We all know it's healthy to drink lots of water, but sometimes too much water can be dangerous.
It turns out marathon runners are at particular risk of running into that danger zone. A study in the New England Journal of Medicine followed nearly 500 runners in the Boston Marathon and found that 13% developed a condition called hypothermia, which is caused by over-hydration. The condition causes sodium levels in the blood to drop dangerously low, and can lead to confusion, lethargy and twitching and in rare cases, coma and death.
The bottom line according to the Harvard study is runners need water, but don't over-compensate. The best way to prevent this is to check your weight before and after a long distance run. If you're gaining several pounds during the run, you may be drinking too much water and you could be at risk.