We are made up of over 60% water and every cell in our body requires water just to function. Even the smallest degree of water loss can impair both physical and mental function. If you go for a run and at the start weigh 72.5kg and then after 20 miles weigh in at 70kg then this equates to a 4% body weight loss. A 4% body weight loss will mean that your exercise performance will be cut by about 25% - not something you need during a long race! There is no doubt that water is essential to life, but still there are many people that don’t drink enough.