Adequate water intake on a daily basis is critical for health. We need to replace the 1.5 litres we lose every day through breathing, sweating and going to the bathroom. Therefore on average we should be drinking 2 litres of fresh, clean, pure water everyday, or 25-35ml per kg body weight.
When you run, you will produce heat, which must be dealt with to avoid overheating. Your body deals with this by producing sweat, which cools the body down. Sweat rates vary from person to person and day-to-day depending on the weather conditions, intensity and the duration of the exercise.
When we sweat, we not only lose water but electrolytes as well, in particular sodium, potassium and magnesium, which play a critical role in controlling the balance of fluid in your cells, tissue and blood. You will need to ensure you keep your water levels topped up and it is advisable to also top up with electrolytes too so that you can restore fluid balance correctly.
Keeping hydrated will result in a better performance and reduction in early fatigue. During your training, it is advisable to estimate your sweat rates and therefore your need for rehydration. This is simply done by weighing yourself before your training and afterwards. Please note that this is only a best guess and take into consideration the intensity and duration of the training session as well as the ambient temperature and compare this to your race day conditions.