A recent study done by the Department of Orthopaedics and Sports Medicine examined the influence of hydration on physiological responses and running speed during trail running. The authors found that the Dehydrated Group had significantly slower times, greater perceived effort, and higher gastrointestinal temperatures (important stuff if you are using heart rate as a gauge for exercise effort and performance - your heart is pumping more blood to your organs and not to your muscles!)

Get and Stay Hydrated.

Credit to Journal of Strength & Conditioning Research:
November 2011 - Volume 25 - Issue 11 - pp 2944-2954

Dr. Evan Kahn