Although not a concern for the average person with two good kidneys, hyponatraemia is a
diagnosis increasingly found in
marathon (26.2
miles) and
ultra-marathon (50+
km) runners.
Conventional running wisdom emphasizes drinking lots of water while racing (and holds that water is more or less as good as various enhanced sports drinks and supplements), which is fine advice for most races. However, the very prolonged nature of marathons and ultra-marathons, with runners going for hours at full effort, can result in serious sodium depletion through sweat, which can actually be exacerbated by the amount of water runners will take in during the race to stay hydrated. Until recently many marathoners who collapse have been incorrectly tagged as being dehydrated and given more water, which too can delay recovery.
The moral of this story is: if you intend to run for more than 90 minutes at a pop, take a few pretzels along or drink a bit of an electrolyte containing sports drink (Gatorade or what have you) along with your water.