Background
Individuals of South Asian (SA) descent are at an increased risk of type 2 diabetes (T2D) and metabolic dysfunction compared to their White European (WE) counterparts1. Heat therapy, such as hot water immersion (HWI), has been purported to induce health benefits whilst meta-analyses show equivocal results regarding glycaemic control in various populations2. No study has investigated whether populations of distinct racial backgrounds exhibit different glucose tolerance responses following repeated heat therapy. This study investigated the effect of repeated HWI on glucose tolerance in SA and WE individuals using an oral glucose tolerance test (OGTT).
Methods
Following Loughborough University ethical approval and written informed consent, ten WE and ten SA, recreationally active males (mean (SD) age 23 (5) vs 24 (2) years; BMI: 25 (3) vs 25 (3) kg/m2, respectively) completed an OGTT before (PRE) undergoing 10 sessions of 60-minute (30-minute shoulder-level; 30-minute waist-level) HWI (39 °C) over 14 days. The OGTT was repeated between 24 and 48 hours of the final HWI session (POST). Capillary glucose concentrations were measured fasted and at regular intervals up to 2 h following a 75 g oral glucose load3.
A mixed design ANOVA assessed the effects of chronic HWI on glucose tolerance, peak [glucose] and area under the curve (AUC) between racial groups, with Bonferroni corrected post hoc pairwise comparisons to examine within- and between-race differences.
Results
For glucose tolerance, there was no significant main effect of condition, race or condition × race interaction (all p > 0.05). However, a significant condition × time × race interaction (p < 0.001) indicated that HWI affected the time course of the [glucose] responses to HWI differently between SA and WE individuals. Post hoc analysis revealed that in PRE-HWI, SA individuals had higher [glucose] than WE at 30 min (p = 0.035). During POST-HWI, SA exhibited higher [glucose] than WE at 60 min (p = 0.005). Within-race comparisons showed that SA experienced reductions in [glucose] POST-HWI at 15 min (p = 0.011) and 30 min (p = 0.019) compared to PRE-HWI, while WE individuals showed no significant within-group changes (p > 0.05).
For peak [glucose] and AUC, there was no significant main effect of condition, race or condition × race interaction (all p > 0.05).
Discussion
Heat therapy may influence the time course of glucose responses in SA individuals, however, the absence of changes in peak [glucose] and AUC suggests that these effects may not translate to overall improvements in glucose tolerance. The observed reductions in early postprandial [glucose] in SA individuals could indicate subtle alterations in glucose handling, but the lack of similar changes in WE individuals suggests potential racial differences in metabolic adaptations to heat therapy. These differences may be influenced by variations in insulin sensitivity and/or hormonal responses4.