Dehydration: how much should we drink per day?

As the northern hemisphere approaches the summer months, many of us begin to think about increasing our fluid intake to prevent dehydration. But exactly how much should we be drinking a day? Does the amount vary depending on what we are drinking? And what happens if we don’t drink enough fluid?

Water image

How much should you drink per day?

The Department of Health estimates that we should drink around 1.2 litres of fluid per day, according to the NHS. (1)This equates to eight 150ml (around 5-oz) glasses.

The literature on fluid intake can be confusing. This is because some sources refer to the amount of fluid we should intake through beverages alone; whereas other sources refer to the total water intake.

This difference may be partly responsible for the confusion over how much water it is necessary to consume on top of the water found in your food.

Twins test effects of water intake on skin health and beauty - BBC

Twin experiment on consumption on water in addition to that provided by food. Source BBC

According to a study carried out in the U.S. and reported by the Food and Nutrition Board of the Institute of Medicine of the National Academies, drinking beverages made up 81 percent of total water intake on average. Thus, water in food provided approximately 19 percent of water intake for those surveyed. (2)

The NHS reports an even higher amount of water coming from foods – around 1 litre per day. (1) The explanation for this difference probably depends on what foods the statistic was calculated on. The comparison of the water percentage provided by food is going to depend on your diet.

Your recommended fluid intake actually depends on how much food you consume. On average, people need 1-1.5ml of water for each calorie they consume. (3) For a person consuming 2000 calories this corresponds to 2-3 Litres of fluid per day.

The amount you should drink will also depend on how much exercise you are doing. The NHS recommends that, in addition to your normal daily intake, you should consume up to 1 litre (or 2 pints) of fluid for each hour of exercise you carry out. (4)

Water intake should also be increased during pregnancy. This is partly due to pregnant women being advised to increase their calorie intake. It is also to maintain fetal circulation, amniotic fluid and a higher blood volume. (3)

Essentially then, how much you should drink per day will vary from person to person. It will depend on a number of factors including what food you are eating and how much exercise you are doing.

How do different fluids vary the amount we should drink?

Caffeine is a diuretic which means that it makes your body lose water, i.e. it makes you urinate more frequently. Consequently, drinks that contain caffeine, such as coffee, tea, cola and energy drinks may have a negative effect upon your water intake. (5)

Specifically, a study published in the European Journal of Epidemiology found that water losses due to caffeine corresponded to 1.17ml per mg of caffeine. (6) This works out at a loss of 163.8ml per mug of filter coffee (based on it containing 140mg caffeine). (7)

The study also found that alcohol caused water losses of 10ml per gram of alcohol. (6)

Based on an average glass of wine, the alcohol content will cause you to lose more fluid than is provided by the glass itself!

This is estimated for a large (250ml) glass of wine of average strength (13%).(8) The alcohol content of the glass is 32.5ml, which equates to 25.6g of alcohol (based on 100% ethanol having a density of 0.789g/ml).(9)This means that the body will lose 256ml of fluid, which is a higher volume than the glass of wine itself!

What happens if we don’t drink enough fluid?

In essence, not drinking enough fluid leads the body to become dehydrated. One may initially have only mild dehydration and feel thirsty and uncomfortable, but dehydration can escalate and become very serious. In severe cases, dehydration can even result in death. (10)

Dehydration can have a range of effects upon the body including:

  • Headaches
  • Light-headedness or dizziness
  • Lower volume of urine output
  • Dark-coloured , strong-smelling urine
  • Lack of energy
  • Muscle weakness
  • Dry mouth, eyes and lips
  • Fast heartbeat (1, 10, 11)

There may be other longer term health problems that are caused by dehydration:-

  • Constipation
  • Kidney Stones
  • Fatigue (10)

A recent study, published in the British Journal of Urology, stated that increasing water intake can reduce risk of kidney stones. (12)

April Cashin-Garbutt

Written by

April Cashin-Garbutt

April graduated with a first-class honours degree in Natural Sciences from Pembroke College, University of Cambridge. During her time as Editor-in-Chief, News-Medical (2012-2017), she kickstarted the content production process and helped to grow the website readership to over 60 million visitors per year. Through interviewing global thought leaders in medicine and life sciences, including Nobel laureates, April developed a passion for neuroscience and now works at the Sainsbury Wellcome Centre for Neural Circuits and Behaviour, located within UCL.

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Comments

  1. Simon Simon Australia says:

    Sitting slouched behind our computers in an airconditionned office all day, it is too easy to go without drinking enough water!

    Even though the cooler is just feet away, I used to find realize that the whole day had gone by without a glass of water. Coke or coffee are not acceptable substitutes!

  2. David Roberson David Roberson United States says:

    The number and conclusions are inaccurate.  

    What is coffee?  What is tea?  What is water?  What is soda?  What is diet soda?

    Coffee is water with a ground bean in it.  If you swallowed a couple of those beans and drank a 12oz glass of water, does it count towards your water intake?  A cup of coffee is generally under 100mg of caffeine.  That means even if the number involving liquid loss was correct, you would still hydrating at nearly 1 cup for ever 1.5 cups you drank.  

    Tea is water with an herb in it.  If you ate the tea leaves and then drank the water, would you count the water?  There is 26mg of caffeine per cup.  This means even if the incorrect caffeine dehydration numbers are to be believed, it is like drinking water.

    If you eat a candy bar with a water, does it negate the hydration?  No, that is stupid.  Neither does a table spoon of sugar and a small amount of caffeine.  Diet sprite, ginger ale, and the like will hydrate you exactly like water will.  Diet caffeinated sodas are nearly identical too.

    Finally, the loss of water doesn't occur at that rate for people who regularly drink caffeine.  In all likelihood, if you wake up today and drink 4 cups of coffee then you probably do that every day.  The loss is minor to you because your body processes caffeine differently.  

    If you put a packet of flavor in your bottled water then you have just made diet caffeine free soda.  Carbonation doesn't change hydration.  

    Just some food for thought.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of News Medical.
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