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How Much Water Do You Really Need To Drink?

Posted on 16 March 2023

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We all know the importance of drinking enough water throughout the day. Good hydration helps to prevent urinary tract infections (UTIs), kidney stones, headaches, constipation, dizziness and more. Some research even suggests that good hydration is associated with slower biological ageing. You’ve probably heard the advice to drink eight glasses of water per day. It’s an easy goal to remember, but it’s not very precise. Factors like body weight and perspiration can have a significant impact on the amount of water you need. When you drink your water is also relevant – you certainly wouldn’t want to drink all eight glasses at once. Can’t we simply trust the body’s natural thirst response to determine how much and how often we should drink? In this article, we’ll cover some of the research concerning water, its effects on the body, and how much of it you should consume.

Why Water Is So Important

Water is the most versatile solvent known to humanity – more different substances can dissolve in water than in any other liquid. All of the biochemical reactions within the body take place in an environment that is mostly composed of water – if they did not, then many of the reagents involved would be unable to mix and react with each other. If we ever discover life elsewhere in the universe, it is more than likely that this life will also use water as their universal solvent.


In addition to acting as a medium in which all the chemicals within the body can interact, the properties of water are also essential for allowing cells and proteins to maintain their shape. The water contained within each cell can be thought of like air or water filling a balloon, exerting an outward force that resists external pressure from the environment and prevents the cell from collapsing.

Dehydration And You

The amount of water within the body, and particularly within individual cells, needs to be tightly controlled within a certain range. By mass, about 60% of the adult male body and 55% of the adult female body is composed of water (this sex difference is due to differences in stored fat, which cannot mix with water). If someone is drinking less water than they are losing (mainly through sweat, urine, faeces and indeed their breath), then the total amount of water within the body begins to decrease. Blood, which is about 90% water, will occupy slightly less volume within the circulatory system, resulting in decreased blood pressure. It will also become slightly thicker. Individual cells, meanwhile, will begin to lose water and shrink. Cells are able to resist the loss of water by taking up more electrolytes, but their ability to do this has limits.

Water content differs between different tissues, is dependent on sex and body composition, and will fluctuate throughout the day.

Once someone has lost 15-20% of their body water, blood volume and pressure has decreased enough to severely impact the delivery of nutrients to cells, while the thickness of the blood is sufficient to damage blood vessels and organs. As the body loses water, the concentration of electrolytes increases, which can disrupt the function of the nerves and muscles. It is at this point that dehydration starts to become a deadly threat.

If you’re currently reading this article online, your living conditions are probably such that dying of thirst isn’t something you’ll ever have to worry about. However, mild dehydration may have long-term health effects that reduce one’s life expectancy. In one recent study, researchers measured participants’ blood sodium levels after they had been fasting for 12 hours. This meant that blood sodium should only have been affected by their water consumption, not their salt intake. They found that people with higher blood sodium concentrations (implying lower water intake) were up to 60% more likely to develop chronic disease than those with the lowest sodium levels. They also found that higher sodium levels correlated with more rapid epigenetic ageing, suggesting mild dehydration may even make you age faster. This could be related to the finding that dehydrated cells are less able to resist and repair damage.

Risk of having a biological age greater than one’s chronological age (relative to those with a blood sodium of 137-142mmol/l). Biological age calculated based on 9 biomarkers (left) and 15 biomarkers (right).
Middle-age high normal serum sodium as a risk factor for accelerated biological aging, chronic diseases, and premature mortality

Long-term health effects aside, many studies have found that even a 1% decrease in body water content can have a measurable effect on cognitive performance and mood. Similarly mild dehydration can have a significant impact on physical performance during exercise. As with many aspects of health, there may be a sliding scale between nonoptimal, unhealthy and dangerous levels of dehydration.

What Is Optimal Hydration?

What’s the best way to ensure your level of hydration never drops below what is optimal? Different people are of course going to lose water at different rates based on factors like temperature, exercise, and the consumption of diuretics like caffeine. However, based on scientific research, a general guideline is that someone who is not exercising should drink around 235 ml of fluid per hour on average during the first 10 hours they are awake. For the remainder of the day, they should consume fluids as desired.

Why the 10 hour timeframe? The function of the kidneys, which are responsible for controlling how much water is reabsorbed and how much is lost in the urine, is affected by the time of day. The kidneys’ rate of filtration and urine production is higher during the first 10 hours of the day than it is for the remainder of the day. This is probably due to our need to retain water and electrolytes during the night, when we are asleep and not taking in any new water or nutrients.

Urine production is strongly circadian (meaning it follows a 24 our pattern).
Circadian rhythms and the kidney

This level of fluid intake is mostly independent of body weight, but will of course increase during exercise, in hot environments and following caffeine intake. There’s no hard rule for how much fluid intake should increase under these conditions, but there is a simple research-based formula called the Galpin equation that can be used to calculate average fluid intake during physical exertion. It still won’t be accurate for everyone because water loss during exercise depends on many individual factors, but it can give you a rough estimate of necessary fluid intake.

Why Not Just Drink According To Thirst?

If regulating water intake is so important, why can we not simply trust our body’s natural thirst response, which we evolved for this specific purpose? It’s a perfectly valid question, and one that is not possible to answer with complete certainty, but it’s probably for a similar set of reasons that we shouldn’t always trust our body’s natural hunger response. Evolution doesn’t care that much whether we get kidney stones when we are 60, and we evolved in an environment without immediate access to water 100% of the time, so having a more tightly regulated thirst response may not have been worth the evolutionary cost. Whatever the reason, experiments have shown quite convincingly that drinking according to thirst is not enough to maintain optimal levels of hydration – especially with older age, which is associated with an impaired thirst response.

How Much Water Is Too Much?

Water intoxication is quite rare, and is more common among athletes and soldiers who may drink too much water while undergoing intense physical training. It can also occur when people drink too much water while taking drugs with an antidiuretic effect (meaning they make the kidneys reabsorb more water). The kidneys are usually capable of removing about a litre of water from the body per hour. Case studies suggest that most people who developed water intoxication had been drinking 1.5-2 litres of water per hour on average for an hour or three before their symptoms emerged. In other words, it’s not something most people are likely to do by accident while aiming for 235 ml per hour.

There’s no single rule for everyone when it comes to how much water you should drink, but we hope this article gives you a more precise idea about optimal hydration than common guidelines. 

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    The Hydration Equation: Update on Water Balance and Cognitive Performance:

    Hydration and physical performance:

    Circadian rhythms and the kidney:

    Hyponatremia caused by excessive intake of water as a form of child abuse:

    How to Optimize Your Water Quality & Intake for Health:

    Title image by KOBU Agency, Upslash

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