Hew-Butler, T. D., Tam, N., Nolte, H., & Noakes, T. (2009). Maintenance of total body water despite body weight loss during an ultramarathon. ACSM 56th Annual Meeting, Seattle, Washington, Presentation Number 2348.

This study determined if: 1) body weight loss equated to total body water loss during prolonged endurance exercise, and 2) plasma sodium, osmolality, and volume were maintained with ad libitum food and fluid intakes, despite significant (>2%) bodyweight loss from pre- to post-race. Athletes (N = 10) participating in an 80-km mountain footrace consented to bodyweight (BW), venous blood, and saliva measurement 2-days, immediately pre-, and post-race. Changes in body mass, plasma sodium and potassium concentrations, osmolality, and volume were determined. Food and fluid were allowed ad libitum and quantified by researchers stationed at 13 checkpoints.

Eight Ss completed the 80 km footrace with a mean finishing time of ~691.4 minutes and fluid intake of ~4.8 liters. A significantly greater decrease was noted in % bodyweight loss versus % total body water loss immediately post-race minus 2-days pre-race. Significant differences were noted between the change in % bodyweight loss in both the post- minus immediately pre-race condition and the post- minus 2-day pre-race condition when compared with the pre- minus the 2-days pre-race condition. No significant differences were noted in the change in plasma sodium concentration or plasma volume when the three testing conditions were compared.

Implication. Total body water, plasma sodium, plasma osmolality, and plasma volume were maintained during an 80 km mountain footrace despite significant bodyweight loss. These findings suggest that recommendations to replace 100% bodyweight losses during prolonged endurance running may promote total body water dysregulation. While body weight decreases in an ultramarathon race, ad libitum drinking appears adequate to preserve important fluid functions and statuses.

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