Dehydration really can disrupt brain function

I often advise individuals to avoid dehydration at least in part because I’ve noticed how it can provoke lethargy, particularly mental lethargy. There is some evidence which supports this observation, in the form of studies in which individuals have been made dehydrated through exercise, sometimes in heat. Such studies have generally found brain function is impaired in such situations. The problem with these studies, though, is that we can’t be sure if it’s the dehydration, or the heat, or the exercise or a combination of these things that is responsible for a dropping off in mental function.

I was therefore interested to read the summary of a recently published study [1] in which men were subjected, on separate occasions, to each of three test conditions:

1. 40 minutes of walking on a treadmill with quite a steep incline (5 per cent) at a speed of 5-6 km/hr in a temperature of about 28 degrees centigrade (about 82 Fahrenheit). The men were also treated with the drug frusemide – a diuretic that speeds dehydration.

2. Same conditions as above, but instead of being given frusemide, the men were treated with a placebo.

3. Same conditions as above, while the hydration status of the men was maintained (in other words, they were not allowed to become dehydrated).

Basically, conditions 1 and 2 were designed to induce dehydration of two severities (the one with frusemide being more severe), and condition 3 was designed to act as a control (where individuals expended about the same amount of energy in the same heat but did not suffer dehydration). The study subjects were subjected to tests of mental function, fatigue and mood.

Dehydration was found to reduce vigilance and working memory (the ability to actively hold information in the mind needed to do complex tasks such as reasoning, comprehension and learning). Fatigue during exercise was worse during dehydration. Measures of tension and anxiety were greater too.

What this study shows is that dehydration does indeed have the capacity to affect brain function and mood, and therefore it pays to stay topped up with water. Our requirements for water vary according to a variety of factors including weather, temperature, how active we are, how much we sweat and how much water we get via what we eat. A good guide, in terms of water intake, is to drink enough to keep urine pale yellow in colour throughout the course of the day.


1. Ganio MS, et al. Mild dehydration impairs cognitive performance and mood of men. British Journal of Nutrition. 7 June 2011 [epub]

13 Responses to Dehydration really can disrupt brain function

  1. Feona 8 July 2011 at 6:54 pm #

    I agree with Deborah and Wendy – and it’s interesting that all this emphasis on drinking water came along just as the bottled water industry took off! The point is to take in enough fluid, which can come from where you like, including tea, coffee, fruit juice and the liquid content in food. As for flushing out toxins – yes indeed, that’s what your kidneys are for.

  2. john 8 July 2011 at 7:15 pm #

    Are we not supposed to at least drink 8 glasses of water whether in the summer or winter, especially in the summer. Coffees, teas and other drnks do not count. John Borg

  3. roynj 8 July 2011 at 7:54 pm #

    Does eating FRESH FRUIT substitute for drinking Water to some degree? Or is there too little liquid in fruit?

    From the paleo perspective–humans do not have a tongue to lap up water like other animals, thus hinting that they got liquid some other way–fruit and veg? Thanks

  4. mike 8 July 2011 at 8:25 pm #

    It’s furosamide. at least here in the states :). More seriously, it’s hard to measure pale yellow, especially when supplements tend to make the urine a deeper yellow. Is there an alternative, better, in fact, method of self-measurement. As per my doctor’s orders I try to minizimize fluid intake after 6 pm to minimize the sleep-time effects of BPH. Could that explain my late night crankiness, and bad dreams?

  5. mike 8 July 2011 at 8:31 pm #

    Oops, I meant furosemide. Two wrongs……

  6. Wendy McMullan 8 July 2011 at 9:14 pm #

    I have read articles by urologists saying we do not need to drink more than one litre of water per day, as we get a large amount from our food. To the frequent argument that large amounts of water “flush out toxins”, they say that the kidneys do this quite efficiently.

