Clutching a bottle of water is now considered a normal part of daily life. People often make a health resolution to drink more fluids, hoping for it to bring them a host of health benefits including better skin and improved digestion. But how much water should you drink a day, is it really necessary to consume several litres, and can you drink too much?
Dr Juliet McGrattan looks at the importance of water, how fluid balance works and what really happens to your body if you don't hit your daily fluid intake:
What does water do in the body?
Did you know that your body is around 50 to 60 per cent water? If you weigh 60kg then you are made up of about 30 litres of water. All your organs, tissues and cells contain water, even your bones. It’s essential for life and we all need to consume it to stay alive.
Water plays lots of different roles in the body including:
A building material for cells, tissues and organs (your brain is 75 per cent water).
Clearing waste from the body – urine and faeces contain water.
Transporting oxygen and nutrients around the body.
Maintaining blood pressure.
Regulating body temperature.
Keeping your mucous membranes such as airways and gums moist.
Forming body fluids such as saliva and stomach juices.
The body likes to be in homeostasis which means that everything is in the right balance for it to perform at its best. Maintaining the right fluid balance is a vital part of this. It keeps a tight regulation on fluid balance and day to day, the amount of water in the body only varies by a few per cent.
💧 Water in
We get water into our bodies from three sources:
The body makes a small amount of its own water from metabolism.
Fluids - most of the water we consume comes from what we drink.
Food - around 20 per cent of our daily water intake comes from the food we eat. Fruits, vegetables and soups are high in water content – a cucumber is 95 per cent water.
When we ingest water in fluids or food it is very quickly absorbed in our stomach and intestine. Within five minutes, some of it will have reached our blood stream. Water then moves from the blood into our tissues and cells.
💧 Water out
We lose water from our body in a number of ways:
Breathing – you can see this in action when you breathe out on a cold day.
Sweating – we all sweat daily but this increases significantly during exercise.
Urine – 500mls to 2500mls of urine passed per day is normal.
Faeces – a small amount, (around 200mls per day) is lost in normal stool; excessive amounts can be lost with diarrhoea.
What we call insensible losses – those that you can’t easily measure, such as water lost through breathing and sweating, account for around 750mls of water lost per day.
Regulating fluid balance
The body likes to keep the fluid balance tight but how does it achieve this when what we take in and what we lose varies so much every day?
There are two functions at play here, thirst and kidneys:
When our body needs more water it tells us. Messages go the brain from different cells around the body to inform it that water levels are low. The brain gives us the sensation of thirst which drives us to seek a top-up.
Sometimes though, we drink when we aren’t thirsty. We drink because we’re eating and it helps the food go down, we drink because it’s a work break and we always have a cup of coffee at 11am, we drink because we’re on a night out with friends. Thirst can’t be and isn’t the only way our body regulates our fluid balance.
Our kidneys are the most important way our body controls our water levels. They can both retain and lose water. Kidneys contain millions of nephrons. These are microscopic sieves that filter out the waste and toxins in our blood and send them out of the body in urine. The kidneys use sodium (salt) to move water around. It’s a known fact that water moves to more concentrated areas, this is known as osmosis.
If the body wants to top up its water levels, it takes back sodium that has gone through the sieve. This increases the concentration of the blood and results in water moving into the blood too. Conversely, if the kidney wants to lose water, it doesn’t take back this sodium, sodium levels in urine are higher and the amount of water lost in the urine increases.
How much water should you drink?
The amount of water that you need to drink varies so much day-to-day and is influenced by many factors including:
Age – fluid requirements decrease as you age.
Gender – women require slightly less water than men.
Body composition – fat contains much less water than muscle so people with more body fat require less water and those with high muscle mass need more.
Climate – hot weather makes you sweat more and need more water.
Activity levels – higher levels of exercise require higher levels of fluid intake.
As a result of these differing requirements, it’s hard to set an exact daily water intake target and guidelines for adults vary hugely:
European Food Safety Authority 2010 recommend 2.5 litres per day for men and 2.0 litres for women.
The Food and Nutrition Board 2004 (US and Canada) give an adequate intake (AI) of 3.7 litres per day for men and 2.7 litres for women.
There are many calculations that involve using your body weight as a guide for how much water you need. For example, take your weight in kilograms and divide by 30, eg 60kg divided by 30 = 2 litres.
Remember that these amounts include the water that you get from your food as well as your fluids. In terms of how much actual fluid you should drink, the NHS guide is six to eight glasses per day which is around 1.2 litres.
What type of liquid counts?
Any type of liquid is fine; it doesn’t have to be plain water. Milk, fruit juices, tea and coffee all count towards your daily fluid goals. Caffeinated drinks will make you need to pass urine more quickly but won’t dehydrate you and do count towards your daily intake. It’s less clear how hydrating or dehydrating alcohol is so it shouldn’t be relied on as part of your fluid intake.
Are you drinking enough water?
When your body doesn’t have enough water it tells you. A report by the World Health Organisation in 2004 gives the following consequences of not drinking enough water:
1% dehydration causes thirst.
3% dehydration gives a dry mouth.
4% dehydration results in a fall in work capacity.
5% dehydration causes difficulty in concentration, headaches and sleepiness.
7% dehydration can lead to collapse.
10% dehydration is life threatening.
Mild long-term dehydration is linked to constipation, an increased risk of urinary tract infections and kidney stones. Its effects on other body systems are still being investigated.
Simply drinking when you feel thirsty will keep the balance right. You can also keep an eye on the colour of your urine. Aim for a pale yellow colour. Urine is darker first thing in the morning when it is more concentrated but a big drink with breakfast will counteract the overnight reduction in fluid intake.
Can you drink too much water?
It’s best to spread your fluid intake over the day into six or eight small drinks than to have just one or two large ones.
Kidneys can cope with a lot of fluid but excessive water consumption over a short period of time could lead to over-hydration and water intoxication. For example, four litres of water over 24 hours is fine, but that same amount over one or two hours would be dangerous.
There are some medical conditions where a doctor may suggest you limit your fluid intake. These include:
Congestive cardiac failure
The main problem with over-hydration is that it affects the sodium levels in the body and upsets fluid balance. Hyponatraemia means low sodium levels and people who do endurance sports are at particular risk. Sodium (salt) is lost through sweat and significant amounts are lost during an endurance event.
Drinking large quantities of plain water without replacing any of the lost salt (or other electrolytes) effectively leads to blood dilution. In the early stages this causes cramps, nausea and headaches but it can progress and become life threatening. It is easy to avoid by not drinking large amounts of plain water and replacing electrolytes through sports drinks, food or salt tablets.
Your body is great at maintaining a good fluid balance.
Be guided by your thirst.
There’s no need for continual sipping but drink water regularly throughout the day.
How much water you need will vary with the weather and how much exercise you’re doing.
Last updated: 01-04-2021
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