Weight Change and Hydration Status During a 161-km Ultramarathon.

Abstract

Weight loss during ultramarathon competition is often considered a useful predictor of performance, hydration status, and hyponatremia. The validity of this assumption has not been well studied. PURPOSE: To examine the correlation between weight change and performance, hydration and serum sodium in a 161-km ultramarathon. METHODS: This was a prospective observational study. Runners were weighed at pre-race check-in, the half-way point, and at the finish line. Height was self reported and recorded at pre-race check in. Willing finishers underwent urine analysis and provided a blood sample for serum sodium analysis. RESULTS: Out of 672 runners who started the race, 351 (52%) successfully completed the race under the 30-hour cut-off time. Weight was measured on 658, 518, and 344 runners at pre-race, half-way, and finish line, respectively. Urinalysis (dipstick) was done on 70 runners (51 finishers), and blood samples were obtained from 84 runners (66 finishers). The majority of runners (starters and finishers) were male (∼84%), and the average age was 40.5 years (range 19-74). Mean weight and BMI were 73.1 ± 10.3 kg and 23.3 ± 2.3 kg/m2, respectively. Mean weight change for runners was -2.1 ± 2.1% and -0.84 ± 2.3% of their pre-race weight at mid-point and finish line, respectively. Based on linear regression analysis, there was a significant correlation between percentage weight change from pre-race to finish line and finish time (P=0.016, r2=0.02). There was no statistically significant relationship between weight loss and urine specific gravity (P=0.124, r2=0.05), or between weight loss and serum sodium concentration (P=0.183, r2=0.03). CONCLUSIONS: Significant weight fluctuation is common during ultramarathons. Our data show that weight change may be a weak predictor of race performance and that faster finish times tend to occur with more weight loss. It appears that weight change is not a useful predictor of urine specific, nor is it useful as a predictor of serum sodium. More study is needed to determine if weight change during an ultramarathon is correlated with adverse patient outcomes. This study was funded in part by the ACSM Clinical Sports Medicine Endowment Grant.