Introduction
Water makes up a large part of our body weight. It is essential for our overall health. This includes how our central nervous system works, especially the brain. About 75% of the brain is water. So it is obvious that hydration affects brain function. Hydration has a profound impact on our cognitive abilities and emotional well-being. Water is vital for many metabolic processes. It helps regulate temperature and maintain electrolyte balance.
Water balance
Our bodies always work to keep a balance between water intake and output. If this balance is disturbed, it can lead to problems. For example, hyperhydration happens when we drink too much water. This can cause headaches, nausea, and even convulsions. We get dehydrated when we don't drink enough water. This can cause different health problems.
This article explores how hydration affects our cognitive performance and mood. This link is key for public health. It is crucial for vulnerable groups, like the elderly and children. We will explore the science behind these effects. We'll see how they work and what they mean for keeping our brains healthy and working well.
The Physiological Impact of Dehydration
Dehydration occurs when there is an insufficient intake of water, leading to a state of low water intake levels. This can cause various physical issues. These include headaches, kidney stones, and other kidney problems. It can also lead to declining health and issues with the digestive and cardiovascular systems.
Sustained dehydration can increase the risk of kidney stones and urinary tract infections. It may also cause long-term vasoconstriction, which can increase the chances of hypertension and stroke. Our bodies maintain fluid balance through sophisticated homeostatic processes. These primarily involve the renal system, which adjusts urine production, and the sensation of thirst, which encourages us to drink water. In response to fluid imbalance, key hormones come into play, most notably vasopressin (antidiuretic hormone) and angiotensin II, which are central to the homeostatic regulation of fluid.
It has been proposed that dehydration could lead to increased levels of cortisol, a hormone released during stress responses. Increased cortisol levels have been shown to negatively impact memory function and processing speed, potentially contributing to memory-related cognitive deficits during dehydration. Preliminary neuroimaging studies have also suggested that dehydration can affect the very structure and function of the brain, with observations of total brain volume shrinkage in dehydrated individuals.
Dehydration's Toll on Cognitive Performance
Water makes up most cells, even in the central nervous system. So, not drinking enough can hurt how well we think.
Research has looked into these effects. However, results can be inconsistent. This might be because of the different methods and cognitive tests used. This study found that after 24 hours without water, participants showed decreased scores on the portrait memory test. This test measures episodic memory. Dehydration has also been linked to the impairment of vigilance-related attention, as observed in male basketball players. Also, performance on the symbol search test, which measures processing speed, showed reduced performance.
A different study found that dehydrated participants had lower scores on the operation span test, which measures working memory. While these findings suggest impairments in specific cognitive domains, it's important to note that some studies have found no negative effects of dehydration on cognitive performance. The severity of dehydration likely plays a crucial role in the extent of cognitive impairment.
Even mild dehydration can cause small but noticeable problems. The effects depend on the situation, whether you're a pilot needing sharp focus, a golfer steadying your swing, a senior trying to stay alert, or a student taking an exam (studies show those who bring water to tests tend to score better!)
The Impact of Dehydration on Mood
Beyond cognitive performance, dehydration can also take a toll on our emotional state. The study found that after water restriction, participants felt less energetic and had lower self-esteem. They also reported more fatigue and higher total mood disturbance (TMD). Research links dehydration to changes in alertness, calmness, and confusion.
Even mild dehydration can lead to mood changes. It may cause lower positive mood ratings and higher negative mood symptoms, such as tension, anxiety, and fatigue. These findings show that staying well-hydrated is important for both clear thinking and feeling good.
The Restorative Power of Water 
Luckily, rehydration seems to reverse the harmful effects of dehydration on brain function, mood and thinking. The study showed that drinking water after a period of dehydration led to improvements in processing speed (as measured by the symbol search test) and working memory (operation span test). The study found that 1000 mL of purified water is the best amount for young adults to boost cognitive performance after dehydration. Rehydration also resulted in decreased fatigue and TMD, and an increase in vigour.
A proposed psychological mechanism for these benefits involves the idea that alleviating thirst through water consumption frees up limited attentional resources, which were previously occupied by the physiological state of thirst, thus allowing for improved performance on cognitive tasks.
Subjective thirst can affect how fast people respond when drinking water. Importantly, the expectancy of water alone does not seem to influence cognitive performance, suggesting a physiological basis for the benefits of rehydration.
