Fluid loss during prolonged efforts causes – in case of insufficient drinking – a decrease is circulating blood volume. The blood gets more viscous then. In order to maintain oxygen supply the heart has to work harder which may be noticed by the increase of the heart rate. Not drinking during efforts causes a clearly higher heart rate compared to the same circumstances in which the athlete does drink. In other words: by not drinking, or drinking too late or too little performance capacity decreases. Also the moment of exhaustion comes sooner.

A good fluid balance during exercise is of major importance. The biggest nutrition mistake during sports activities one can make is drinking too little. The human body consists for more than 60% of water. Fluid has several important functions such as:

  • dissolving and transporting numerous substances
  • regulation of heat loss
  • excreting toxic substances with sweat and urine.

During physical exercise a lot of heat is produced. In order to avoid overheating the body will begin to sweat (perspire). An athlete may lose much water by perspiration.

Tough exercise in warm conditions may cause a sweat loss of up to 3 liters per hour. When this loss is not compensated in time it will cause diminished performance capacity.


With a fluid loss of more than 6% of the body weight the consequences may be disastrous. A slight fluid loss of 2% may already lead to diminished performance. With a loss of 2 to 6% strength and endurance diminish by about 30%.

An athlete of 70 kg will not be capable of maintaining his performance level with a fluid loss of 3 to 4 kg, which is 4 to 6% of his body weight.

Fluid loss in % of the Body Weight Consequences
2% Disturbed heat regulation. Body temperature rises. Thirst.
3% Decreased endurance capacity
4 - 6% Feeling the need to drink decreases. Headache. Diminished power.
6 - 10% Overheating. Muscle cramp. Dizziness. Fatigue. Sortage of breath. Vomiting
> 10% Hard swallowing. Fluid recovery only possible by infusion. Unconsciousness, Coma, Death

Well-trained athletes lose less water and salt during exercise than untrained persons. By sporting frequently the body adapts to a better sweating efficiency. In order to know the extent of fluid loss during exercise in various conditions (training, races, heat and cold) you should weigh yourself before and after the exercise. The differences thus obtained indicate the degree of fluid loss. When an athlete of 62 kg loses 3 kg in total in his workout will not be very efficient. His fluid loss is nearly 5% of his body weight. Such a workout is not effective; you are destroying your body.

Weight loss during exercise is not only caused by dehydration. It is known that glycogen binds quite a lot of water. When these glycogen depots are emptied water will be lost which does not count for dehydration, the more so when a special carbohydrates loading diet was followed. In that case body weight can go up by 2 to 3 kg because of water bound to glycogen. On that basis some weight loss during prolonged exercise is acceptable.

Drinking in the Tour de France              Long time ago

There is a golden rule for drinking: drink regularly during exercise and do not wait till you are thirsty. When you drink for the first time when you get thirsty, you are mostly too late. When you get thirsty a weight loss 2-3% may already be the case. Practice shows that very often athletes drink too little. The average compensation of fluid loss through drinking is about 30-50%


The osmolarity of a drink is the most important factor that decides the speed of passage through the stomach and the intestines and from there the absorption by the body, or more specific by the blood. Osmolarity indicates the number of particles, consisting of carbohydrates, salts and minerals dissolved in the fluid, compared to the number of particles in the blood. Drinks may be hypotonic, isotonic or hypertonic when compared to the blood.

Hypotonic: the drink contains fewer particles than blood

Isotonic: the number of particles equals that of the blood

Hypertonic: the drink contains more particles than the blood.

Also see the separate article 087-a OSMOSIS.


Water does not hold any particles dissolved in it. Therefore the speed of passage through the stomach is highest. But drinking water during prolonged intensive efforts does have some disadvantages. The taste of it is not very agreeable, which will not prompt the athlete to drink regularly. This is a disadvantage because the feeling of thirst is already less during exercise. After water a hypotonic sports drink passes through the stomach rather fast. Hypertonic drinks pass rather slowly, because they are first diluted, causing a full and sloshing feeling in the belly.


Because the lost salts and carbohydrates should also be replenished during prolonged exercise a hypotonic drink is ideal. The drink contains small quantities of carbohydrates and minerals. Very suitable are diluted fruit juices and hypotonic commercial drinks. During exercises shorter than 1.5 hours water is suitable. Mineral water is better than water from the tap. Normal tap water contains fewer minerals.

By excessive drinking hypotonic water a serious disturbance of the electrolytes balance may arise. This electrolytes disturbance is called water intoxication.