Marathons can be bad for your health,scientists have warned after a study found 80 per cent of competitors suffer kidney~1 injury because of dehydration~2.Researchers said that although the kidneys of the participants in the 26.2 mile race fully recovered within two days,their findings raise questions concerning the potential~3 long-term impact at a time when marathons are increasing in popularity. Marathons can be bad for your health, scientists have warned after a study found 80 per cent of competitors suffer kidney ~ 1 injury because of dehydration ~ 2.Researchers said that though the kidneys of the participants in the 26.2 mile race fully recovered within two days , their findings raise questions concerning the potential ~ 3 long-term impact at a time when marathons are increasing in popularity.