Leptin responses to physical inactivity induced by simulated weightlessness

Area of Science:

• Endocrinology
• Human Physiology
• Space Medicine

Background:

• Physical inactivity, such as head-down bed rest (HDBR), significantly impacts body composition (BC).
• Leptin plays a crucial role in regulating BC by influencing fuel homeostasis.
• Understanding hormonal responses to HDBR is vital for mitigating adverse health effects during prolonged inactivity.

Purpose of the Study:

• To investigate the effects of a 7-day HDBR protocol on leptin and counterregulatory hormone levels.
• To determine if HDBR influences key metabolic markers such as insulin and glucose.
• To explore potential gender-specific responses to HDBR-induced hormonal changes.

Main Methods:

• A 7-day head-down bed rest (HDBR) study involving 8 males and 8 females.
• Daily fasting blood sampling for analysis of leptin, insulin, norepinephrine, epinephrine, growth hormone, cortisol, nonesterified fatty acid (NEFA), and glucose.
• Body composition (BC) was measured using H(2)(18)O dilution; energy intake was monitored.

Main Results:

• HDBR led to significant increases in leptin and insulin levels in both men and women.
• The insulin-to-glucose ratio also increased, suggesting potential insulin resistance.
• Growth hormone, catecholamines, and cortisol levels remained unchanged; NEFA decreased, and glucose decreased only in women.

Conclusions:

• HDBR elevates leptin and insulin levels independently of stress responses, body composition changes, or energy intake variations.
• The observed hormonal changes, particularly the increased insulin-to-glucose ratio, correlate with the development of insulin resistance in men.
• These findings necessitate consideration for longer HDBR periods, accounting for potential impacts on energy intake and body composition.