AB Studies show that vasomotor and sudomotor activities are compromised in individuals with Type 1 diabetes mellitus (T1DM), which could lead to impaired skin blood flow (SkBF) and sweating during heat stress. However, recent work suggests the impairments may only be evidenced beyond a certain level of heat stress. Purpose: We examined T1DM-related differences in heat loss responses of SkBF and sweating during exercise performed at progressive increases in the requirement for heat loss. Methods: Sixteen adults (10 males and six females) with (T1DM, n = 8) and without T1DM (control, n = 8) matched for age, sex, body surface area, and fitness cycled at fixed rates of metabolic heat production of 200, 250, and 300 W[middle dot]m-2 in the heat (35[degrees]C and 20% relative humidity). Each rate was performed sequentially for 30 min. Local sweat rate (LSR, ventilated capsule), sweat gland activation (modified iodine paper technique), and sweat gland output were measured on the forearm, upper back, and chest, whereas SkBF (laser Doppler) was measured on the forearm and upper back. Results: Despite a similar requirement for heat loss, LSR was lower in T1DM on the forearm and chest relative to that in the control. Reductions were measured in the second (forearm: 0.68 +/- 0.14 vs 0.85 +/- 0.11 mg[middle dot]min-1[middle dot]cm-2, P = 0.004; chest: 0.58 +/- 0.08 vs 0.82 +/- 0.12 mg[middle dot]min-1[middle dot]cm-2, P = 0.046) and third exercise bouts (forearm: 0.75 +/- 0.11 vs 0.98 +/- 0.12 mg[middle dot]min-1[middle dot]cm-2, P = 0.005; chest: 0.66 +/- 0.1 vs 1.02 +/- 0.16 mg[middle dot]min-1[middle dot]cm-2, P = 0.032). Differences in forearm LSR were the result of a reduction in sweat gland output, whereas the decrease in chest LSR was due to lower sweat gland activation. SkBF did not differ between groups. Conclusions: We show that T1DM is associated with impairments in heat dissipation during exercise in the heat, as evidenced by attenuated LSR. However, these differences are only shown beyond a certain requirement for heat loss
