Recent evidence suggests that heat acclimation (HA) positively affects metabolic health in humans. HA has been shown to decrease fasting plasma glucose and insulin concentrations, inflammation markers, free fatty acids and cholesterol levels. In a previous study, we found that both rate of glucose disappearance and endogenous glucose production (EGP) decreased in a fasted state upon HA in overweight middle-aged men, which indicates improved liver metabolism. This suggests that beneficial effects of HA could at least partly be due to enhanced liver function, but underlying mechanisms remain uncertain. To gain better understanding of the metabolic effects of HA, this study aimed to assess the effect of acute whole-body heat exposure on liver insulin sensitivity, as well as parameters of metabolic health and thermoregulation in middle-aged overweight individuals.

11 overweight non-diabetic volunteers (n=6 male/5 female, 56.5±3.2y, BMI 29.6±1.8kg/m², fasting plasma glucose 5.6±0.4mmol/L) participated in this study. After a standardized dinner and overnight fast, participants underwent two times a 3-hour 1-step hyperinsulinemic-euglycemic clamp with a low dose of insulin (10mU/m²/min) and primed-continuous intravenous infusion of [6,6-2H2]glucose (0.04mg·kg^–1·min^-1), during either heat exposure (H) or thermoneutrality (TN). Thermal exposure was applied with a water-perfused suit (H:39˚C; TN:32˚C). Rates of glucose appearance and disposal, EGP, plasma metabolites, hematocrit and parameters of thermoregulation (e.g. core body temperature , mean skin temperature) were assessed at baseline and during insulin stimulation.

To the best of our knowledge, this is the first study performing an insulin-clamp during thermal exposure. Preliminary results (N=8) show that core body temperature and mean skin temperature were successfully increased by 0.8±0.5˚C (p=.008) and 2.2±0.7˚C (p=.000) during H vs. TN in the insulin-stimulated stable period. As the analysis is ongoing, parameters of blood chemistry and stable isotope analysis will be presented at the conference.