We’ve all heard the advice to “drink more water.” But what does the research actually say about how changes in water intake affect our mood, and does it matter how much you habitually drink in the first place? A landmark peer-reviewed study published in PLOS ONE (Pross et al., 2014) addressed exactly this question, yielding striking findings that challenge a one-size-fits-all approach to hydration.
Who Are “High” and “Low” Drinkers?
Before diving into the findings, it helps to understand how researchers defined their groups. The study recruited habitual high-volume drinkers (HIGH), defined as those with fluid intakes of at least 2 liters per day, and low-volume drinkers (LOW), defined as those consuming less than 1.2 liters per day. These weren’t extreme outliers; they represent a real spectrum of everyday drinking habits found in the general population.
During a two-day baseline period, HIGH participants consumed 2.5 liters of water per day, and LOW participants consumed 1 liter per day. Over the following three controlled intervention days, HIGH had their water intake restricted to 1 liter per day, while LOW had theirs increased to 2.5 liters per day. To measure mood, researchers administered several validated scales, including the Bond & Lader Visual Analog Scale, the Profile of Mood States (POMS), the Karolinska Sleepiness Scale, and a Thirst & Emotional Visual Analog Scale, at multiple time points throughout each day.
What Happens When High Drinkers Cut Back?
The results for the HIGH group were sobering. Restricting water intake in high drinkers led to a significant increase in thirst and decreases in contentedness, calmness, positive emotions, and vigor/activity. In other words, it wasn’t just physical discomfort; their emotional landscape shifted meaningfully in a negative direction.
Habitual high drinkers who were forced to reduce their daily water intake reported being thirstier, less calm, less content, less vigorous and described experiencing fewer positive emotions. These weren’t trivial fluctuations. The changes in calmness and positive emotion reached statistical significance, suggesting a genuine psychological cost to under-hydrating when your body is accustomed to more.
The researchers had anticipated this outcome based on prior dehydration literature. It was hypothesized that a water decrease in HIGH would negatively impact sleep/wake-related moods, consistent with the effects seen in fluid deprivation studies. The data bore this out and extended the finding beyond just sleepiness into broader emotional well-being.
What Happens When Low Drinkers Drink More?
The LOW group told a different, but equally compelling, story. The intervention demonstrated that a daily water intake increase led to a significant mood improvement in habitual low drinkers, who reported less fatigue, less confusion, less thirst, and who tended to be less sleepy.
Specifically, increased water consumption in the LOW group resulted in a significant decrease in confusion-bewilderment scores, and lower sleepiness was also observed as a trend. For people who chronically underdrink, simply bringing their intake up to a more adequate level appears to have a meaningful positive effect on how alert, clear-headed, and energized they feel day-to-day.
The hypothesis had been that increased water intake would positively affect alertness and happiness ratings in LOW, as previously demonstrated in adults and young children, and the results supported this expectation.
(Part 2 will continue in a future edition of The Enterprise for the conclusion of this article.)
Note: For references, see the article posted at www.LifeandHealth.org.
(Submitted by Betty Dean. Written by Ezra Otieno, MPH. Used by permission from www.LifeandHealth.org. Courtesy of LifeSpring – Resources for Hope and Healing, Stuart, VA)
