How Much Water Do You Need to Drink Every Day?

Curious how much water you should drink? Maybe you have read the headlines that you need 8 glasses of water a day, but are unsure what the evidence is behind this recommendation? Or perhaps you are worried you are not drinking enough and could be dehydrated? Is it possible to drink too much water and what is water intoxication? In this article I share the evidence about how much water you need to drink every day.

am i dehydrated? how much water should i drink a day

How Much of Your Body is Water?

Approximately 60% of your body is made up of water, with two thirds within the cells, and the remainder in extracellular spaces, between the cells. Gender, age, physical activity and body fat percentage can all influence the total percentage of water within your body. Water plays a vital role within your body, ensuring that you have enough blood volume, your kidneys and heart are able to function, and support the biochemical reactions that happen at a cellular level.

How Does Your Body Regulate Water?

Just like blood sugar, temperature and many other important functions in our body, total body water is tightly regulated by a process called homeostasis. This means the ability to maintain a relatively stable internal state, that persists despite external variation. Simply, this means that when you lose enough water, your body can sense this and responds by stimulating a thirst sensation so that you drink. Conversely, if you drink too much, then your body will produce less anti-diuretic hormone (ADH), also called vasopressin; this is the hormone that stops you needing to get up to pee all night. If you produce less ADH, then you will produce more urine and pee out any excess fluid. Using this system of thirst and hormonal control, your body is able to keep your total body water within a very tight safe range.  

What is Fluid Balance and Why Does it Matter?

Determining hydration status depends on understanding fluid balance. This means the amount of fluid consumed and produced, minus the amount of fluid lost and excreted. Most fluid loss is easy to calculate as this is via urine. But even if you didn’t eat, drink or move an inch, you would still lose water during the day. Fluid loss that is unmeasurable is called ‘insensible fluid loss’ and occurs through passing stool, humidification of your breath and sweating. These water losses are estimated to be approximately 800ml per day for an average healthy adult. As a by-product of biochemical reactions, 400mls of water is produced by your body every day.

Therefore, this means that in addition to urine, your net water loss is approximately 400mls of water per day. If you don’t replace this, then you will become dehydrated.

What is dehydration?

Technically dehydration is where you have lost salt and water. However, most people who aren’t a nephrologist, use this term when they have lost more water than they have taken on board, and so are in water deficit. More correctly, this isn’t dehydration, but volume depletion, as you are only short of only water, not water and salt.

There is a spectrum from mild to severe and life-threatening volume depletion, leading to headaches, dry mouth, dry eyes, dry skin, feeling light-headed, poor concentration and passing less urine. It occurs more frequently in hot weather as your insensible losses increase.

Water loss can happen more easily if you have:

  • Diabetes
  • Diarrhoea and / or vomiting
  • Drunk too much alcohol
  • Have a high temperature of 38˙C or more
  • Have sweated a lot after exercise

Why Does Water Matter?

In order for your body to function optimally, your total body water levels need to be in a tightly controlled range. If you don’t respond to the thirst sensation and drink, or you are unable to drink, or unable to keep fluids down, then mild volume depletion can progress.

What Does Water Loss Look Like in a Child?

Babies and children are not able to recognise or say they are thirsty in the same way, but you can recognise that they need more by the following symptoms and signs:

  • Dry mouth and tongue
  • No tears when crying
  • Fewer wet nappies than usual
  • Sunken eyes
  • Irritable
  • Lethargic and floppy
  • Sunken fontanelle (soft spot on the top of their head)

If your child has any of these, you should seek medical attention and try to encourage them to drink.

Tips for Rehydrating

If you are dehydrated to take fluids onboard little and often. This is especially important with children with diarrhoea and vomiting, as often if they drink a large volume, then have a rebound vomit. Instead try having 5mls of water or oral rehydration therapy (ORT) every 5 minutes.

When Should You See a Doctor?

It can be difficult to tell when mild dehydration starts to progress, so it is always best to get checked out if you are concerned. If you have any of these symptoms, you need to have a medical review:

  • you have had diarrhoea for 24 hours or more
  • can’t keep down fluids
  • have bloody or black stool
  • are irritable, disorientated or much sleepier and less active than usual.
can you drink too much water? water intoxication

How Much Water do You Actually Need to Drink?

You have probably seen the headlines that you should be drinking 8 glasses of fluid a day. Instead, the average Briton is estimated to drink less than 1 glass of water per day. Where does the 8 glasses a day come from and what is the evidence?

Research was carried out in the 1950s and an approximation of how much water was needed to replace fluid loss was determined for children using the 4 – 2 – 1 principle. These calculations are based on fluid needed per hour based on body weight. For the first 10kg of body weight 10ml/Kg/hour is needed. For the next 10Kg of body weight, 20ml/Kg/hour is needed, with 1ml/Kg/hour for any subsequent kilo thereafter. So for an average 70Kg adult, this equates to [(10×4) + (10×2) + (50×1)] = 110ml per hour. Over a 24 hour period, this is 2640ml (110×24). An average glass contains approximately 300mls. 2640mls is 8.8 average sized glasses.

Another simpler way of calculating approximate fluid requirement for adults only, is to multiply 0.033 by weight in Kg (70Kg. x. 0.033 = 2.31L).

But really all you need to remember to do is drink if you are thirsty, as drinking more than you need is not associated with additional health benefits.

Remember if it is hot, or you are sweating more due to a temperature or physical exercise, you will need more.

Do All Drinks Count as Fluid Intake?

While water helps to hydrate you, some drinks such as tea and coffee are diuretics and so can actually stimulate you to pee more out instead of hydrating you. Alarmingly for many people their fluid consumption is comprised of sugary or fizzy drinks of juices. Coconut water is a great source of potassium, and the unsweetened varieties are low in carbohydrates and can be hydrating instead of drinks marketed for sports hydration. But aim to have your primary drink as water, as evidence suggests this is good for rehydration, even after intensive exercise.

Can I Drink Too Much Water?

It is possible to drink too much water, although if you are healthy it’s rare to voluntarily drink too much. This is because if your kidneys are healthy, they will pee out most of the excess water. However, they will eventually reach a point where that is not possible, and the finely tuned concentration of salts in your body will become diluted. This is called water intoxication, and is very dangerous. It can lead to fits, and even death. If you have kidney disease this can increase your risk. Other conditions are also associated with drinking too much, for example you might have heard of psychogenic polydipsia, which is increased water intake (10-15 Litres a day) after taking certain drugs like MDMA and ecstasy. This quantity of water overwhelms the kidneys. Also if you do endurance exercise and drink lots of water, it’s really important to take on board salts too, as these are lost in sweat. Just drinking water only could lead to dilution of your cellular salt concentrations, leading to water intoxication.

water content of food and preventing dehydration

Water Content of Food

Some of the water you take into your body is stored in food. This is one of the reasons why if  you eat lots of fruit and vegetables, you might not need to drink quite so much. Examples of food with a high water content are:

  • cucumbers – 96% water
  • lettuce – 96% water
  • celery – 95% water
  • strawberries – 92% water
  • watermelon – 92% water

Fruit and vegetables also contain vitamins, minerals and polyphenol antioxidant compounds.

If you enjoyed this article you might find what are the health risks of microplastics useful to read for how you store your water. Do you drink it out of plastic bottle, glass or metal bottle?


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