One of the聽stranger聽things聽I鈥檓聽known for among my friends is聽that I聽never get thirsty.聽Ever.聽Not after exercising, not聽even聽after a sauna.聽Unless I actively remind myself, I simply forget to drink.聽It鈥檚聽only in the聽extraordinarily聽hot聽UK聽weather of late that聽I鈥檝e聽begun to wonder聽whether聽this biological聽blind spot聽is聽contributing to my聽recent聽brain fog and general聽lack of聽cognitive sparkle.聽Is dehydration聽to blame, and if so,聽how much should聽I聽drink聽to聽keep my brain working at its best?聽
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,聽in聽one study,聽29 adults聽completed a bunch of cognitive tests on two occasions, each following 12 hours without food or water.聽Before one testing session,聽they聽were offered a聽500-millilitre聽bottle聽of water聽and were told they could drink as much as they wanted.聽People聽聽that required them to聽suppress a quick gut feeling and instead聽engage聽slower, more聽reflective thinking聽after they had drunk water.聽This result held regardless of聽how thirsty or聽motivated they felt.聽
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This聽was the first聽study聽to聽suggest聽that water might聽influence聽this聽ability, known as聽鈥渃ognitive reflection鈥�,聽but聽the study was small,聽so it is聽difficult to聽say why.聽One聽possibility, suggest the researchers,聽is that聽dehydration reduces聽blood flow to the anterior cingulate cortex,聽which is a brain聽region聽we use聽to聽inhibit聽actions.聽Dehydration聽can also聽increase聽blood levels of聽cortisol,聽a聽stress hormone聽that has been shown to聽interfere with聽decision-making.聽
In聽other聽studies, dehydration聽wasn鈥檛聽found聽to have聽a significant聽impact on聽cognitive聽performance, but participants聽reported聽that the聽. To investigate聽further,聽聽at Kings College London and his colleagues聽asked聽10 students聽to聽spend 90 minutes聽cycling on聽a聽stationary bike聽鈥� once聽wearing regular exercise wear and another time聽wearing a聽black bin聽liner聽next to their skin,聽followed by聽a聽chemical warfare protective聽suit聽and聽finally聽a tracksuit.聽鈥淭he idea was this was impermeable,聽so they would sweat lots,鈥� says Kempton. 鈥淚n [this] condition, they lost over 1 kilogram of weight in 90 minutes.鈥�
Although聽that聽sounds聽fairly extreme, Kempton reckons you might lose this amount of water on聽a hot day聽if you聽didn鈥檛聽drink. The students drank 500聽ml of water the night before the trial and another 500 ml just before they began exercising.聽The聽team聽then used fMRI to聽measure activity in the students鈥� brains during a聽test of聽executive functions like planning ahead and spatial聽processing.聽
The students performed聽no聽worse when聽dehydrated,聽but聽.聽While dehydrated, completing the task聽produced聽greater activation in their frontoparietal聽network, involved in attention and solving聽problems. This suggests that their brain was聽working聽harder to聽achieve聽the same result, says Kempton.听听
Harder work means increased metabolism and energy use.聽Kempton鈥檚聽hypothesis聽is聽that聽if dehydration uses up more energy, performance may eventually begin to suffer. 鈥淲e use the analogy of a car using up more petrol to go the same distance聽as聽a more efficient car 鈥� we would expect it to run out of gas faster,鈥澛爃e聽says.聽
Perhaps I鈥檝e聽been running聽low on聽gas lately, so to speak, trying to聽maintain聽my usual聽level of functioning with a聽parched and聽increasingly聽inefficient brain.聽But just聽how dehydrated聽might you need to be before performance聽actually declines?聽聽
础听聽found聽that dehydration can cause small but significant deficits in cognitive performance 鈥� particularly for tasks involving attention, executive聽function聽and motor coordination 鈥� when聽water loss聽exceeds聽2 per cent聽of聽body mass. For a 70聽kg person, that鈥檚 about 1.4 kg.聽
