Hot water immersion acutely increases postprandial glucose concentrations

Background: Chronic hot water immersion (HWI) confers health benefits, including a reduction in fasting blood glucose concentration. Here we investigate acute glycaemic control immediately after HWI. Methods: Ten participants (age: 25 ± 6 years, body mass: 84 ± 14 kg, height 1.85 ± 0.09 m) were immersed in water (39ºC) to the neck (HWI) or sat at room temperature (CON) for 60 min. One hour afterwards they underwent an oral glucose tolerance test (OGTT), with blood collected before and after HWI/CON and during the 2 h OGTT. Results: Glucose incremental area under the curve (iAUC) during the OGTT was higher for HWI (HWI 233 ± 88, CON 156 ± 79 mmol·L-1·2h, P = 0.02). Insulin iAUC did not differ between conditions (HWI 4309 ± 3660, CON 3893 ± 3031 mU·L-1·2h, P=0.32). Core temperature increased to 38.6 ± 0.2°C during HWI, but was similar between trials during the OGTT (HWI 37.0 ± 0.2, CON 36.9 ± 0.4°C, P=0.34). Directly following HWI, plasma average adrenaline and growth hormone concentrations increased 2.7 and 10.7-fold, respectively (P < 0.001). Plasma glucagon like peptide-1, peptide YY and acylated ghrelin concentrations were not different between trials during the OGTT (P > 0.11). Conclusions: HWI increased postprandial glucose concentration to an OGTT, which was accompanied by acute elevations of stress hormones following HWI. The altered glycaemic control appears to be unrelated to changes in gut hormones during the OGTT.