LAUSR

To the Editor: Although regular exercise is recommended for type 1 diabetes management [1], exercise in hot conditions may pose a health concern [2]. This is primarily because even patients without neuropathy display impaired cutaneous vasodilation [3] and sweating [4], especially during vigorous exercise [5], which may increase dry heat gain by reducing bloodborne heat delivery to the skin and attenuate evaporative heat loss. The resulting reductions in total heat loss (dry + evaporative heat loss) can elevate heat illness risk by exacerbating body heat storage and the subsequent increase in body core temperature [6]. However, since those previous studies [3–5] measured cutaneous vasodilation and sweating at only a handful of small surfaces (~1–3 cm) on the body, it remains unclear whether such impairments translate into clinically meaningful decrements in whole-body total heat loss (i.e. from all body surfaces). We therefore used our unique direct air calorimeter (the gold standard method for measuring whole-body heat exchange) [7] to examine the effects of type 1 diabetes on whole-body total heat loss and body heat storage for the first time during light, moderate and vigorous exercise in the heat. Following written informed consent, 28 habitually active young adults (aged 18–37 years) with (ten men, four women) and without (ten men, four women; control) type 1 diabetes, but of similar age, aerobic fitness and body morphology, participated in an experimental trial approved by the University of Ottawa Research Ethics Board. Participants with type 1 diabetes had been diagnosed ≥5 years earlier, had an HbA1c of 6.2–9.2% (44–77 mmol/mol), and no diagnosed diabetesrelated complications. Participants were instructed to arrive well hydrated, having followed their normal insulin therapy (type 1 diabetes: pump, n = 8; injections, n = 6), having eaten their usual breakfast and having abstained from exercise, alcohol, caffeine and anti-inflammatory drugs for >24 h. After confirming euhydration (urine specific gravity: <1.025), participants entered the direct calorimeter (35°C, relative humidity ~20%; humidex 36°C) wearing shorts, sleeveless top (women) and sandals. Participants completed 30 min of seated rest and three 30 min bouts of semirecumbent cycling at metabolic heat production rates of 200 (light), 250 (moderate), and 300 W/m (vigorous), each followed by 30 min recovery. These work rates represented ~37%, 47% and 56% of peak aerobic power, respectively, and ensured the heat load was matched between groups. Indirect calorimetry (Moxus system, AEI Technologies, Bastrop, TX, USA) was used to derive metabolic heat production (metabolic rate – external work). The direct calorimeter measured whole-body dry and evaporative heat exchange [7]. Body core temperature was measured via the oesophagus (n = 9 per group) or rectum (n = 5 per group) using a Mon-a-therm Temperature Probe (Mallinckrodt Medical, St Louis, MO, USA). Blood samples were drawn from an indwelling catheter during the final 5 min of rest and each exercise period from the type 1 diabetes (n = 14) and control (n = 12) participants, and analysed for blood glucose concentrations (mmol/l) by an external laboratory (Dynacare, Brampton, ON, Canada). Data are reported as means over the final 5 min of rest and each exercise period. Body heat storage (kJ) during each * Glen P. Kenny [email protected]