By Jean – sports doctor

Dehydration is the insufficient supply of water to those parts that need it during exercise: the brain, blood circulation and the heart, muscles, tendons. If the brain lacks water, body temperature rises, alertness and lucidity are impaired.

Reflexes are diminished, and headaches can occur. In the event of insufficient water, the muscles heat up while less hydrated fibres can be torn. Poorly hydrated tendons get inflamed – this is called tendinitis.

If less fluid circulates in the blood, blood gets thicker, and thus the perfusion of muscles during exercise is less efficient. If the body lacks water, the heart beats faster (tachycardia). One interesting number should be noted: if the body loses 1% of its weight in water, we lose 10% of muscle power. Hence, a 70 kg athlete who loses 2 litres of water, or 3% of his weight, sees his power drop by 30%.



Muscle contraction itself produces heat (like the pistons of an engine) and therefore a loss of water. By sweating, the body loses water. And climatic conditions can exacerbate dehydration: the sun, ambient heat. If it’s hot and humid, dehydration is magnified because the main means for lowering temperature is disrupted: sweat evaporation. And if it’s humid, sweat won’t evaporate.


Preventing dehydration:

Drink before, during and after a sporting activity. Here are the main rules so that during exercise, water intake quickly reaches the muscles:

-drink little by little from the start of the activity

-if the drink contains salt (sodium), muscle cells are better hydrated (salt retains water)

-if the pH of the drink is close to 7, liquid in the stomach reaches the muscles faster

-The most effective drinks to prevent dehydration during exercise are mineral drinks

-A key rule: during exercise you must drink before you get thirsty, because when you’re thirsty, that means the body has already lost water.