Water water everywhere

How much water do we actually need to drink?

Water Water Everywhere


Fundamentally water is the most important nutrient.


It’s essential for our bodies, without it we can last several days before being unable to function and the normal bodily processes cease to work, leading to death. But water is also one of the most contentious areas in nutrition and with so much crap out there you might just feel like you’re drowning in it. Fear not, the following article goes through the facts about water.

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Why do we need water?


We need water as an ingredient in the reactions that occur throughout our body and we use it as a way of transporting other nutrients and chemicals throughout the body. We use it as a means of getting rid of waste products, which is probably what we most associate our water needs with. While replacement of the water we lose as urine and poo is important it's not the actually the main loss of water as shown in the table below. Contrary to what is commonly portrayed, our urine output accounts for just under half of the total water output of our bodies each day. So in reality we lose slightly more water each day in other ways than down the toilet.

The table below shows the average input and outputs of fluid from the body.

Adult Man

Adult Female

Intake (mls)

Fluids

1950

1400

Water in food

700

450

Metabolic water

350

250

Total

3000

2100

Output (mls)

Urine

1400

1000

Sweat

650

420

Exhaled Air

320

320

Insensible losses

530

270

Faeces

100

90

Total

3000

2100

So how much water do we need?


How much water we need to drink is one of those basic questions that seems it should have a simple answer. Fundamentally it does, and the answer is we need to drink water enough so that our bodies can function properly day after day.

How much water this actually equates to is affected by so many variables that to say we need to drink “X mls per day” is virtually impossible. Environmental factors also play a part, exercise and the ambient temperature will affect the amount we sweat and therefore our water losses will increase.

Water intake not only refers to water we drink as a liquid. Most food is mainly water and this ‘water’ is just as valid as a source of intake as drinking a liquid. Most fruits are 60 - 90 % water. Fibre in fruit and vegetables also retains water in the bowels and will in turn increase the water lost down the toilet, this is also the mechanism that increased fibre in the diet helps to soften the stools.

We also ‘make’ water as a by product of the chemical reactions that occur within our bodies constantly. This is how camels survive long periods without drinking. The fat stored in their humps is metabolised to release water which meets their bodies needs.


What about my 8 glasses of water a day?


The classic 8 glasses of water per day epitimises the type of nutribabble bullshit this site was set up to fight. There are a number of theories as to where this came from including an american ‘nutritionist’, an advertising campaign and a publication from the American Food and Nutrition Board. No matter where it came from, what we do know is that it’s not based on any evidence and essentially, it holds no water.

The current guidelines on water consumption are 3 litres per day for men and 2.2 litres per day for women from the US guidelines. The European guidelines recommend 2.5 litres per day for men and 2 litres per day for women.

Now obviously there is a large difference between the two figures which gives us the first clue that there may be some difficulty in coming up with figures like this and they shouldn’t be used as gospel as they so often are. That is not to say the figures are wrong but that they must be used for what they were intended with an understanding of their limitations.

Water Bottle

Guidelines are exactly that, guidelines. It would be lazy to use these figures as gospel and shows a poor understanding of their origins and intent.


Both the US and European guidelines are based on studies that ask people to recall their fluid intake over the last 24 hours. It’s a fairly crude way of estimating people’s fluid intake but it is what it is. There are lots of factors that make the information obtained in this way less reliable such as recall bias, we have a further post covering this in more detail and won’t go into this here.


What do the guidelines mean then?


The figures in the guidelines came about in the following way. The average fluid intakes from the large studies were calculated and then they added on roughly 20% to account for water in food. The result was then presented as an “adequate intake”. This was a rough guide to an amount of daily water intake that would be ‘adequate’ for a general healthy population. People with certain conditions and illnesses are not included in the recommendations.


The crucial point here is that an adequate fluid intake is not the same as a ‘minimum requirement’. The minimum requirement is what we need for our bodies to function normally. These guidelines are often portrayed as a “minimum” requirement, often by people who would like to sell you something to help you reach that “minimum requirement”.


Consider the guidelines another way and using these figures as a ‘minimum requirement’ becomes an absurdity. We know they are derived from the average reported intake of thousands of people with some extra added to this. Since its an average, by definition 50% of the people are drinking less than the average. If we take this as a ‘minimum requirement’ this means 50% of the people are not drinking the ‘minimum’ water requirement which should lead to dehydration. If we extrapolate this to the population it suggests that up to 50% of the population are not meeting their ‘minimum’ fluid requirements each day which is highly unlikely. There may be some who say that perhaps large portions of the population are in fact “dehydrated”, we would welcome any evidence of this given the current difficulty in assessing hydration at a population level that has been summarised by Popkin when he states in his review article “We have no acceptable biomarkers of hydration status at the population level”.


So how much should I drink?


Much like other nutrition guidelines there isn’t a definitive answer to that question. The guidelines are educated guesses of ‘adequate intakes’ and are not meant as a ‘minimum intake’. We haven’t discussed hydration status or measuring it in this article however our bodies have a very sophisticated thirst mechanism for a reason. We are unlikely to go far wrong in normal day to day life if we listen to it without all the marketing on top.

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