Variations in body morphology explain sex differences in thermoeffector function during compensable heat stress

Area of Science:

• Exercise Physiology
• Human Thermoregulation
• Environmental Physiology

Background:

• Sex is often considered an independent factor influencing cutaneous vascular and sudomotor responses during heat exposure.
• However, the role of morphological differences, specifically body surface area to mass ratio, in mediating these sex-related responses remains unclear.

Purpose of the Study:

• To investigate whether sex-related differences in cutaneous vascular and sudomotor responses during exercise are explained by variations in the body surface area to mass ratio.
• To determine if sex independently modulates thermoeffector function when heat-loss requirements are equivalent across individuals.

Main Methods:

• Assessed vasomotor and sudomotor functions in 60 individuals (36 men, 24 women) with diverse specific surface areas.
• Subjects underwent rest and steady-state cycling in controlled, compensable environmental conditions (28°C, 36% RH).
• Metabolic heat production rates were standardized per unit of body surface area to ensure equivalent heat-loss requirements and mean body temperature changes.

Main Results:

• Mass-specific surface area was a significant determinant of both vasomotor and sudomotor responses, explaining 10-48% of inter-individual variability.
• Positive correlations were observed between specific surface area and forearm blood flow/vascular conductance, particularly in women.
• Negative correlations were found between specific surface area and sweat rates in both sexes.
• After accounting for morphological differences, sex explained only 5% of the inter-individual variability in thermoeffector responses.

Conclusions:

• Sex differences in thermoeffector function during compensable exercise are largely morphologically dependent, not intrinsically sex-dependent.
• The ratio of body surface area to mass is a critical factor in regulating cutaneous vascular and sudomotor activity during heat stress.
• These findings challenge the notion of sex as an independent modulator of thermoregulation when morphological factors are considered.