  7. Susan 8 July 2011 at 11:30 pm #

    This is a timely article. I’m always dehydrated after driving from coastal California to the high desert of Arizona. This trip generally results in bad dreams, burning eyes, bloody nose and feeling awful for a few days. Even a humidifier doesn’t alleviate the misery. Two days after I arrived for my current visit, I began adding a serving of white rice or sweet potato to my evening meal and really think that helps. Normally, I would eat only VLC–meat and green vegetables. Of course, I’m also drinking more water, but apparently I need more carbs to hold the water where it belongs. It’s good to know that my bad dreams probably are caused by dehydration. Thanks for all your hard work and helpfulness, Dr. Briffa. They’re definitely appreciated. Any thoughts on my addition of a little carbohydrate? Hehe, carbo-hydrate. That’s funny!

  8. Jane 8 July 2011 at 11:32 pm #

    We are dealing with very hot weather here right now and so many people around me are feeling very lethargic. I’m sure most of us are dehydrated!

  9. Deborah Booth 9 July 2011 at 1:14 am #

    I think that has been proved to a load of tosh! The “keep your urine pale” is a far better guide as we are all so different… I dont sweat much even when very hot, I have tried all that 8 glasses of water a day, and frankly needed a comode strapped to my person to function!! And what the heck is wrong with tea????

  10. Esther Oppenheim 10 July 2011 at 9:21 am #

    Can you give us details about the size of the study, the funding for it and the provenance of the Authors? The journal belongs to the Nutrition Society which is the organisation ot the nutrition industry, so we have to be suspicious of ulterior motives and it costs a small fortune to get a subscription or even to access one paper.

  11. Jill H 10 July 2011 at 8:03 pm #

    Feona “and it’s interesting that all this emphasis on drinking water came along just as the bottled water industry took off!”

    I don’t know whether this is the case or not – but what an important connection to make. From my own experience keeping hydrated is probably one of the most important and simple steps I can make to keep feeling healthy. I certainly know when I am dehydrated – my head aches – I feel light headed and indeed my memory gets worse. But I also have changed my thinking about keeping hydrated and, as Dr Briffa has said in his concluding paragraph, I also feel that what is eaten has a bearing and that if I am eating green leafy veg and lots of plant based foods this does count to the water taken in along maybe with my one coffee and cups of tea – green, black and herbal as well as drinking plain water. But going back to Feona’s point, perhaps one of the biggest (hopefully) changes we will see this century is the realisation that there is no ‘away’ to ‘throw’ when we throw away petroleum based plastic – made to last forever and designed to be used once and one of the biggest problems for the most important body of water on our planet – our Oceans – is the amount of plastic trash( plastic water bottles and plastic bags being right up there) finding its way into it – harming the animals that live in it – no fun watching an elephant seal pup coughing up a plastic bag and impacting our own life support system – no animals lives on this planet without healthy oceans. So many simple but important decisions to make like taking a reusable stainless steel water bottle instead of buying endless plastic ones – that may indeed be very profitable to the food industry and going back to using reusable bags like canvas bags . is the website of at artist, Angela Haseltine Pozzi from Bandon Oregon who has started a community project collecting all the trash that washes ashore on her beloved beaches. Well worth a look at the video she has made – she has used her gift as an artist to make giant sculptures of ocean creatures out of all the trash – they are beautiful and alarming at the same time.

  12. Steve Tindall 11 July 2011 at 3:40 pm #

    It’s really simple: Just keep drinking constantly all day; a few sips every half an hour. Forget about quantity, as long as you are in the rhythm of regularly taking in water your body will automatically direct you to to drink the right amount for your needs.

  13. Chris 15 July 2011 at 6:47 pm #

    Interesting article in New Scientist (9 July 2011, p36; Guardians Of The Oceans: Givers Of Life) how large sea mammals enrich and preserve mass and diversity of species lower down the food chain, despite that they may feed off them. It is a well constructed and reasoned argument from marine biologist, Steve Nicol, that is almost completely counter-intuitive, and is definitely worth a read. While slightly off the topic of de-hydration it sits well with Jill H’s concerns.

Leave a Reply