Sequential exercise in triathletes: variations in GH and water loss

Area of Science:

• Exercise Physiology
• Endocrinology
• Sports Science

Background:

• Growth hormone (GH) is implicated in regulating water loss via sweating during exercise.
• Sequential exercise protocols involving cycling and running present unique physiological challenges, including postural variations.

Purpose of the Study:

• To investigate the relationship between GH secretion, water loss, and plasma volume changes during sequential cycling and running exercise.
• To determine the influence of exercise order and postural transitions on these physiological responses in elite triathletes.

Main Methods:

• Eight elite triathletes completed two randomized trials: cycling followed by running (C1-R2) and running followed by cycling (R1-C2).
• Each 20-minute segment was performed at ~75% maximal oxygen consumption, with blood sampling for GH and plasma volume, and water loss estimation.
• Incremental cycle tests determined maximal oxygen consumption for intensity standardization.

Main Results:

• GH increases were significantly lower during the running segment when it followed cycling (R2) compared to cycling after running (C2).
• Total water loss was significantly lower in the C1-R2 trial compared to the R1-C2 trial.
• Plasma volume decreased significantly after initial cycling (C1) and running (R1), but showed a significant positive change in R2, suggesting fluid retention.

Conclusions:

• Lower GH increases during running after cycling may reduce sweating, contributing to less plasma volume reduction.
• Postural changes during the transition from cycling to running appear to influence the GH response.
• Athletes should monitor hydration during the initial running phase (R1) to mitigate potential dehydration during subsequent cycling (C2).