Do ultra-runners in a 24-h run really dehydrate?
BACKGROUND
Loss of body mass during a 24-h run was considered to be a result of dehydration.


OBJECTIVE
We intended to quantify the decrease in body mass as a loss in fat mass or skeletal muscle mass and to quantify the change in hydration status.


METHODS
Body mass, fat mass, skeletal muscle mass, haematocrit, plasma sodium and urinary specific gravity were measured in 15 ultra-marathoners in a 24-h run.


RESULTS
Body mass decreased by 2.2 kg (p = 0.0009) and fat mass decreased by 0.5 kg (p = 0.0084). The decrease in body mass correlated to the decrease in fat mass (r = 0.72, p = 0.0024). Urinary specific gravity increased from 1.012 to 1.022 g/mL (p = 0.0005).


CONCLUSIONS
The decrease in body mass and the increase in urinary specific gravity indicate dehydration. The decrease in body mass was correlated to the decrease in fat mass and therefore not only due to dehydration.
Knechtle
B
B
Gesundheitszentrum St. Gallen, Vadianstrasse 26, 9001, St. Gallen, Switzerland. beat.knechtle@hispeed.ch
Wirth
A
A
Knechtle
P
P
Rosemann
T
T
Senn
O
O
D000273:Adipose Tissue; D001823:Body Composition; D015992:Body Mass Index; D003681:Dehydration / Q000209:etiology / Q000503:physiopathology*; D006801:Humans; D008297:Male; D008875:Middle Aged; D010807:Physical Endurance / Q000502:physiology*; D012420:Running / Q000502:physiology*; D012885:Skinfold Thickness; D013046:Specific Gravity; D014556:Urine / Q000737:chemistry