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Are you getting enough fluids?

Mark Fardon explains the facts, fiction and provides some practical advice on fluid intake.

An adult human is made up of 55 to 65% water, which is equivalent to about 9 gallons (40 litres) and is vital to sustaining life and is a critical ingredient in all body cells and fluids. We can last a lot longer without food (up to 80 days) than water (3 to14 days). Part of the reason for this is that we can store substantial energy reserves as fat, which has a high ratio of energy to its volume, but water cannot be compressed. Unfortunately, fat is not easy to convert to energy for the sort of levels of "fuel" consumption required by an athlete. This is why we use carbohydrate-based diets.

For any power output, we need to transport "fuel" (glycogen) from its "tank" (liver) to the "engine" (muscles). The fluid is the most efficient way of transporting this "potential power". In the case of a car, it is the oil that lets the bearings run smoothly, for us it is blood. If you think of oil in a car, it becomes less efficient as it gets old, petrol gets used up and needs replenishing, and you need water in your radiator to stop the engine from overheating and seizing. In all these cases the key to the best and most efficient performance is to keep the fluids fresh and topped up!

So how much do we need and what grade should it be?

Like cars, humans are not all the same. Look at us! We get everything from clapped-out bangers, which would not pass an MOT, to high-performance dragsters that go like rockets, but would struggle with a 3-point turn. In between there are family cars, off-roaders, container trucks, motorcycles and a mass of others; but all have one thing in common - they need the right fluids to run, and we are the same.

So, effectively water is a catalyst that lets us work properly:

  • It transports nutrients such as glycogen, salts and oxygen to the right parts of the body
  • It removes waste products such as lactic acid from muscles through the bloodstream and from the kidneys in the form of urine
  • It helps regulate our body temperature between strict limits in the form of sweat
  • It lubricates the joints and eyes
  • It assists in breathing
  • It maintains blood pressure and volume

On this basis, we must maintain our levels of fluid when stressing the "engine" (body) in a sport.

How much water do we use in a day?

It depends on who we are, what we are doing and where. An average, an inactive individual of 75kg (about 12stone) in a mild climate, will use about 2½ litres of water a day (see table below); that is half-a-gallon.

Daily water input Daily water losses
Food 1000ml Urine 1250ml
Drinks 1200ml Faeces 100ml
Metabolism 350ml Skin 850ml
    Lungs 350ml
Total 2550ml Total 2550ml

How much do we need to be drinking?

The answer is always to make sure it is enough. Fluid loss is a great short-term way of appearing to lose weight, but it is only temporary and can have severe effects on athletic performance. 2% loss will affect your performance, and a 5% loss could lead to as much as 30% loss of power output. That may sound like quite a lot. For our 75kg individual, it would mean 4 kgs of water, but if they were 2% below optimum to start, then the loss of just over 2kg will have them hitting the 95% barrier. On a hot day on a bike, this can happen very quickly due to sweat loss.

The thought of having to drink half-a-gallon of water each day while you are at rest may seem a bit daunting - it is. This figure includes the water content of foods, which can be quite substantial. For instance, on a "per edible portion" basis (i.e. not per 100g), tomatoes can have a water content of 93g and white fish 82g, while crisps have just 2g. However dried dates, which are a good energy source, have 12g water but potatoes (another good source of complex carbohydrate, through starch) have 79g water per portion. The solution is to take an interest in what you eat and drink. When you exercise, you will lose a lot of weight through the fluid losses detailed above. However, there is also a bit of weight loss by using up energy in the form of glycogen that is formed from fat, protein and carbohydrate intake.

But how much do we lose?

As an example, a 10-mile cycling time-trial will use about 500 Calories, which constitutes about 65grams (2.3oz). If you lose more than that - you are dehydrating.

Regulation of body water balance

The following table is a recommended daily intake of water for a sedentary individual with a body mass of 70 to 75kg living in a temperate climate. (Intake = food + drinks + metabolism)

Age Intake ml/kg body weight
(i.e. For 75kg individual)
12 years 58 4.35 litres
20 years 35 2.625 litres
50 years 35 2.625 litres
60 years 30 2.25 litres

The water content of food - per edible portion

Tomatoes 93g water
Melon 92g water
Milk 88g water
White fish 82g water
Potatoes 79g water
Low fat spread 50g water
Butter, margarine 16g water
Dried dates 12g water
Crisps 2g water

Why do we sweat?

Exercising muscles produce heat, but the body's temperature needs to keep within safe limits, around 37 to 38° C. To do this, we work in a similar way to a car's engine. We use a radiator for water-cooling. Sweating is the primary way of getting rid of heat from the body (by evaporation). Unlike a car's system, which is sealed and so re-circulates cooled water, ours allows the fluid to evaporate and so needs regular topping up to maintain performance.

