Hydration and mental performance

Water makes up nearly three-quarters of the brain's composition, so it makes sense that the brain needs to avoid losses to maintain optimum working performance. Interestingly, a group of athletes who lost two percent of their weight in fluid took a hit on their attention and decision-making skills. Researchers found a reduction in map recognition, grammatical reasoning, mental maths, and proofreading – this is the highest level of cognitive functioning, the so-called executive function. And the effect got worse as the athletes became more dehydrated.

While dehydration in young people leads to fatigue and low mood, the elderly have less cognitive reserve, and this can lead to a decline in cognitive performance. When researchers looked at the results of several studies, dehydration had a significant effect on attention and executive function.

Unlike the young, over a long period of time, this may not readily be reversed by adequate hydration, it may cause permanent damage to the brain. Dehydration makes the elderly more vulnerable to dementia, and speeds up cognitive decline if they already have dementia.

Memory performance often goes hand-in-hand with attention, and it seems that short-term memory and working memory are under particular threat from dehydration. This is how we remember the address we've just looked up or do sums in our head, rather than long-term memory of childhood experiences or what year we graduated and so on.

When university students sitting for exams were analyzed, it was found that those who had brought water in with them scored 5 to 10% higher grades than those without – this was after they had factored out expectations from previous coursework results.

A study from the University of Connecticut found that even a 1 - 2% dehydration caused feelings of low mood in a group of women tested with moderate exercise to bring on mild dehydration. Another found that feelings of self-esteem and vigor were negatively affected by a sensation of thirst when participants were deprived of water for 36 hours (although given food). These changes in mood were entirely reversed as soon as participants had rehydrated.

Hydration isn't just about water. Your body also needs salts (known as electrolytes such as sodium or potassium) to work optimally, and these can be lost through excessive sweat, fast breathing, or not replacing what you pee out.

If sodium levels in the blood become low, water is pulled into the body's cells, causing them to swell up. Brain cells are particularly at risk, and at certain levels of low sodium (hyponatremia), this may cause headaches, fatigue, confusion, irritability, seizures, and coma.

With longer-term hyponatremia, risk of problems with attention, steadiness in walking, risk of falls, and cognitive impairments can seep in. However, these can be reversed if the sodium is corrected slowly and safely. While this is the more dramatic end of things, it suggests that we should replace electrolytes alongside water to keep our brains in tip-top shape.