If you're training for a marathon, a proper plan for water intake   is important. Of course, there's the risk of dehydration   . But athletes now know they can also get into trouble by drinking too much. Too much water intake can lower 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. Their goal is to try to keep someone from not getting dehydrated by more than 2 percent of their body weight.  One technique for deciding how much water you need is to get the exact numbers. Runners have to weigh themselves before and after a run to determine how much water they've lost. If their weight drops by more than 2 percent, they have not drunk enough water.  Hyponatremia   occurs when runners drink so much water that blood salt levels drop off. A study published last year tested 488 runners who completed the Boston Marathon and found 13 percent of them had dangerously low blood salt levels. The first sign that runners may notice is slightly swelling in the hands. They can't get their rings off, and then they might feel sick. They may not remember where they are. In fact, most runners get enough salt to get back to normal levels by eating just one meal after a run. Contrary to the old advice that runners should drink as much as they can to prevent dehydration, the new research has shown that the body is a remarkable machine that actually tells you via thirst when you need water. The author of this passage is primarily concerned with   _  .
the signs and treatment for dehydration the long distance runners who need help how to manage water intake during a marathon a 28-year-old woman who died after a Boston Marathon