Runners who take part in day-long "ultra-marathons" experience much of their fluid loss in the first 8 hours of the race, demonstrating the importance of early hydration, a study has found. Published in a recent edition of the Clinical Journal of Sport Medicine, researchers in this latest study repeatedly weighed 52 ultra-marathoners (runners in day-long marathons) over the course their race, which was either a 12- or 24- hour event.
The scientists discovered that, on average, runners in the 12-hour marathon lost nearly 3% of their body weight. In the 24-hour group, the average weight loss was 5%. In each group, the majority of this fluid loss happened in the first 8 hours of the race, with the most rapid weight changes seen in the initial 4 hours. Among 24-hour marathoners, a second dip in body weight between hours 16 and 20 of the race was also measurable.
According to authors of the study, the findings suggest that ultra-marathoners should concentrate on getting enough fluids in the earliest part of the race to prevent a steep weight loss. Researchers caution that losing even 1% of body weight due to dehydration can impair normal cardiovascular function and body-temperature regulation.
Conversely, marathoners also need to be careful about ingesting too much water, as this can lead to a potentially serious condition called hyponatremia. Hyponatremia occurs when blood sodium levels drop too low. While it often causes fairly mild symptoms like cramps and nausea, severe cases can lead to disorientation, seizures and even death. Researchers note that "ultra-endurance" athletes are at particular risk of developing hyponatremia as a result of over-hydration.
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