It鈥檚聽worth bearing in mind that not聽everyone is affected聽equally.聽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.聽Prolonged聽, including increased frailty, heart disease,聽poor聽oral聽health聽and worse recovery from surgery. It聽also appears to聽worsen聽symptoms of dementia, possibly because聽, and makes聽it聽harder to clear聽these聽proteins from the brain聽鈥撀爐wo processes implicated in the聽progression of dementia.聽
Young聽children are also聽especially聽vulnerable, due to immature renal development, extra physical exertion, quicker fluid loss through sweat and聽a reliance聽on adults to provide聽their聽drinks.聽Still,聽I had聽assumed, given the聽increased聽awareness of聽being well聽hydrated聽(you聽can鈥檛聽send your kids anywhere without a water bottle permanently attached to them)聽this聽wouldn鈥檛聽be a problem in the UK or other high-income countries. Yet聽聽suggest that聽many聽school children聽around聽the world聽.听听
In December, a study of聽9- and 10-year-olds in the UK found that聽17 per cent were dehydrated at the beginning of the school day, and that聽聽鈥� despite having free access to water.聽
This increase in dehydration correlated with聽worsening聽mood between morning and afternoon and was associated with poorer performance on memory tests.聽Several聽other聽studies have聽linked dehydration in children with聽. Small聽intervention studies聽have also shown that聽giving children a drink before聽a聽test聽can improve聽visual attention, visual聽memory聽and mood.听听
This makes me wonder聽whether聽my own聽defective聽thirst cues could be a problem for my whole family. The聽researchers聽behind the UK聽study聽argue聽that聽it鈥檚聽not enough for聽children聽to have聽access to water throughout the day, because聽they聽are overly reliant聽on external cues聽鈥搇ike聽parents and teachers reminding聽them to drink.听听
As for how much we should聽be drinking for maximum brain health:聽鈥淲e probably shouldn鈥檛 get too neurotic about this,鈥� says Kempton. 鈥淚t鈥檚 good to remember we also get water from food as well as drink鈥� but it鈥檚 definitely something to bear in mind.鈥澛�
We often hear聽that we should drink聽eight聽glasses, or 2 litres,聽a day, while for people aged 14 or over. In reality, there鈥檚 no single answer.
础听huge global study from 2022 of more than 5000 people aged between聽8 days聽and聽96聽years聽found聽that this blanket recommendation of聽eight聽cups聽. It found that聽water turnover 鈥� the amount entering and leaving the body each day 鈥撀爓as highest in聽men between the ages of 20 and聽30,聽averaging聽around 4.3聽litres聽of water聽a聽day, while聽in聽women aged聽25聽to聽60聽it averaged聽around 3.4聽litres.聽
Crucially, some of this value comes from water produced聽by聽metabolism,聽and聽in higher-income countries, around half of what is left聽can often be聽provided by food.聽
For a typical person in the聽US or Europe, that leaves a rough drinking requirement of around聽,聽according to聽聽at the University of Aberdeen, who led the study.聽Obviously, pregnancy, lifestyle factors,聽age聽and exercise, among other things,聽can shift these聽numbers substantially.聽
But聽given聽how trendy聽it鈥檚聽become to carry聽around聽enormous聽canisters of water,聽it鈥檚聽fair聽to say聽that anyone diligently draining one each day may be overdoing it. Children,聽on the other hand,聽may聽benefit聽from more聽encouragement聽from adults to drink regularly.聽
As for me,聽I鈥檓聽fairly certain聽I fall on the wrong side of the equation and that I should increase my own intake 鈥� and that of my children.聽Although聽perhaps聽our聽shared聽love of cucumber and strawberries, which are聽both more than 90 per cent water, is聽doing some heavy lifting.听听
I聽won鈥檛聽get too聽stressed聽about it, but聽I will be adding a few more glasses to my daily routine, in the hope of recovering a little of that cognitive sparkle.聽
