I’m sure we’ve all heard the advice to drink 8 glasses of water per day. But is this backed by evidence? Read on to find out!

Why do we need fluid?

Fluid is essential for survival and health. In fact, around 50-70% of the human body is made up of fluid, which varies at different ages (1, 2).

Important functions of fluid in our body include (2):

• waste removal

• regulating temperature

• transporting nutrients

• joint lubrication

• kidney and urinary tract health

• digestive health and reducing constipation

• vital component of cells and blood etc.

Dehydration occurs when 3% of body weight is lost due to lack of fluid (3). But even 1-2% loss of fluid can lead to issues with memory, concentration, mood, fatigue and physical performance (4). More severe and prolonged dehydration can lead to death if not addressed quickly and safely (5).  

Is there evidence to back the 8 glasses of water per day rule?

It isn’t clear where the advice to drink 8 glasses of water per day came from, but it has spread far and wide.

The amount of fluid we need varies between different people based on:

• body size

• amount of movement

• heat exposure

• individual factors like medical conditions, medication, pregnancy, breastfeeding etc.

So a rule that everyone should be having exactly 8 glasses of water per day doesn’t make sense, especially as we can get our fluid from a variety of sources, not just water.

Health professionals sometimes use equations or adequate fluid intake guideline amounts for estimating fluid requirements. These are very much an estimation as a starting point, and there are issues with their reliability (6, 7, 8, 9).   

Based on these guidelines and calculations, estimated adult guidelines for daily fluid intake often come out between around 1600 – 2500ml (2, 10). Using a standard 250ml glass or cup, this works out as around 6 to 10 glasses/cups per day. 

So 8 glasses of fluid per day might suit some people who require 2 litres per day, but there are lots of individual variations to consider and as highlighted these equations and guidelines only provide a rough estimation to begin with. 

How can I tell if I’m staying hydrated?

Since equations and calculations of fluid intake are limited and can’t provide the full picture, it’s important to pay attention to signals from your body when it comes to hydration.

Thirst is an obvious place to start. When we’re thirsty our mouth becomes dry and we might notice some of the other signs of dehydration like fatigue, headache, lightheadedness, dizziness or issues with concentration or physical performance. 

Although responding to thirst is important, some studies suggest that when we start drinking we can stop feeling thirsty before our body is fully rehydrated (2, 11). On the other hand, some research in athletes has found that drinking to thirst is a very effective and safe strategy (12, 13).  

Another method that athletes sometimes use is to weigh themselves before and after training to calculate their fluid losses and how much fluid they need to rehydrate (14). 

But the best way to check hydration is to look at the colour of our urine (15). Pale yellow urine indicates being well hydrated, but anything darker than this indicates dehydration. Dehydrated urine may also have a stronger smell as it’s more concentrated. Clear urine is also a sign of good hydration, but if this is consistently clear it can be a sign of overhydration.

You can find an example of a urine colour chart here form NHS Inform Scotland

It’s also important to be aware that more fluid is needed in certain circumstances, such as (2):

• hot weather

• being more active

• when pregnant (increases by ~1 glass per day)

• when breastfeeding (increases by 2-3 glasses per day)

• replacing fluid losses from diarrhoea or sweat (e.g. when you have a fever)

On the other hand, there are cases where people are advised to restrict or limit fluid intake. For example, if your body is retaining too much fluid due to heart failure or kidney disease.

What counts towards daily fluid intake?

All fluids count towards fluid intake, including:

• water

• milk

• juice

• flavoured drinks

• fizzy drinks

• tea

• coffee

However, UK public health advice includes limiting the intake of sugary drinks and limiting fruit juice to one small glass per day (16). Pregnant women are also advised to limit caffeine intake to 200g per day, which is roughly 2 mugs of instant coffee or 3 cups of tea (16). 

Although caffeine can increase urine output, the fluid in caffeinated drinks like tea and coffee makes up for this which means they aren’t dehydrating (17, 18). 

Whereas alcohol can make us pass more urine and be more dehydrating, especially when consumed in higher amounts (19). Alcohol also isn’t recommended on a daily basis or in large amounts due to the variety of health risks associated with this (20).

Food actually provides fluid as well. It’s estimated that foods provide around 20-30% of our fluid intake in the UK from food like soup, stews, ice cream and fruit and vegetables that have a higher water content (2). 

We can also lose electrolytes like sodium when sweating a lot or from severe or ongoing diarrhoea. So in some cases replacing electrolytes using food, sports drinks, sports supplements or rehydration solutions. Severe dehydration can require medical support in order to rehydrate safely. 

Summary

Staying hydrated and consuming enough fluid is undoubtedly vital for our health. 

The rule that we all need to consume exactly 8 glasses of water per day is a myth, as although it may work for some people, it can’t be applied to all people and situations and we can consume fluid from all types of drinks, as well as many foods. 

The best way to stay on top of hydration is to keep an eye on the colour of your urine, if this is darker than pale yellow it means you need to rehydrate. Paying attention to thirst and other signs of dehydration is also important.