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Sports Science Dehydration - Anatomy Fitness
Date: 11 May, 2009
Contact: Anatomy Fitness

Sports Science - Dehydration

In my last article, I talked about overhydration and how it affects your body. In this article well look at dehydration and why its important to make sure you drink enough.

Cyclists often have to carry their fluids with them, and/or plan where and when they are going to stop and drink something. Whether training for or competing in an event, you cant ignore how important fluids are. Even short events require planning leading up to them so that you are properly hydrated leading into competition, and dont suffer ill effects of too much or too little water.

Drinking while cycling can be a balancing act (pardon the pun); you need to offset how much fluid you need with where you are going to get that fluid from. If this sounds basic, Im not going to apologise: its important!

The Importance of Staying Hydrated While Training and Competing

Your body is made up of approximately 60% water. When you are exercising, you sweat, which causes water loss from your body. It is important to keep hydrated when exercising, otherwise you may start to feel the effects of this water loss (dehydration) most notably:




        Fatigue or weakness.

        Dry mouth


These symptoms occur when you lose approximately 2% of the fluid in your body, although every person is different. They are easily treated by drinking water if you keep on exercising without replacing the fluid lost from your body, more serious symptoms of dehydration could appear (at around 5% fluid loss) such as:

  • Increased heart rate
  • Increased respiration
  • Decreased sweating
  • Decreased urination
  • Increased body temperature
  • Extreme fatigue
  • Muscle cramps
  • Nausea
  • Tingling of the limbs

If you dont replace the fluid in your body, not only will the above symptoms start occurring, but it also increases the likelihood of injury. If you arent functioning as you should, then it becomes much easier to lose your footing or your balance and injure yourself. If you are exercising and feel thirsty, then drink water or a sports drink straight away.  It is very important to drink if you feel thirsty, but remember, prevention is always the best cure.

Best course of action

Your best course of action when exercising is to stay hydrated. This means that you need to plan ahead and make sure that you have plenty of fluid available while you are exercising, and make sure that you drink at regular intervals.

How much fluid do I need?

I covered this is my last article about overhydration, but it is worth repeating. So in case you missed it:

Each person differs according to bodyweight and intensity of exercise. The best way to work out how much water to drink during a typical training session is to follow a set formula.

For your typical training session, weigh yourself before and after. Work out how much weight you lost during the session. Add this to the amount of water you drank during that session to give your total sweat loss.


Subject X had a 90 minute training session

  1. Weight before training: 85kg
  2. Weight after training: 84.25kg

Therefore he lost 0.75kg during training (1kg lost is equal to 1000mL of water lost)

Subject X had 450mL of water during is training session

Therefore his total sweat loss was 1.2kg (0.75kg + 0.45kg) = 1200mL

Your total sweat loss for the session is equal to the amount of fluid you should drink for a similar session.

Subject X will need to consume 1200mL of water during similar training and competition sessions, he/she should spread this over the session, for example, drink 200-250mL every 15minutes if possible.


To work out percentage of bodyweight lost in that session, take the total sweat loss and divide it by your pre-session weight, then x by 100.


% of body weight lost = (1.2 kg/85 kg) x100 = 1.4%


What about hydration after my training session?

To check how much further hydration you need after a session, follow the steps above to work out total sweat loss for the session. You need to drink 1.5x this amount after a session to be fully recovered before your next training session.


Subject Y was training for a triathlon and has two training sessions each day.

  1. Weight before first training session: 65kg
  2. Weight after first training session: 63.9kg

Total fluid lost: 1100mL (lost 1.1kg)

% of body weight lost = (1.1kg/65kg) x 100 = 1.7%

To fully replace fluid losses before the next training session, Subject Y needs to drink 1100mL x 1.5 = 1650mL

Water or sports drink?

If you are training for less than 60 minutes per session, then water is a great choice. You could also drink a sports drink, although it is not absolutely necessary. If you are training for longer than 60 -90 minutes per session, then ideally you should be drinking a sports drink while training that consists of 4-8% carbohydrate and 500-700mg/L of sodium.

The carbohydrate and sodium are important as they aid in absorption and retention of fluid in the body.  Glucose and sodium are co-transported across the intestine. Once it has entered the bloodstream, the osmolality is changed by the carbohydrate so that water is drawn across also.  The sodium also helps to improve palatability, stimulate thirst and aids water retention in the body.

Tips for keeping hydrated:

  • Start drinking early.
  • Drink more fluid before and during exercise to reduce fluid loss and have less to replace
  • Drink an appropriate sports drink following training to immediately help replace lost fluid.
  • Have the right amount of fluids (water, sports drink) kept aside so you can monitor how much you am drinking
  • Get into a routine of taking fluid on board.



Exercise safely: keep yourself properly hydrated.

Click to enlarge...

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