BACKGROUND:

Individuals of South Asian (SA) descent are at an increased risk of cardiovascular disease (CVD) compared to White European (WE) counterparts¹. This health disparity may be partially explained by impaired vascular function within the SA population². Heat therapy, such as sauna bathing or hot water immersion (HWI), has been shown to improve vascular function³ and reduce all-cause mortality⁴. However, it is unknown whether populations of distinct racial backgrounds such as WE and SA respond differently to chronic heat therapy. Therefore, we compared the effect of repeated HWI on cutaneous microvascular function and mean arterial pressure (MAP) between individuals of WE and SA descent.

METHODS:

Ten healthy, recreationally active WE and SA males (age: 23 ± 5 vs. 24 ± 2 years; body mass index 25 ± 3 vs. 25 ± 3 kg/m², respectively) had forearm and toe cutaneous vascular conductance (CVC) responses during post-occlusive reactive hyperaemia (PORH) and local heating (LH) measured (Pre). Thereafter, 10 sessions consisting of 30 minutes of shoulder-level then 30 minutes of waist-level HWI (39.0°C) over 14 days were completed. For the first (H1) and last (H10) HWI sessions, MAP was measured at rest and during 30-, 45- and 60-minutes of immersion, with forearm and toe CVC responses remeasured within 48 hours of H10 (Post). Two-way mixed-model repeated measure ANOVAs were conducted to investigate the effect of HWI between racial groups.

RESULTS:

Baseline forearm and toe CVC were similar Pre and Post HWI (p ≥ 0.53), although SA had lower baseline forearm CVC (p = 0.01) but similar baseline toe CVC to WE (p = 0.67), with no Race*HWI interaction effects (p ≥ 0.16).

During PORH, forearm and toe peak CVC as a percentage of baseline (p ≥ 0.42), CVC area under the curve (p ≥ 0.55) and PORH index (p ≥ 0.44) were unchanged by HWI. Furthermore, a greater forearm PORH index in SA (p = 0.03) was the only racial difference and there were no Race*HWI interaction effects (p ≥ 0.08).

In response to 42°C LH, forearm (p = 0.04) and toe (p = 0.04) CVC were elevated following HWI; during 44°C LH toe CVC was greater (p = 0.02), whereas forearm CVC was unchanged (p = 0.12). There were no racial differences (p ≥ 0.42) or Race*HWI interaction effects (p ≥ 0.37) during LH. Resting and throughout immersion, MAP was unchanged by HWI (p ≥ 0.33) and did not differ between races (p ≥ 0.54), with no Race*HWI interaction effects (p ≥ 0.39).  

CONCLUSION:

Forearm and toe microvascular reactivity to LH were improved by HWI in individuals of WE and SA descent, however, microvascular responses during PORH were unchanged. Although HWI did not influence MAP, chronic adaptions are purportedly underpinned by repeated acute responses⁵, highlighting longer-term HWI may be required for beneficial reductions in MAP. Despite HWI similarly improving microvascular function between racial groups, these results are particularly relevant for SA individuals given the elevated CVD risk, impaired vascular function and reduced likelihood of meeting physical activity guidelines identified within this population^1,2.