When you train it is absolutely critical that you remain hydrated.  Being well hydrated:

• Assists with recovery
• Avoids dehydration
• Helps you optimize your training

This graph shows exactly this positive benefit of hydration in action.  The graph shows two separate training days (from one of our clients) and compares the effects of being hydrated as opposed to becoming increasingly dehydrated.  As you can clearly see, the blue line represents speed which is controlled by monitoring heart rate.  As the run went on, the clients speed had to decrease in order to keep the heart rate in the correct zone, due to dehydration effects.  In contrast the pink graph shows the client drinking water during the session, and over the course of the session was able to maintain a much higher average speed (at the given heart rate).

So why did this happen?  When you become dehydrated your blood gets thicker (because the plasma loses its water content).  This is medically known as increase viscosity.  As the blood gets thicker it also becomes harder to pump to your muscles and needs to beat faster to deliver the essential oxygen to keep going.  Hence the athlete will either need to slow down or can continue at this pace, which could lead to adverse training effects.

If you apply this principle over several weeks and months of consistent training, a person who continually stays well hydrated will get in many more quality hours of training than someone who is dehydrated.

Now the question is who do you want to be?