Hyponatremia

"For a long time it was thought that dehydration was a potential danger for people engaged in extended vigorous activity Thus, athletes were encouraged to drink lots of water while engaged in active sport. The trend toward extensive hydration has spread throughout society; many people carry water bottles everywhere and dutifully keep well hydrated.



To help prevent overhydration, the number of water stations
such as this one has been reduced in many marathon events.
Source

"It turns out, though, that in some circumstances, drinking too much water is a greater danger than not drinking enough. Excess water consumption can lead to hyponatremia, a condition in which the concentration of sodium ion in the blood is too low. In the past decade at least four marathon runners have died from hyponatremia-related trauma, and dozens more have become seriously ill. For example, a first-time marathon runner named Hillary Bellamy, running in the Marine Corps marathon in 2003, collapsed near mile 22 and died the next day. One physician who treated her said that she died from hyponatremia-induced swelling, the result of drinking too much water before and during the race.

"The normal blood sodium level is 135 - 145 mM (millimolar). When that level drops to as low as 125 mM, dizziness and confusion set in. A concentration below 120 mM can be critical. Low sodium level in the blood causes brain tissue to swell. Dangerously low levels can occur in a marathon runner or other active athlete who is sweating out salt at the same time that excessive salt-free water is being drunk to compensate for water loss. The condition affects women more than men because of their generally different body composition and patterns of metabolism. Drinking a sport drink, such as Gatorade, which contains some electrolytes, helps to prevent hyponatremia.

"Contrary to popular belief, dehydration is not as likely as overhydration to present a life-threatening situation, though it can contribute to heat stroke when the temperature is high. Athletes frequently lose several pounds in the course of extreme workouts, all in the form or water loss, with no lasting adverse effects. When, for instance, Amby Burfoot ran in the Boston Marathon in 1968, his body weight went from 138 to 129 pounds during the race. He lost 6.5% of his body weight while winning the men's competition that year. Weight losses of this magnitude are typical of elite marathon runners, who produce tremendous amounts of heat and sweat and cannot afford to slow down for much drinking."

Passage taken from Brown, et al. (1009). "Drinking Too Much Water Can Kill You" in

"Chapter 4: Aqueous Reactions and Solution Stoichiometry". Chemistry: The Central Science, 11ed.

Upper Saddle River, NJ: Pearson Prentice Hall. p. 147.