There is such a thing as too much water; excessive intake can dilute levels of sodium in the blood.
University of Connecticut researcher Douglas Casa says people get into trouble when they try to follow set requirements for hydration.
Scroll below for more tips on how to figure out what's the right amount to drink for your body.
If you're training for a marathon or an Ironman, a hydration plan is important. Of course, there's the risk of dehydration. But athletes now know they can also get into trouble by drinking too much. Excessive water intake can dilute levels of sodium in the blood. The death of a 28-year-old woman following the Boston Marathon caught the attention of many runners and led to new research.
Experts advise long distance runners to replace the liquids they sweat out.
"Our goal is to try to keep someone from not getting dehydrated by more than 2 percent of their body weight," says Douglas Casa, a researcher at the University of Connecticut's Human Performance Laboratory.
One technique for calculating how much fluid you need is to get an accurate scale. Runners can weigh themselves before and after a run to determine how much water weight they've lost. If their weight drops by more than 2 percent, they have not consumed enough fluid.
Hyponatremia occurs when runners drink so much liquid that concentrations of sodium in the blood drop off. A study published in the New England Journal of Medicine last year tracked 488 runners who completed the Boston Marathon and found 13 percent of them had dangerously low blood salt levels.
The first symptoms that runners may notice is minor swelling in the hands. "They can't get their rings off, then they might get nausea and dizziness. They may not remember where they are" says Dr. Lewis Maharam, who directs the International Marathon Medical Directors Association.
Most runners get enough salt to restore normal levels by eating just one meal after a run, and most never need medical attention. But with a spate of reported cases of hyponatremia, Maharam's group has a new guideline for hydrating.
The recommendation is contrary to the old advice that runners should drink as much as they can stomach to prevent dehydration.
"The new research has shown that the body is a remarkable machine that actually tells you via thirst when you need fluid," says Maharam.
Performance-oriented runners may prefer the more exacting scale-weighing technique. Casa recommends that runners use that method until they start to get a good estimate of how much water they sweat out during a typical training run.