How Water Influences the Brain
The precise mechanisms through which hydration status affects brain function are still being investigated, but several possibilities have been proposed. From a psychological perspective, the limited attentional resources theory suggests that feelings of thirst and the physiological state of dehydration compete for our cognitive resources, reducing the capacity available for other mental processes.
Drinking water can help reduce dehydration. This may free up resources and improve thinking skills. On the physiological front, dehydration might affect cardiovascular reactivity and cerebral blood flow, indirectly impacting cognitive function.
As mentioned earlier, the potential role of increased cortisol levels during dehydration could also negatively affect memory and processing speed. During dehydration, the body's homeostatic response involves hormones like vasopressin and angiotensin II, which can influence many substrates and neurotransmitters in the brain6 …. Finally, neuroimaging studies have indicated that dehydration may lead to temporary changes in brain volume, which could underlie some of the observed cognitive effects.
Drawbacks in the science
Scientists are finding more proof that drinking water affects how our brains work, but different studies keep getting mixed results. Some show big impacts from dehydration, while others find smaller effects. The confusion likely happens because researchers use different methods to test this. For instance, some studies may guess hydration levels, rather than testing if people are dehydrated. This would be done with a urine test, rather than assuming based on water intake.
Dehydration is induced through diverse methods, including heat stress, exercise, and fluid restriction, and these different approaches might have confounding effects. Fluid restriction, however, is considered to more closely resemble routine voluntary dehydration. Cognitive assessments in studies differ greatly. Some say that broad neuropsychological tests might miss the subtle effects of nutrition changes.
The lack of objective measures of hydration state in some studies also poses a limitation, as it's not always clear to what extent participants are dehydrated or euhydrated. Future research should prioritise standardising methodologies, including the use of objective hydration measures such as urine indices (osmolality, specific gravity) and serum osmolarity. Exploring the brain's activity during hydration changes with neuroimaging is important.
Identifying the best conditions for hydration to affect cognitive performance and investigating mediating factors that might influence the effects of water intervention are important avenues for future research. We also need longitudinal studies to see how hydration habits affect cognitive health over the long term.
Water Intake Recommendations for Brain Function
Several authoritative bodies have issued guidelines for adequate daily water intake. The European Food Safety Authority (EFSA) recommends 2.5 L of fluids per day for men and 2.0 L per day for women. However, the Institute of Medicine (IOM) suggests even higher intakes. It is important to note that the World Health Organisation (WHO) and other guidelines related to cognitive health do not currently include specific water intake recommendations. Fluid needs can vary a lot. They depend on the climate, how active you are, and your body.
Therefore, encouraging individuals to monitor their hydration levels using indicators like urine colour may be a practical approach.
Practical Tips
To ensure adequate hydration for brain function follow these practical tips to increase daily water intake:
- Carry a water bottle and refill it regularly.
- Drink water before, during, and after physical activity.
- Make water readily available at home and work.
- Be mindful of thirst cues, but also drink regularly even if not feeling thirsty.
- Include water-rich fruits and vegetables in your diet.
Maintaining good hydration is particularly relevant for vulnerable populations. In older adults, there is a potential link between reduced hydration and greater cognitive decline. For children, adequate hydration can impact cognitive performance in school settings6 …. The PREDIMED-Plus study, conducted in older Spanish individuals, suggests a potential link between hydration status (measured by serum osmolarity) and 2-year changes in cognitive performance.
Conclusion
The evidence reviewed underscores the crucial role of hydration in maintaining optimal cognitive ability and mood. While severe dehydration has clearly been shown to cause cognitive deficits and mood disturbances, even milder forms can have noticeable impacts.
On the other hand, adequate water consumption and rehydration can lead to improvements in various cognitive functions, particularly attention, processing speed, and working memory, as well as a more positive emotional state.
Given the brain's high water content and the fundamental role of water in physiological processes, it is imperative to be mindful of our hydration status as a simple yet powerful factor influencing our cognitive and emotional well-being.
More research is needed to understand the underlying mechanisms and create clear, evidence-based hydration guidelines. However, the existing evidence strongly suggests that promoting adequate hydration is a valuable strategy for supporting brain health and has the potential to inform public health intervention programmes aimed at improving quality of life.
To learn more about how nutrition can improve your brain power, head over to this article on superfoods
References
Effects of hydration status on cognitive performance and mood
Water intake, hydration status and 2-year changes in cognitive performance
Effects of hydration status on cognitive performance and mood
