Timing of acute passive heating on glucose tolerance and blood pressure in people with type 2 diabetes: a randomized, balanced crossover, control trial

Type 2 diabetes mellitus (T2DM) is characterized by chronic hyperglycemia and progressive insulin resistance, leading to macro and microvascular dysfunction. Passive heating has potential to improve glucose homeostasis and act as an exercise mimetic. We assessed the effect of acute passive heating before or during an oral glucose tolerance test (OGTT) in people with T2DM. Twelve people with T2DM were randomly assigned to 3 conditions:1) 3 h OGTT (CON); 2) 1 h passive heating (40 °C water) 30 min before an OGTT (HOT-OGTT); and 3) 1 h passive heating (40 °C water) 30 min after commencing an OGTT (OGTT-HOT). Blood [glucose], insulin sensitivity, extracellular heat shock protein 70 (eHSP70), total energy expenditure (TEE), heart rate (HR), systolic (SBP) and diastolic blood pressure (DBP) were recorded. Passive heating did not alter blood [glucose] (CON, 1,677 (386) a.u.; HOT-OGTT, 1,797 (340) a.u.; OGTT-HOT, 1,662 (364) a.u.; P = 0.28), insulin sensitivity (P = 0.15), or SBP (P = 0.18), but did increase [eHSP70] in both heating conditions (CON, 203.48 (110.81) pg·mL^-1; HOT-OGTT, 402.47 (79.02) pg·mL^-1; OGTT-HOT, 310.00 (60.53) pg·mL^-1; P < 0.001), increased TEE (via fat oxidation) in the OGTT-HOT condition (CON, 263 (33) kcal; HOT-OGTT, 278 (40) kcal; OGTT-HOT, 304 (38) kcal; P = 0.001), increased HR in both heating conditions (P < 0.001) and reduced DBP in OGTT-HOT condition (P < 0.01). Passive heating in close proximity to a glucose challenge does not alter glucose tolerance but does increase [eHSP70] and TEE, and reduce blood pressure in people with T2DM.