Scientifically speaking, dehydration occurs when you’re using more water than you’re taking in, usually due to sweating, excessive vomiting/diarrhea/urination, or insufficient water intake. Among other symptoms, dehydration causes a reduction in blood plasma, which increases blood viscosity, or thickness. This makes it more difficult for your heart to pump blood efficiently, causing an elevated heart rate and a reduction in sweating; an increase in core temperature, and an increase in the glycogen utilization rate. None of these symptoms could be considered ideal for athletic performance.

But how much dehydration can you tolerate before your performance truly suffers?

Source: Can You Perform While Dehydrated? – TrainingPeaks