James' World 2

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One of the stranger things I’m known for among my friends is that I never get thirsty. Ever. Not after exercising, not even after a sauna. Unless I actively remind myself, I simply forget to drink. It’s only in the extraordinarily hot UK weather of late that I’ve begun to wonder whether this biological blind spot is contributing to my recent brain fog and general lack of cognitive sparkle. Is dehydration to blame, and if so, how much should I drink to keep my brain working at its best?

Water is, needless to say, essential for the brain. It helps neurons transmit electrical impulses, delivers vital nutrients to power cells and flushes out toxins while we sleep, among other things. Unfortunately, pinning down its exact effect on cognition is challenging, partly because most studies on hydration and brain function are fairly small. Nevertheless, together they offer some intriguing clues.

For instance, in one study, 29 adults completed a bunch of cognitive tests on two occasions, each following 12 hours without food or water. Before one testing session, they were offered a 500-millilitre bottle of water and were told they could drink as much as they wanted. People scored better in tests of judgement and decision-making that required them to suppress a quick gut feeling and instead engage slower, more reflective thinking after they had drunk water. This result held regardless of how thirsty or motivated they felt.

This was the first study to suggest that water might influence this ability, known as “cognitive reflection”, but the study was small, so it is difficult to say why. One possibility, suggest the researchers, is that dehydration reduces blood flow to the anterior cingulate cortex, which is a brain region we use to inhibit actions. Dehydration can also increase blood levels of cortisol, a stress hormone that has been shown to interfere with decision-making.

In other studies, dehydration wasn’t found to have a significant impact on cognitive performance, but participants reported that the tasks felt more effortful when they were dehydrated. To investigate further, Matthew Kempton at Kings College London and his colleagues asked 10 students to spend 90 minutes cycling on a stationary bike – once wearing regular exercise wear and another time wearing a black bin liner next to their skin, followed by a chemical warfare protective suit and finally a tracksuit. “The idea was this was impermeable, so they would sweat lots,” says Kempton. “In [this] condition, they lost over 1 kilogram in weight in 90 minutes.”

Although that sounds fairly extreme, Kempton reckons you might lose this amount of water on a hot day if you didn’t drink. The students drank 500 ml of water the night before the trial and another 500 ml just before they began exercising. The team then used fMRI to measure activity in the students’ brains during a test of executive functions like planning ahead and spatial processing.

The students performed no worse when dehydrated, but their brain activity told a different story. While dehydrated, completing the task produced greater activation in their frontoparietal network, involved in attention and solving problems. This suggests that their brain was working harder to achieve the same result, says Kempton.

Harder work means increased metabolism and energy use. Kempton’s hypothesis is that if dehydration uses up more energy, performance may eventually begin to suffer. “We use the analogy of a car using up more petrol to go the same distance as a more efficient car – we would expect it to run out of gas faster,” he says.

Perhaps I’ve been running low on gas lately, so to speak, trying to maintain my usual level of functioning with a parched and increasingly inefficient brain. But just how dehydrated might you need to be before performance actually declines?

A 2018 analysis of 33 studies found that dehydration can cause small but significant deficits in cognitive performance – particularly for tasks involving attention, executive function and motor coordination – when water loss exceeds 2 per cent of body mass. For a 70 kg person, that’s about 1.4 kg.

It’s worth bearing in mind that not everyone is affected equally. Older people are particularly sensitive to dehydration, in part because our sense of thirst diminishes with age, and kidneys can become less efficient at conserving water. Prolonged dehydration in older adults is associated with several conditions, including increased frailty, heart disease, poor oral health and worse recovery from surgery. It also appears to worsen symptoms of dementia, possibly because dehydration can cause proteins to misfold, and makes it harder to clear these proteins from the brain – two processes implicated in the progression of dementia.

Young children are also especially vulnerable, due to immature renal development, extra physical exertion, quicker fluid loss through sweat and a reliance on adults to provide their drinks. Still, I had assumed, given the increased awareness of being well hydrated (you can’t send your kids anywhere without a water bottle permanently attached to them) this wouldn’t be a problem in the UK or other high-income countries. Yet several studies suggest that many school children around the world regularly arrive at school dehydrated.

In December, a study of 9- and 10-year-olds in the UK found that 17 per cent were dehydrated at the beginning of the school day, and that this increased to 40 per cent by the end of the day – despite having free access to water.

This increase in dehydration correlated with worsening mood between morning and afternoon and was associated with poorer performance on memory tests. Several other studies have linked dehydration in children with poorer attention, processing speed, accuracy and concentration. Small intervention studies have also shown that giving children a drink before a test can improve visual attention, visual memory and mood.

This makes me wonder whether my own defective thirst cues could be a problem for my whole family. The researchers behind the UK study argue that it’s not enough for children to have access to water throughout the day, because they are overly reliant on external cues – like parents and teachers reminding them to drink.

As for how much we should be drinking for maximum brain health: “We probably shouldn’t get too neurotic about this,” says Kempton. “It’s good to remember we also get water from food as well as drink… but it’s definitely something to bear in mind.”

We often hear that we should drink eight glasses, or 2 litres, a day, while some National Health Service websites recommend as much as 3 litres a day for people aged 14 and over. In reality, there’s no single answer.

A huge global study from 2022 of more than 5000 people aged between 8 days and 96 years found that this blanket recommendation of eight cups seldom matches our actual needs. It found that water turnover – the amount entering and leaving the body each day – was highest in men between the ages of 20 and 30, averaging around 4.3 litres of water a day, while in women aged 25 to 60 it averaged around 3.4 litres.

Crucially, some of this value comes from water produced by metabolism, and in higher-income countries, around half of what is left can often be provided by food.

For a typical person in the US or Europe, that leaves a rough drinking requirement of around 1.5 to 1.8 litres, according to John Speakman at the University of Aberdeen, who led the study. Obviously, pregnancy, lifestyle factors, age and exercise, among other things, can shift these numbers substantially.

But given how trendy it’s become to carry around enormous canisters of water, it’s fair to say that anyone diligently draining one each day may be overdoing it. Children, on the other hand, may benefit from more encouragement from adults to drink regularly.

As for me, I’m fairly certain I fall on the wrong side of the equation and that I should increase my own intake – and that of my children. Although perhaps our shared love of cucumber and strawberries, which are both more than 90 per cent water, is doing some heavy lifting.

I won’t get too stressed about it, but I will be adding a few more glasses to my daily routine, in the hope of recovering a little of that cognitive sparkle.

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