NEW YORK (Reuters Health) - Runners who take part in day-long “ultra- marathons” experience much of their fluid loss in the first 8 hours of the race — pointing to the importance of staying hydrated early on, a study has found.
Ultra-marathons last anywhere from 12 to 24 hours, and although runners can rest, eat and drink when they need to, they are at high risk of becoming seriously dehydrated.
In the new study, researchers in Taiwan repeatedly weighed 52 ultra- marathoners over the course their race, which was either a 12- or 24- hour event.
They found that, on average, runners in the 12-hour marathon lost nearly 3 percent of their body weight. In the 24-hour group, the average weight loss was 5 percent, according to findings published in the Clinical Journal of Sports Medicine.
In both groups, 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, according to the researchers, led by Dr. Wei- Fong Kao of the Veterans General Hospital-Taipei.
The 24-hour marathoners also showed a second dip in body weight between hours 16 and 20 of the race, the researchers note.
The findings suggest that ultra-marathoners should concentrate on getting enough fluids in the earlier part of the race if they want to prevent a steep weight loss, according to Kao’s group.
Losing even as little as 1 percent of body weight due to dehydration can impair normal cardiovascular function and body-temperature regulation, the researchers point out.
“Hence, to maintain body weight,” they write, “fluid intake should be optimized in the first 8 hours for both 12- and 24-hour runners and in 16 to 20 hours for 24-hour marathon runners.”
Marathoners also need to be careful about ingesting too much water, however, as this can lead to a potentially serious condition called hyponatremia. Hyponatremia arises 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.
Research suggests that “ultra-endurance” athletes are also at risk for becoming overhydrated and developing hyponatremia.
SOURCE: Clinical Journal of Sports Medicine, March 2008.