Risk Factors for HyponatremiaThe study looked at several factors, and found that those who were slowest (having more time to drink on the course), who were very low body mass, and who gained weight during the event were the ones most likely to develop hyponatremia.
The study found that it didn't matter whether the racer drank both water and sports drink, or whether they took nonsteroidal antiinflammatory drugs (such as ibuprofen or naprosyn) or were female.
Water and Sports DrinkThese conclusions are a concern for long distance walkers. The old advice was "drink, drink, drink," and to drink sports drink that had salt replacement in order to prevent hyponatremia. However, this study suggests that even if drinking sports drink there is a risk of hyponatremia from too much fluids. Walkers should continue to use sports drink or salty snacks rather than only drinking water, but it may not be the only solution to prevent hyponatremia.
Weigh Yourself Before and After a Long WalkThe best test for whether you are drinking too much on a long walk is to weigh yourself immediately before and after your walk. If you gained weight during the walk, you are drinking too much. You should drink enough water and sports drink to maintain weight, but not gain. A weight loss shows that you are risking dehydration from lost water.
A Pint is a PoundEvery pint (16 oz. bottle) of water weighs a pound. A liter of water weighs 1 kilogram. If you have lost weight during your walk, or gained it, adjust how much you drink next time you walk.
Drink When ThirstyThe USATF 2003 guidelines for track and field athletes say to drink when thirsty. The old advice was not to wait until thirsty, but fears about hyponatremia made them change the advice. Use "drink when thirsty" and weigh yourself before and after your longer walks to see what is right for your body.
Urination as a GuideIf your urine is very dark yellow after a long walk, or you can't urinate at all, it is a sign of dehydration. If your urine is colorless or very pale, then you may be drinking either enough or too much - use the weighing method to determine whether you are drinking too much.
Christopher S.D., et.al. "Hyponatremia among Runners in the Boston Marathon." N Engl J Med 2005; 352:1550-1556. April 14, 2005