Fluid homoeostasis during prolonged low-intensity walking on consecutive days

Area of Science:

• Exercise Physiology
• Human Physiology
• Sports Science

Background:

• Understanding fluid homeostasis during prolonged physical activity is crucial for athletes and military personnel.
• Previous research has focused on extreme heat or dehydration, with less attention on sustained moderate-intensity exercise in cooler conditions.

Purpose of the Study:

• To examine the effects of consecutive days of moderate-intensity walking on fluid balance and plasma volume in male subjects.
• To assess changes in serum electrolytes and osmolality during and after prolonged exercise.

Main Methods:

• Six male subjects walked 37 km daily for four consecutive days at a consistent intensity (17% of maximum oxygen uptake).
• Diet (carbohydrate breakfast, mixed diet) and fluid intake (ad libitum water) were recorded.
• Body weight, 24-hour urine output, plasma volume, and serum constituents (sodium, osmolality) were monitored.

Main Results:

• Body weight remained stable throughout the 4-day study.
• A significant increase in plasma volume (21.3%) was observed over the 4 days, with the largest increase on day 1.
• This plasma volume expansion persisted for at least 4 days post-exercise.
• Serum sodium concentration showed a tendency to increase during exercise and return to baseline overnight.

Conclusions:

• Sustained moderate-intensity walking leads to an expansion of plasma volume, suggesting an adaptive mechanism for fluid homeostasis.
• The observed plasma volume increase is maintained even after exercise cessation.
• While serum sodium fluctuates, overall serum osmolality remains stable, indicating effective regulation of hydration during prolonged exercise.