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The Myth of Eight Glasses

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, these individuals report improvements in energy and vitality, just for drinking more water. A couple of studies bear this out. In one study in men, mild dehydration was found to impair vigilance and working memory (the ability to actively hold information in the mind for tasks such as reasoning, comprehension and learning) [2]. In another study, this one in women, mild dehydration was found to impair mood and concentration, and make headaches more likely[3].

Also, it is perhaps worth bearing in mind that, by and large, drinking sufficient quantities 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 eight 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. 2013;97(3):455-62

2. Ganio MS, et al. Mild dehydration impairs cognitive performance and mood of men. British Journal of Nutrition. 2011;106(10):1535–43

3. Armstrong LE, et al. Mild dehydration affects mood in healthy young women. Journal of Nutrition. 2012;142(2):382–8

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

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