A drink of water can improve or impair mental performance depending on small differences in thirst

Food and water are requirements for life, but eating and drinking have physiological costs as well as benefits. In the short-term a meal presents a challenge to bodily homeostasis and in response substantial physiological resources are deployed in digesting and absorbing the ingested materials and in utilising and storing the ensuing rush of nutrients into the blood (Woods, 1991). One consequence is a dip in mental (and physical) performance after eating; for example, the ` post-lunch dip'' (Craig, 1986; Smith et al., 1991). It is more surprising that the present study found that even the consumption of water can impair mental performance. In recent unpublished studies investigating thepsychostimulant properties of caffeine and caffeine-containing drinks we used plain tap water and no drink (` nothing'') as control treatments. In some studies these treatments had similar effects, but unexpectedly in others performance was found to be poorer after water than after nothing. The performance tasks were a demanding rapid visual information processing task (RVIP) as used here, and a simple reaction-time task.Wewere not able to find an explanation for these different results, although in the light of the present experiment it seems likely that differences in initial thirst were at least partly responsible (unfortunately, appropriate data on thirst were not collected). The observation that a drink of water can impair performance was confirmed and extended in the present experiment. Participants performed the RVIP task (Smit & Rogers, 2000) after consuming either nothing, or a small (120ml) or a moderate (330ml) volume of tap water, chilled in a refrigerator and served at 10 C. These volumes are equivalent to the amount of fluid contained in a standard wine glass and a standard soft-drink can, respectively. The 30 female and 30 male participants (mean SD age 26 7 years, and weight 68 11 kg) were randomly assigned to one of these three treatments. They were instructed to follow their usual pattern of activity, eating and drinking (except alcohol was not permitted) up to the commencement of their 2-h test at either 11:00 a.m. or 3:00 p.m. The first 55min were occupied with an explanation of the study schedule, a familiarization session and pre-treatment measurement of RVIP performance, mood and thirst. Five min were allowed for drink consumption (participants assigned to the ` nothing'' condition sat quietly), following which the same measurements were repeated after two (mood and thirst only), 25 and 50min. Toilet breaks were permitted between measurement sessions. In the RVIP task, single digits (0±9) were presented in the centre of a computer screen at the rate of 100 per min (no inter-stimulus interval). Participants were instructed to press the spacebar on the computer keyboard as quickly as possible when they detected any unbroken sequence of three odd or three even digits. There were eight possible ` hits'' per min and the task lasted for 6min. Pre-treatment thirst, measured on a 9-point scale (` not at all'' to ` extremely'' thirsty) 2min before water consumption, was included as a between-subjects factor in the analysis of the results (median split of participants based on these pre-treatment thirst ratings). Thirst ratings for ` low'' and ` high'' thirst participants were (Mean and SD) 2.9 1.3 and 7.2 0.9, respectively, and time since last drink (measured to the time of the pre-treatment thirst rating) was 124 76 and 185 168 minutes, respectively. Consumption of water reduced thirst in low and high thirst participants in a dose-dependent fashion (main effect of treatment, p 0.2). The mood ratings revealed immediate, but not sustained, ` alerting'' and ` revitalising'' effects irrespective of the participants' initial (pre-treatment) level of thirst (Fig. 1a). For example, the main effect of treatment for alertness ratings was significant at 2min, F(2, 54) 3.82, p 0.028, but not at 25 and 50min after consumption ( p> 0.3 for all main and interaction effects of thirst and treatment). In contrast, the effect of water on RVIP performance was very different between high and low thirst participants (Fig. 1b). When initial thirst was high, there was a dose-related improvement of performance, but when initial thirst was low there was a dose-related impairment of performance, thirst by treatment interaction F(2, 54) 7.59, p 0.001. Pre-treatment performance (%hits) did not differ as a function of thirst or ` treatment'' and the thirst by treatment interaction was also non-significant ( p> 0.3). Post-treatment performance shown in Fig. 1b was analysed with pre-treatment E-mail of first-named author: peter.rogers@bristol.ac. uk