Chronic heat exposure for health and exercise performance – cardiovascular research heats up

Flow-mediated dilatation (FMD) is a method used to measure endothelial-dependent vasodilatation, quantified by the change in conduit artery luminal diameter during post-occlusive hyperaemia. The hyperaemic response is predominately governed by nitric oxide (NO)-mediated vasodilatation secondary to endothelial shear stress; therefore, FMD is commonly used as an index of endothelial function and indirect measure of NO bioavailability. Additionally, FMD has been negatively associated with risk of future cardiovascular (CV) events in healthy adults (Shechter et al. 2014), which justifies investigation into strategies aimed at enhancing endothelial function. In a recent article published in The Journal of Physiology, Brunt and colleagues (2016b) examined the effects of chronic heat exposure on CV health. Young, sedentary but otherwise healthy participants underwent 36 hot or thermoneutral water immersion sessions over an 8-week period with CV measures (blood pressure, carotid artery wall thickness, dynamic arterial compliance, pulse wave velocity, brachial artery FMD and endothelial-independent vasodilatation) assessed at baseline and 2 week intervals thereafter. Of importance, the authors demonstrated for the first time that chronic exposure to an elevated core temperature (38.5°C) resulted in increased brachial artery FMD and reductions in arterial stiffness, carotid artery wall thickness and blood pressure. As stated in the article, this work built upon a 30 year prospective study which found a negative association between sauna use and incidence of CV disease. The increases in FMD of > 5% at study completion were both statistically and clinically relevant (a 2% increase in FMD corresponds to a 15% reduction in CV disease risk), and were higher than previously reported increases observed after 6 weeks of sprint interval training or endurance training (∼1% and ∼2% increases in popliteal artery FMD, respectively) (Rakobowchuk et al. 2008). Therefore, repeated heat exposure provides an impressive strategy for improving vascular function and reducing risk of cardiovascular disease in an already healthy cardiovascular system. Further, repeated heat exposure may have a greater effect on brachial artery FMD in diseased or at risk (e.g. pre-hypertension) populations than in healthy participants due to a large degree of endothelial dysfunction associated with CV disease; however, this warrants further investigation. Additionally, the use of sauna or hot tubs may be a safer or more accessible treatment strategy compared to regular exercise in these diseased populations. Though Brunt and colleagues (2016b) did not specifically aim to investigate changes in NO bioavailability, changes in endothelial-dependent vasodilatation but not endothelial-independent vasodilatation suggests that the changes in CV measures were due to increased local vasodilators, particularly NO. In a following paper, Brunt and colleagues confirmed that chronic heat exposure improves microcirculation via increased NO bioavailability, as an NO synthase inhibitor was capable of abolishing the effect (Brunt et al. 2016a). Research using nitrate supplementation as an ergogenic aid also revolves around the supplement's ability to influence endothelial-dependent vasodilatation. The use of exogenous nitrate supplementation to increase NO bioavailability for the specific purpose of aiding sport performance is a hotly debated topic, and current literature has been criticized for a lack of standardized supplementation protocols and small or inappropriate sample populations used to evaluate the ergogenic effects. This obfuscates firm conclusions on the topic (Allen, 2015). Based on our own data and early reports, our research group believes that improvements in endothelial function in elite athletes will be small (even negligible) with increased NO bioavailability, but it is possible that performance may still be meaningfully increased. Similarly, in regards to its ability to increase plasma nitrite concentrations, ischaemic preconditioning has also been discussed as a potential ergogenic aid with conflicting empirical support (Incognito et al. 2016). A potential mechanism of ischaemic preconditioning is the ability to increase skeletal muscle blood flow during exercise by preserving NO bioavailability, which has been shown through measures of brachial artery FMD after ischaemic stressors and strenuous exercise (Incognito et al. 2016). The ability of repeated heat exposure to increase resting brachial artery FMD (Brunt et al. 2016b) and therefore NO bioavailability, may present as an alternative strategy for improving skeletal muscle blood flow during exercise in thermoneutral environments. However, whether resting levels of brachial artery FMD are associated with improved muscle blood flow during exercise remains unknown, as do the influence of additionally layered factors such as sex, training status, diet and fitness. The use of heat exposure has classically been used in athletics to acclimate individuals prior to competing in hot environments, which promotes more effective regulation of core temperature, increases sweat rates and increases plasma retention. As Brunt et al. (2016b) noted, the minimal effective dose of passive heat exposure for altering CV or exercise performance measures has not been determined. A CrossTalk debate between Minson & Cotter and Nybo & Lundby (Minson & Cotter, 2016; Nybo & Lundby, 2016) has discussed the effects of heat acclimation on exercise performance in cool and hot environmental conditions, yet there remains considerable equipoise within the literature as well as variability in heat acclimation protocols between studies. Nonetheless, the increased NO bioavailability found by Brunt et al. (2016a) suggests another potential mechanism by which heat acclimation may improve a number of CV risk factors and alter exercise performance. We suggest that future studies investigate the effects of chronic heat exposure on skeletal muscle blood flow during exercise to provide insight of the potential integration of heat exposure to exercises strategies for CV disease and in athletic training regimes as an ergogenic aid. None declared. All authors have approved the final version of the manuscript and agree to be accountable for all aspects of the work. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

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