Objective: The principal objective of this study was to evaluate serial weight changes in athletes during 12- and 24-hour ultra-marathons and to correlate these changes with athletic performance, namely the distance covered.
Design: This was a prospective study.
Setting: The 2003 Soochow University international ultra-marathon.
Participants: Fifty-two race participants.
Interventions: 12- or 24-hour ultra-marathon.
Main Outcome Measurements: Body weight changes were measured before, at 4-hour intervals during, and immediately after the 12- and 24-hour races.
Results: Significant overall decreases in body weight were apparent at the conclusion of both races. The mean relative body weight change after the 12-hour race was −2.89 ± 1.56%, ranging from 0 to 6.5%. The mean relative body weight change after the 24-hour race was −5.05 ± 2.28%, ranging from −0.77% to −11.40%. Of runners in the 24-hour race, 26% lost greater than 7% of baseline body weight during the race. During both the 12- and 24-hour races, the greatest weight change (decrease) occurred during the first 4 hours. Weight remained relatively stable after 8 hours, although a further decrease was apparent between 16 and 20 hours in the 24-hour participants. Weight change had no bearing on performance in the 12-hour race, whereas weight loss was positively associated with performance in the 24-hour race.
Conclusions: Our findings demonstrate that the majority of weight decrease/dehydration in both the 12- and 24-hour races occurred during the first 8 hours. Hence, to maintain body weight, 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.