Acute blood glucose and vascular endothelial responses to passive heating in individuals with type 2 diabetes

Abstract

IntroductionInsulin resistance and hyperglycemia predispose individuals with type 2 diabetes mellitus (T2DM) to endothelial dysfunction and a greater risk of cardiovascular diseases. Numerous animal studies suggest that heat exposure improves glucose control and human studies suggest that it improves markers of vascular health. However, the physiological responses to heat exposure remain understudied in individuals with T2DM, particularly within a therapeutic context.ObjectiveTo characterize blood glucose and vascular endothelial responses to heat exposure in T2DM and determine if heating acutely improves glucose control and endothelial function in this population.MethodsOn separate randomized days, six adults (2 males/4 females, 60 ± 12 years) with T2DM (≥1 year, HbA1c: 7.0 ± 0.5%, fasting glucose: 7.79 ± 1.13 mmol/L) were immersed in control (34°C, 90 minutes) or hot (40–41°C, core temperature ≥38.5°C for 60 minutes) water. Participants were fasted (12 hours) and withheld anti‐diabetic medication (≥24 hours). Blood glucose and insulin as well as brachial artery shear rates were measured at baseline, during immersion and 45–60 minutes after immersion. Brachial artery flow‐mediated dilation (FMD) was measured at baseline and 45 minutes after immersion. Blood glucose control was evaluated during an oral glucose tolerance test (OGTT, 75 g) 60 minutes after immersion. Core (Tcore, rectal) and mean skin (Tskin) temperatures, heart rate (HR), systolic (SBP) and diastolic (DBP) blood pressures were measured throughout the protocol.ResultsCompared to control, hot water immersion increased Tcore (+1.44°C [1.05, 1.83], P<0.01), Tskin (+2.36°C [1.21, 3.51], P<0.01), HR (+35 bpm [26, 43], P<0.01), and decreased DBP (−7 mmHg [−2, −13], P<0.01) and SBP (−9 mmHg [−1, −16], P=0.03). Hot water immersion increased antegrade (+71 s−1 [35, 107], P<0.01) and decreased retrograde (−15 s−1 [0.9, 3], P=0.06) shear rates. Nonetheless, FMD was unchanged during both control (−0.74% [−4.33, 2.86], P=0.81) and hot (−1.93% [−5.53, 1.66], P=0.30) water immersion. Blood glucose decreased progressively throughout the control protocol (−1.3 mmol/L [−0.8, −1.9], P<0.01), whereas hot water immersion attenuated this decrease (+0.1 mmol/L [−0.5, +0.6], P=0.93). Hot water immersion did not alter glucose (P=0.17) and insulin (P=0.99) area under the curve during the OGTT.ConclusionThese preliminary results suggest that the physiological responses associated with heat exposure do not translate into acute improvements in blood glucose control or brachial artery endothelial function in individuals with T2DM.Support or Funding InformationCardiometabolic Health, Diabetes and Obesity Research Network and Diabetes Quebec.