The effect of repeated hot water immersion on microvascular function, glycaemic control and inflammation in White European and South Asian males

Individuals of South Asian (SA) descent display a higher risk for cardiovascular disease and type 2 diabetes mellitus than their White European (WE) counterparts. Heat therapy, such as hot water immersion (HWI), can improve microvascular function and glycaemic control, although effects across racial groups are unknown. This study compared how repeated HWI influenced microvascular function, glycaemic control and markers of inflammation in males of WE and SA descent. Ten WE and SA males completed ten 60 min HWI sessions over 14 days. Before and after HWI, forearm and great toe cutaneous vascular conductance (CVC) responses to postocclusive reactive hyperaemia and local heating (LH) were measured, resting blood samples were collected, and an oral glucose tolerance test (OGTT) was conducted. Baseline great toe CVC did not differ between racial groups (P = 0.670), but forearm CVC was lower in SA (P = 0.010). For postocclusive reactive hyperaemia, forearm and great toe peak CVC and area under the curve were unchanged by HWI (P ≥ 0.300), whereas CVC during 42°C LH increased at both sites after HWI (P ≤ 0.037), as did great toe CVC during 44°C LH (P = 0.021). Glucose and insulin concentrations were elevated in SA during the OGTT (P ≤ 0.035); glucose concentration and peak insulin reduced after HWI in SA (P ≤ 0.024), but not in WE. The interleukin-6:interleukin-10 ratio was unchanged by HWI (P = 0.159), but elevated in SA (P = 0.006). Repeated HWI increased microvascular responses to LH to a similar extent between racial groups and reduced early-phase OGTT glucose concentrations in SA, although insulin sensitivity was unchanged. These findings support HWI as a health-promoting intervention.