in Thirst, Appetite (2001) 36, 57-58.

Valtin H., Drink at Least Eight Glasses of Water a Day. Really? Is There Scientific Evidence for “8x8”? American Journal of Regulatory Comparative Physiology (2002) 283, 993-1004.