Quantifying the effect of water intake on mood

Water is essential for human health. People that aren’t taking in enough fluids quickly feel the consequences. Research often tries to answer the question: what is “enough” water? What are the results of too little water? Different people have quite a wide range^[1] of fluid consumption habits, and optimal fluid consumption has long been an area of debate.

Adults typically have around 60% of their bodyweight composed of water. Water isn’t just essential for basic bodily functions like proper digestion, it may also have an effect on mood and emotional state. Previous research found that fluid deprivation negatively impacts mood under extreme conditions, like extreme dehydration, heat, or intense physical exercise. However, there is only limited data examining the effect of mild dehydration or fluid restriction on mood.

These studies found that mild dehydration caused a decrease in alertness^[2] and an increase in fatigue^[3], headaches, and difficulty concentrating^[2]. There were also a limited number of trials testing the effect of drinking a specific volume of water on mood and cognitive performance. In one study^[4], participants completed a cognitive test after drinking nothing, 120 mL, or 330 mL of water. When people were thirsty, drinking water improved performance. When thirst was reported as low, water was detrimental to performance. Another study^[5] assessing cognitive performance found that drinking 150 mL of water twice, compared to no water (after an overnight fast with no food or drink) had no effect on cognitive performance, though self-reported alertness was significantly higher after water ingestion. However, in another study^[6] assessing cognitive performance and mood after a 200 mL drink of water found no differences in mood in the water compared to the no water group.

Based on limited existing data, the researchers hypothesized that:

1) In high volume water drinkers, a decrease in water consumption would have a negative impact on mood, especially aspects of mood that relate to sleep, like fatigue and vigor.

2) In low volume water drinkers, an increase in water consumption would increase alertness and happiness.

Though some earlier evidence suggested these hypotheses were correct, they had yet to be tested in a large, rigorous study. The study under review examined how changes in water intake affected mood states (such as anxiety and happiness) and physiological sensations (such as sleepiness and thirst) in two groups of participants with different habitual fluid intake levels: 2-4L/day, which were considered high volume fluid drinkers (HIGH), and <1.2L/day, which were considered low volume fluid drinkers (LOW).

There is a lack of data on how changes in water intake affect mood. Previous research tended to test the mood effects of more extreme dehydration, and sample sizes were often small.