Water – how much is too much?

Fluids 1

Is water good for you?

Some health hippie nutters say you have to drink 8 litres of water a day.  Not only is this bad for the environment (plastic water bottle excess), but it may also be a waste of money.


Usually, there is nothing wrong with drinking a bit of water. It seems common sense to ensure you are hydrated when busy running around.

So should you drink when you are thirsty or is that too late?

Reassuringly, it seems thirst does occur before you get dehydrated. So if you drink when your thirsty things will go well.

The sensation of thirst is regulated by a combination of hormones (ADH) and physiological ‘osmoreceptors’.  You can trust your body to tell you that you are thirsty:

Thirst Osmoreceptor

Problems can occur when you believe the myth that you need to hydrate yourself to an oceanic degree because excessive amounts of water can be fatal.

In 2007 Jennifer Strange, a mother of three children, entered a radio based competition.

The contest involved drinking as much water as possible.

Hold your wee for a wii” asked contestants to drink as much as possible without going to the toilet.  After 7 1/2 litres of water the Wii was one but Mrs Strange collapsed.

Despite attempts at saving her life in hospital she succumbed to severe brain swelling and she died.

So the lesson is don’t enter a drinking competition and over yourself hydrating can dangerous.

The same phenomenon is also frequently seen in amateur marathon runners who feel the 26.2 mile race requires them to continually drink throughout the race.  Over hydration has led to several deaths in runners.



RSS icon

So how much fluid do you actually need, medically speaking?

This is pretty important.  As a nurse or doctor when you are working out how much a fluid a patient will need if they cannot eat for some reason we need to ask a few questions:

  • What are the basic daily requirements for fluid?  
    • Outputs and Inputs
    • Generally in a well patient ‘insensible‘ (physiological) losses of water from metabolism will be higher than ‘insensible gains‘ by about 500ml in an adult.
  • Are they sick and using up more fluids than normal?  
    • Sick patients will have higher requirements for fluids and water replacement.
  • How much sodium and potassium do they need?
    • Not too much, but enough.
    • A fasting patient should have:
      • 1L of saline (0.9%) – provides 150mmol of Sodium and unfortunately a little more chloride than desired
      • 2L Dextrose 5% (effectively water after you use the sugar)
      • A dash of potassium (30-60mmol/day)
    • Measure the potassium regularly in hospitalised patients
  • How much fluid is in the human body?
    • There is between 50 and 60% water (about 42L).
    • The percentage of water correlates with ‘lean’ mass.
    • Lean young males have a higher water percentage (closer to 60%) compared with fat older females (closer to 50%).

Slide1Where does the replacement fluid go?

  • 1/3 (12L) of the body’s water is in the circulation and interstitial spaces (i.e. in the blood volume and bathing the cells)
    • ‘Normal’ Saline 0.9% when given will stay in this space (distributed equally across the 12L of extra-cellular fluid)
    • Fluids like Blood and Albumin (colloids) will stay in the circulation
  • 2/3 (30L) of the body’s water is in the cells
    • 5% Dextrose ends up equally across the total body water (distributed equally across the 42L of total body fluid including the cells)

RSS icon

The Basic Requirements – Output



The Basic Requirements – Inputs


Fluid 2

Salts (Sodium and Potassium)

Fluid 1


  • Don’t over hydrate if you are running a marathon
  • Don’t buy plastic bottles to over ‘hydrate’ yourself
  • Prescribe fluids to your patients considering their daily needs and ‘insensible’ losses
  • Don’t drink or prescribe excessive amounts of one fluid