When we exercise we lose fluid through sweating and breathing. Exercising increases our body temperature, so to counteract this and prevent dangerous overheating, we sweat to cool down.
If we become dehydrated when exercising, we struggle to maintain our performance levels - exercise becomes harder and fatigue sets in quicker. As we lose fluid, our blood volume drops and body temperature rises. This puts extra pressure on the body because the heart has to pump harder to get the blood around the body, and we have to breathe harder to get enough oxygen.
The amount of sweat we produce increases when we exercise in warmer temperatures, and when we exercise harder or for longer. During a one hour exercise session we can lose a litre of fluid, and even more in hotter conditions.
Body size, gender, fitness levels and other individual factors all affect how much we sweat. The best way to prevent dehydration is to ensure you drink plenty of fluids before, during and after exercise.
Fluids before exercise
Ensuring that you're well hydrated before you begin exercise will help you perform better from the outset. It's recommended that we aim to drink around 400-600ml of fluid about two hours before exercise. This will ensure hydration and allow enough time for excess fluid to be excreted before exercise.
Sports drinks
For most people, water is the best drink before exercise but athletes and people who exercise for long periods or very intensively (for example, marathon running or football) may benefit from using sports drinks. These contain sugar, which is needed by muscles for energy, but for most of us during a moderate session at the gym, or exercising steadily for less than an hour, extra energy is unnecessary.
There are different types of sports drinks, which vary in the speed they hydrate you. It might seem unlikely, but drinks containing some sugars can actually hydrate quicker than plain water. Athletes can really benefit from this rapid hydration, as well as the boost of energy. That extra energy is no good for those of us who exercise to lose weight though - stick to plain water and avoid those extra calories.
Fluids during exercise
We start to lose fluids as soon as we begin exercising so it's a good idea to hydrate yourself early on and at regular intervals during exercise. Research shows that we're likely to drink more if the fluids are flavoured and if we drink from a suitable container, such as a bottle with a sports cap. If you don't like plain water then dilute fruit juice or squash in a water bottle to take with you when you exercise.
After exercise
In order to replace fluid losses after exercise it's recommended that we consume about 1.5 times the quantity of fluids lost during exercise.
You can estimate how much fluid you lose by weighing yourself before and after exercise. Every 1kg lost in weight represents about 1 litre in fluid losses.
So, if you've lost 1kg of weight after exercise, you should aim to drink about 1.5 litres at a comfortable rate (not too fast!) to replace your lost fluid.