Drinking plenty of water is essential during exercise and for optimum recovery. Exercising causes the body to get warmer, so the body tries to cool down by sweating. This causes the loss of water and salts through the skin.
The amount an individual sweats varies from person to person and depends on a number of factors:
Intensity and duration – longer and higher intensity exercise can cause greater sweat loss.
Environmental temperature – in hot, humid conditions sweat loss can increase.
Clothing – the more clothing that is worn, the quicker you are likely to heat up which may cause greater sweat loss.
Genetics – some people are just more likely to sweat than others.
Generally, the more a person sweats, the more they will need to drink.
Small water losses are not harmful. Although, dehydration (water loss in excess of 2-3% body mass) can cause tiredness and restrict performance by reducing strength and aerobic capacity (especially in longer duration exercise), as well as having a negative effect on further exercise sessions.
So, try and stay hydrated before, during and after exercise to prevent dehydration
For more info on staying hydrated be sure to check out the British Nutrition Foundation www.nutrition.org.uk