40-75% of human body mass is water.
Many of us do not drink enough fluid and as such can end up in a state of de-hydration which can be fatal.
Ultimately, water to increase saliva which is needed at the first stage of digestion and also helps to regulate our body temperature.
During exercise, we sweat to help regulate the body temperature which can be dictated by the environment, training duration and training intensity.
Dehydration can reduce the blood flow and puts pressure on the heart which can lead to cardiac conditions such as heat exhaustion.
For athletes, just being as little as 2% dehydrated can have an influence on how you perform and upwards of 10% dehydrated can be fatal.
Ways to prevent this are:
Drink small sips of a bottle of water regularly throughout the day.
Monitor urine levels (see chart below)
The quickest ways to re-hydrate in sport in by drinking orange juice after sport as they contain vitamins and minerals that help rehydrate you.
Consume an isotonic drink between work periods to release electrolytes which will help with muscle function, balance blood acidity levels and pressure and rehydrate you.