Mood & Mental Tasks
Most adults rarely experience dehydration greater than 3 percent during their daily activities. However, even mild dehydration—that is, 1-2%—can affect mood, energy level, and mental awareness. one day to the next. However, it is likely that most adults experience a 1-2% body weight deficit several times each week because thirst is not sensed until we reach a 1-2% body weight loss. Therefore, this article focuses on mild dehydration (1-2% loss of weight).
Controlled Laboratory Studies A Connecticut research team has published the findings of two mild dehydration studies (Ganio et al., 2011; Armstrong et al., 2012). The first involved a 12-hour body weight loss of 1.4% in young women and the second, 1.6% in young men. At those levels of mild dehydration, both groups experienced decreases on mental tasks, compared to control days when they were normally hydrated. Women reported headaches, increased task difficulty, and loss of concentration when mildly dehydrated; they also experienced mood changes in the form of reduced vigor and increased fatigue. Men similarly reported increased fatigue, plus greater anxiety. However, only men experienced poorer performance on computerized mental tasks, during vigilance and visual memory tasks. One year later, a French research group studied the effects of 24-hour fluid deprivation (FD) versus normal fluid intake, on selected mood and physiological factors (Pross et al., 2013). The
participating 20 healthy young women normally consumed 2.0–2.8 liters per day (L/d) of water. FD resulted in increased sleepiness and fatigue, lower levels of vigor and alertness, with increased confusion. These women also were more confused, less calm, and less happy. Interestingly, these mood changes were time-dependent, with greater impairments during the early afternoon (2:00 – 4:00 p.m.). This well-controlled investigation confirmed the findings of their initial study involving mild dehydration, which was published one year earlier (Pross, 2012). Modifying the amount of fluid consumed each day also can alter mood. Pross and colleagues (Pross et al., 2014) evaluated 52 women and men who habitually drank either a low volume (LOW = 1.5 L/d) or a high volume (HIGH = 2.7 L/d). During three controlled drinking days, the water intake of the 22 participants in the HIGH group was restricted to 1 L/day, whereas the water intake of the 30 LOW group participants was increased to 2.5 L/day. No purposeful dehydration was involved. In the HIGH group, the smaller volume of water resulted in statistically significant decreases of contentedness, calmness, positive emotions, and vigor. In the LOW group, drinking more water resulted in decreased fatigue, confusion/ bewilderment, and thirst, with a tendency toward less sleepiness.
A 2013 research investigation discovered the effects of drinking water on the reaction time required to perform a simple fine motor task: pushing a button. (Edmonds et al., 2013). Thirty-eight test participants abstained from drinking any fluid overnight; this resulted in a state of mild dehydration. In the morning, one-half were not given water to drink, and the other half consumed up to 1 L of bottled water, as dictated by thirst. The group that consumed no water had a slower reaction time than the group that drank water. The authors proposed that water consumption released mental resources (i.e., focused attention) that were otherwise occupied with brain processing of the sensations associated with dry mouth and thirst. Complex tasks also may be negatively influenced by dehydration, as shown in a 2015 study conducted in the United Kingdom (Watson et al., 2015). The aim of this study was to measure the effect of mild dehydration on performance during a prolonged, monotonous driving simulation task. This task consisted of a 2-hour continuous drive in an immobile car with a full-size, interactive, computergenerated road projection of a dull, monotonous four-lane dual highway. The computer screen “road” also had a hard shoulder and simulated auditory “rumble strips” (which were incorporated into white lane markings on either side of the highway and a barrier separating JAN/FEB 2017