Do ultra-runners in a 24-h run really dehydrate?

Area of Science:

• Sports Science
• Human Physiology
• Endurance Sports

Background:

• Historically, body mass loss in endurance events was attributed primarily to dehydration.
• Understanding the specific components of body mass lost during prolonged exercise is crucial for athlete health and performance.

Purpose of the Study:

• To differentiate between fat mass and skeletal muscle mass loss during a 24-hour run.
• To quantify changes in hydration status alongside body composition changes.

Main Methods:

• 15 ultra-marathoners participated in a 24-hour run.
• Measurements included body mass, fat mass, skeletal muscle mass, haematocrit, plasma sodium, and urinary specific gravity.

Main Results:

• Body mass decreased by 2.2 kg (p = 0.0009), with fat mass decreasing by 0.5 kg (p = 0.0084).
• The reduction in body mass showed a significant correlation with 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), indicating dehydration.

Conclusions:

• The observed increase in urinary specific gravity confirms dehydration occurred during the 24-hour run.
• The correlation between body mass loss and fat mass reduction suggests that fat loss contributes significantly to overall body mass decrease in ultra-marathoners.
• Body mass loss during prolonged running is multifactorial, involving both dehydration and a reduction in fat mass.