A brief guide to hydration

I read a review paper recently about the assessment of dehydration [1]. The authors conclude that we don’t have very good standardised tests for dehydration, but a reasonable test is to measure heart rate when sitting, and then again immediately on standing. During dehydration the blood volume tends to be lower and blood pressure too.

On standing, there is a tendency for blood to ‘pool’ in the lower body, causing blood pressure to drop further, which may induce a reflex increase in heart rate. If the pulse rate increases by 20 beats per minute or more on standing, this is generally taken as a sign of dehydration. However, as the authors concede, this test is not a particularly reliable way of assessing dehydration, even when blood volume is significantly depleted.

Quite-severe (even life-threatening) dehydration is known to be a potential hazard of certain activities such as mining and endurance exercise, but most of us run a greater risk of milder levels of dehydration on a day-to-day basis. There is surprisingly little good research on the impact of mild levels of dehydration, but my suspicion is that this impairs physical and mental vitality more than we might expect.

Over the years I have spoken to literally hundreds of individuals who say they do not drink much fluid (including water). However, very consistently indeed these individuals report improvements in energy and vitality, just for drinking more water. Now, of course this may just be placebo response. But the consistency of the effect (even in those who are sceptical) tells me there may well be a genuine effect going on here for many.

Also, it is perhaps worth bearing in mind that, by and large, drinking more water is easy to do: it does not take much in the way of organisation or willpower. When a habit is easy to adopt and appears to pay off big, it’s just the sort of new habit that tends to endure.

So, how much water should we drink? 2 litres and 8 glasses-a-day are often quoted, but I think our needs for water are quite individual. Bigger people, for example, on the whole need more water than smaller ones, as do those who take more exercise, sweat more and live in warmer climates.

One quite-decent indicator of our state of hydration is the colour of our urine [2]. The trick is to drink enough fluid to ensure that urine colour stays pale yellow (and non-odourous) throughout the whole of the day. In addition to water, herb and fruit teas are acceptable.

People ask about the dehydrating effects of coffee and tea. These beverages do have a ‘diuretic’ effect and can, theoretically, dehydrate the body. However, as long as enough water (or herb and fruit teas) are being drunk to keep urine pale and non-odourous, then I don’t think the potential dehydrating effects of coffee and tea are a concern.

Just drinking enough to keep our urine pale and non-odourous is enough, I think, to enable us to adjust our fluid intake according to our needs and help optimise our health and vitality.


1. Cheuvront SN, et al. Physiologic basis for understanding quantitative dehydration assessment. AJCN. First published ahead of print January 23

2. Armstrong LE, et al. Urinary indices of hydration status. Int J Sport Nutr 1994;4:265-279

12 Responses to A brief guide to hydration

  1. Paul 13 February 2013 at 9:27 am #

    The ’2 litres a day’ question is interesting.

    Like many ‘facts’ such as men needing precisely 2500 calories a day, nobody can pinpoint the source of such a known ‘fact’ – the 2500 a day is not a ‘need’, but merely a 70 year old observation that men on average ate about 2500 calories a day, and somehow this morphed into a precise requirement, not 2510, or 2490, but 2500. (Presumably, if I accidently ate 2400 calories a day, I would end up weighing about 2 stone after a couple of decades).

    Similarly, when someone bothered to find out where this 2 litres a day came from, it appears to have originated from a US military study where it was recommended that soldiers maintain a level of hydration of 2L a day. For soldiers, and sportsmen, clearly hydration is important – when I have been on my bike on a hot day, I have hit a ‘wall’ where I suddenly couldn’t pedal another foot, and only hydration brought me back to form. However, for someone who is not exercising, or for whom they aren’t sitting on a beach in the sun, I question the ‘need’ for 2 litres a day.

    If you have working kidneys, they will maintain the correct level of hydration whether one consumes 0.5L or 5L, so the precise ‘need’ for liquids may vary from a couple of cups of tea a day to 4 litres of bottled water depending on the weather and level of activity. Averaging the ‘need’ to precisely 2L is not helpful.

  2. Gary Conway 14 February 2013 at 8:58 am #

    Great round-up. My sweet spot seems to be around 3.5L a day and I feel noticeably better when I drink plenty of water. I find the “drink when your thirsty” method causes me to drink too little and leaves me a bit fuzzy. Instead I keep a bottle with me most of the day.

  3. Randall Lightbown 14 February 2013 at 4:00 pm #

    Some very interesting research on dehydration by this man: Tim Noakes
    Challenging Beliefs: Tim Noakes at TEDxCapeTown


  4. kem 14 February 2013 at 7:06 pm #

    Noakes’ “Watterlogged” is a very well researched book on the subject. Nice ancestral point of view.

