Acclimation to intermittent hypobaric hypoxia modifies responses to cold at sea level

Area of Science:

• Human physiology
• Environmental adaptation
• Thermoregulation

Background:

• High altitude residence alters cold stress responses upon return to lower altitudes.
• Altitude-associated stressors (hypoxia, cold, radiation, exertion) interact, complicating thermoregulatory changes.
• Hypoxia adaptation alone may explain some observed thermoregulatory modifications.

Purpose of the Study:

• To investigate if adaptation to hypoxia without cold exposure induces thermoregulatory changes.
• To determine if acclimation to intermittent hypoxia (AIH) leads to cold adaptation at sea level.

Main Methods:

• Five men underwent AIH in a hypobaric chamber (4500-6000 m) for 4.5 days at 24°C.
• Standardized cold air tests (1°C, 2 hours) were conducted at sea level before and after AIH.
• Thermoregulatory responses, including skin temperature, metabolic heat production, heat debt, and shivering, were measured.

Main Results:

• AIH confirmed by increased reticulocyte counts.
• Lower mean skin temperature and heat debt post-AIH.
• Higher mean metabolic heat production and earlier onset of shivering post-AIH.
• Shivering occurred at higher mean skin temperatures post-AIH.

Conclusions:

• Acclimation to intermittent hypoxia induces a normothermic-insulative-metabolic cold adaptation.
• AIH modifies thermoregulatory responses to cold at sea level without direct cold exposure.
• Suggests cross-adaptation of hypoxia adaptation to cold stress response.