The rate of sweating depends on several factors:

  • Work rate - If you indulge in more physical work, you use more energy and create more heat, so need more cooling, which needs more sweat
  • Environmental temperature - If it is warm, your body does not have to work so hard to maintain its temperature. If it is working hard, it will need more cooling than where the environments assisting in the cooling process already. Wind and wind-chill work in the same way. In this respect, a cyclist that is putting out the same wattage as a runner will get colder because they are travelling faster and getting a greater airflow over their sweating body
  • Humidity - The humidity or moisture content of the air affects how easily sweat can vaporise from the skin into the atmosphere. In high humidity, sweating will be less effective because it is harder to transfer to a "wet" atmosphere than "dry". Think of trying to squeeze a sponge out underwater rather than over a bucket, which will remove its water faster? Alternatively, consider which environment a boxer or jockey uses to lose fluid weight quickly, a Turkish bath or a sauna?
  • Body surface area - This is interesting and requires a little maths. You may think a big, fat individual has a greater surface area, so should be able to sweat more than a small person and cool off faster. Unfortunately, they also have a larger core to regulate the temperature because the core size increases at three times the speed of the surface area, the surface area of a large individual will have to work much harder to keep them cool! On the other hand, the small person will be much more prone to hypothermia when they get cold.
  • Hydration status - If you are dehydrated already, your body is going to do a balancing act between avoiding further dehydration and (also possibly dangerous) overheating. One way of doing this would be to stimulate cramps, so you do not use so much energy and do not need so much cooling. Whatever happens, it is unlikely to be conducive the maximum performance, so keep the balance right!
  • Training status - A trained athlete (with adapted muscles, heart and lungs) will be able to perform more efficiently, effectively and therefore more economical than an untrained one. This means they will not need to sweat so much for a given level of effort because their "engine is" better tuned.
  • Clothing - Wearing the right gear may sound obvious, but it is worth keeping up with materials technology. Gore-Tex has made a fantastic difference in the volume and breathability of clothing. Wicking under-vests also make the journey more comfortable and can assist in the process of temperature regulation.

How dehydration can affect performance

It is important to know how well hydrated you are as well as weight and body-fat percentages. This used to be quite expensive and complicated to find out but can now be done with scales that are widely available for less than £100. If that is still too expensive, it may be worth considering buying a set as a group (a sports club). The only caveat would be to make sure they are adequately cleaned between users to avoid risks of passing foot problems such as verrucas between users. The scales work by passing a small pulse between the feet and measuring the resistance, compared with the user's age, height and sex.

Accuracy is pretty good and permits a graphic record of improvements in weight, fat and hydration management.

  • Dehydration by as little as 2% loss of body weight starts to impair performance
  • Dehydration above 5% loss of body weight can decrease work capacity by 30%
  • Heat illness can occur when 5 to 6% of body weight is lost

What happens?

  • Muscular strength/power is affected
  • Prolonged moderate to high-intensity activity is most susceptible to dehydration due to the problem of getting the body to perform at a high level AND take on more fluids to maintain hydration at the same time
  • Perception of effort is increased for the same workload - you feel more fatigued or reduce your self-selected pace
  • Effects occur at all levels of dehydration and increase as the degree of dehydration increases.
  • Myth - "Dehydration can be tolerated" - it cannot, any dehydration will reduce performance.
  • Dehydration reduces mental functioning:
    • Decision making
    • Reaction times
    • Concentration
    • Anticipation
    • Skill delivery
    • Task inaccuracies

To sum up - monitor yourself

  • Record your body weight before and after exercise and allow for different types of weather
  • 1kg weight loss is equivalent to 1 litre of sweat lost
  • Pee test - monitor colour, volume and frequency
  • Keep a record of fluid intake
  • Know your requirements

Article Reference

This article first appeared in:

  • FARDON, M. (2004) Are you getting enough fluids? Brian Mackenzie's Successful Coaching, (ISSN 1745-7513/ 18 / December), p. 6-8

Page Reference

If you quote information from this page in your work, then the reference for this page is:

  • FARDON, M. (2004) Are you getting enough fluids? [WWW] Available from: [Accessed

About the Author

Mark Fardon is a UK Cycling Coach and works in the IT sector. He gained his first coaching qualification in 1981 from BAWLA (British Amateur Weightlifting Association). He went on to gain cycling-specific coaching qualifications with the Association of British Cycling Coaches and the British Cycling Federation. The riders Mark coach age from 12 to 84 years and have ranged from world-class triathletes and mountain-bikers to complete novices, aiming to get fitter and lose weight.