Exercising in environmental extremes : a greater threat to immune function?

Area of Science:

• Exercise physiology
• Immunology
• Environmental medicine

Background:

• Individuals in demanding professions (athletes, military, firefighters) often perform in extreme environmental conditions.
• Exercise in hot conditions elevates core body temperature, stress hormones, and circulating leukocytes compared to thermoneutral conditions.
• The rise in core temperature during exercise is a significant factor in exercise-induced leukocytosis and cytokinaemia.

Purpose of the Study:

• To evaluate the impact of environmental extremes (heat, cold, high altitude, microgravity) on immune function.
• To critically assess the evidence supporting or refuting the belief that exercising in extreme environments compromises immunity.
• To explore the role of core temperature and acclimatization in immune responses to exercise-heat stress.

Main Methods:

• Review of laboratory and field studies investigating immune responses during exercise in various environmental extremes.
• Analysis of studies employing thermal clamp models to isolate the effect of core temperature rise.
• Examination of data on immune cell function, cytokine profiles, and infection incidence in response to heat, cold, high altitude, and microgravity.

Main Results:

• Most evidence does not support the claim that exercising in heat significantly threatens immune function, with limited effects on neutrophil, monocyte, and natural killer cell function.
• Controlled studies indicate that cold exposure, with or without exercise, does not suppress immunity and may have immuno-stimulatory effects.
• While high altitude may be linked to increased infection incidence, confounding factors like acute mountain sickness make conclusions difficult; suppressed cell-mediated immunity in mountaineers and astronauts requires further investigation regarding infection risk.

Conclusions:

• The popular belief that exercising in extreme environments, particularly heat and cold, poses a significant threat to immune function is largely unsupported by current scientific evidence.
• Severe heat stress, as experienced in field studies, may offer greater insights into immune modulation in exertional heat stroke and the effects of acclimatization.
• Further research is needed to clarify the precise impact of environmental extremes on immune resistance to infection, especially in conditions like high altitude and microgravity.