  5. Paul 15 February 2013 at 8:54 am #

    Randall, a very good link summarised by Noakes’ comment that sports hydration is bogus and dreamed up by bottled water marketeers. I think one has to be of an age (I am 48 tomorrow) when one remembers the time before bottled water to understand that the general public’s understanding that we ‘need to hydrate’ is a relatively recent thing, and even young doctors have grown up in a world where this message has been pushed on us relentlessly and naturally believe the marketeers’ claims, because everyone else believes it.

    I also see a parallel with isotonic drinks. Isotonic drinks were developed for athletes, notably Gatorade in the US. This company that developed it was later bought up by a drinks manufacturer, fructose added, and then the drink was sold on the basis it would make you sporty. Here, the parallel is similar. Drinking water was promoted for athletes, and then, on the basis that it was good for athletes, it was promoted to the general public to sell more water.

    And if we want another parallel, in the early 1980s, ‘carb loading’ was promoted for athletes (certainly it was in my rowing club as a new and clever thing to do). This morphed into a general promotion of carbohydrates ‘for energy’, which was directed at the general public (they never promoted fat ‘for energy’, and never promoted carbs ‘for calories’).

    There was a lot of junk science in the 1970s and 1980s, and a lot of it has presumably found its way in the text books that student doctors read. Perhaps Dr Briffa could comment on this?

  6. John Walker 15 February 2013 at 5:11 pm #

    I tend to drink when I am thirsty. Anything else I drink is for pure enjoyment; excluding alcohol; which I am told dehydrates. I can’t say I have ever drunk enough at one sitting to know. I told a Marine that one day. He didn’t believe me! On the subject, I have been advised that any water used to make tea or other beverages should be part of this mystical 2 litres per day. But in all seriousness, as I am presently not as active as I would like to be, I tend to drink when I feel thirsty.

  7. hm 15 February 2013 at 6:07 pm #

    it certainly is variable from person to person. i’ve tried for years to drink the mythical eight glasses a day and just can’t. if it do i get diarrhea,so i don’t. i do have a few cups of herbal tea along with a fairly juicy diet, and always start my day with a big glass of warm water with lemon.

  8. aaadele Bernstein 15 February 2013 at 8:17 pm #

    When I was put on Warfarin my blood was so thick it would coagulate as the nurse tried to take it from my vein. I went on line and found all the most likely culprits and was able to eliminate several super chlorophlll substances from my diet, but that didn’t make much difference. Then I found a statement that the best thinner is water. I added litre of water to my usual intake of herbal drinks and immediately jumped close to the desired Warfarin target.

  9. Helen 15 February 2013 at 8:26 pm #

    Am I right in thinking the symptoms described can also be due to orthostatic hypotension? If so, they are frequently observed in patients with ME, and in cases of hypoadrenalism.

  10. Agate Karevoll 16 February 2013 at 10:25 am #

    I agree with Helen. Hypoadrenalism is generally invisible to doctors, but quite common and devastating. Postural hypotension is an indicator of low aldosterone which reflect on the general state of one’s adrenals. Aldosterone has two known functions: lifting BP when you stand up, and retaining sodium in one’s cellls. When there is too little aldosterone the cells loose salt and potassium follow to maintain balance in the cells. The more you drink the more dehydrated you get, the more you run to pass water all the time. At my worst my BP droppped 40 points when I stood up. The highest part of my body, top of my head, had a scab 10cm across which finally cleared up when I started to take extra salt. If I drink unsalted water I get intensely thirsty after 10 minutes. Pale urin is not always the answer. Apart from myself where failing adrenals have other causes I personally know people who have exercised themselves to Addison’s.
    Doctors became interested in the adrenals in 1918, when it was discovered that influenza attacks them and adrenal extract brought down the death rate y 30%. By 1940 they had more or less
    got a good understanding. With the introduction of hydrocortisone in the 50s mistakes were made, people were overdosed and died. Replacing hormones have been on a back foot since then, yes we get ME from living with too little hormones, then some major attack sends us over the edge. Present day testing in the NSH is mainly ‘the emperor’s new clothes’. Never trust what an edocrinologist tells you, whether you are a patient or a doctor trained by one. No apologies for rant.

  11. Lorraine 21 February 2013 at 4:05 pm #

    Agree that how much water you need depends on how much exercise, sleep, stress, activity, work, etc an individual has/does.

  12. Jane Robinson 21 February 2013 at 10:36 pm #

    As a practitioner of Chinese medicine we always look at our patient’s tongues. We look at the shape, colour, coating, cracks, and moisture, which all provide valuable information about a person’s state of health. It is considered that a ‘normal’ tongue should be pale red, with a thin white coat and wet. If you look at your tongue in the mirror it will soon tell you about your levels of hydration.

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