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  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 http://en.wikipedia.org/wiki/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]