Research shows that over-hydration can actually slow you down.

A new study has found that marathoners who lose the most body weight during the actual race run faster according to an article posted on Web MD. The reason for the weight loss is due to a either a lower fluid intake or a faster rate of fluid loss.

Lead researcher Timothy Noakes, M.D., Discovery Health Professor of Exercise and Sports Science at the University of Cape Town in South Africa, points out that “the clear evidence is that drinking just to thirst is the optimum strategy….Drinking either more or less than to thirst impairs exercise performance.”

Noakes adds that drinking too much can even prove to be fatal, indicating that several runners in the U.S. have died from hyponatremia, the presence of abnormally low levels of sodium in the blood.

Noakes’ study took place at the 2009 Mont Saint-Michel Marathon in France where he sampled 884 marathoners. Of that group, 643 finished and were included in the results. The runners were weighed in at the start and the finish.

For More: Web MD