For a long time dehydration was considered 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, so that today many people carry water bottles everywhere and dutifully keep well hydrated. In some circumstances, however, 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 marathoner 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 brain swelling, the result of drinking too much water before and during the race. The normal blood sodium level is 135 to 145 mM (millimolar). When that level drops to 125 mM, dizziness and confusion set in. A concentration below 120 mM can be critical. Dangerously low levels can occur in any active athlete who is sweating out salt (NaCl) at the same time that excessive amounts of NaCl-free water are being drunk to compensate for water loss. The condition affects women more than men because of differences in body composition and patterns of metabolism. Drinking a sport drink that contains some electrolytes helps to prevent hyponatremia.
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