You lose about 2.4 percent of your endurance by starting a session dry
Pooling 15 studies and 186 people who began 3.6 percent lighter than normal, aerobic performance fell 2.4 percent. How dry they were did not change how much they lost.
What the study found
Turning up dehydrated cost 2.4 percent of aerobic performance
This is about starting a session dry, not about fluid lost during one. Pooling 15 controlled studies and 186 people down 3.6 percent of body mass before they began, aerobic performance fell 2.4 percent and peak oxygen uptake fell the same. The loss did not scale with how dry they were.
The caveat · Drier than most exercisers ever get
An average 3.6 percent of body mass is more dehydration than most recreational exercisers ever reach, and the performance tests averaged 22 minutes. The dehydration was deliberately induced, usually by heat or fluid restriction, which brings its own fatigue on top of the water deficit. The confidence interval on the headline runs from 0.8 to 4.0 percent. Nothing here compares an electrolyte drink with water.