One topic that has received increasing attention in the scientific and endurance sport community over the last few years is training under environmental heat stress. The benefits of training in the heat in preparation for a competition in a hot environment is well-established (3, 23). That process is called heat acclimatisation (or acclimation when performed in an artificial hot environment like a heat chamber), and is an effective means of improving an athlete’s thermoregulatory capabilities in order to tolerate the stress associated with the dual stresses of competitive work outputs and high environmental stress. That is not what this blog is about. Interested readers are directed to our course dedicated to preparing for long-distance triathlon competitions in hot environments (LDT 103).
In this blog we are instead concerned with whether training under environmental heat stress can be used as an additional stressor in order to promote endurance training adaptations relevant to performance in temperate, or cool, conditions. We will firstly discuss a little of the rationale behind this using this approach, before providing details of a study we have recently published. We will conclude with a few thoughts, and acknowledgement that much more research still needs to be done in this field!
What is the rationale behind using heat as a training stressor?
We can think of using heat training to stimulate training adaptations relevant to performance in temperate conditions through the lens of ‘hormesis’. Hormesis refers to using low to moderate levels of stress to stimulate signalling pathways that lead to improved physiological capacity to cope with stress (21). Exercising in the heat stresses a number of physiological systems – thermoregulatory, cardiovascular, metabolic, endocrine, and so on – and the concept of hormesis might tell us that these stresses, provided they are not so large that we cannot absorb them, will trigger positive adaptations to make these physiological systems more robust. Altitude is an environmental stressor that has been used in a similar way for decades; the reduced oxygen content of the thin air at high elevations stresses our oxygen transport pathways and may stimulate beneficial increases in haemoglobin and red blood cell mass (1, 11).
More specifically, the cellular signalling pathways that may be stimulated to a greater extent when training under environmental stress were discussed in detail in a 2018 review article in the prestigious journal Cell Metabolism (5). Whilst all still relatively speculative, it is possible that the greater levels of glycogen depletion expected when training in the heat (4) may lead to increased AMPK activation (26) and adaptive signalling responses (22). Greater lactate accumulation and circulating adrenaline may have effects on expression of PGC-1α, the so-called ‘master regulator’ of mitochondrial biogenesis (2, 9, 18, 19). Exercise training under heat stress might also be expected to increase expression of the ‘heat shock proteins’ (25) which are implicated in managing cellular stress and, possibly, in helping facilitate adaptive processes following training (6, 7). Therefore, there is rationale – albeit theoretical rationale – for why the additional stresses evoked by training in a hot environment might help stimulate endurance-related training adaptation.
A few studies have previously assessed performance responses in temperate conditions following a period of endurance training under environmental heat stress, with quite mixed results (8, 12, 16, 17, 20, 24), although the designs of these studies have differed considerably which makes interpreting the results of the literature as a whole quite challenging. When assessing studies in this field, it is important to consider firstly if there was a ‘control’ training intervention in a temperate environment, and secondly how this intervention was ‘matched’ with the heat training intervention. If the interventions are matched for absolute workloads, it is likely that either the temperate training was particularly easy, or that the heat training was monstrously hard. In these circumstances, we might not consider the training interventions suitably fair comparisons for each other. That being said, a couple of strong studies have been published in the last few years, with these studies appearing to provide evidence that training under heat stress has beneficial effects on haematological parameters that would positively impact the oxygen-carrying capacity of the athlete’s blood (16, 17, 24).
Our recent work
In our study, we wanted to compare performance and metabolic training adaptations to three-week blocks of training performed in either 18 or 33°C (15). We therefore randomly allocated a cohort of endurance-trained cyclists to one of the two interventions. Little to no data had been published on the metabolic effects of training under heat stress, so we took muscle biopsies before and after the intervention to look at citrate synthase enzyme activity, a marker of mitochondrial protein content, and the content of the fatty acid transport protein CD36. We also assessed whole-body fat and carbohydrate oxidation rates during fasted and fed-state cycling, and assessed performance using a 30-min maximal effort time-trial that was conducted immediately following two-hours of moderate-intensity cycling performed after breakfast and with carbohydrate feeding to properly simulate a competitive scenario.
We paid very careful attention to the design and matching of the training interventions. Firstly, we sought to simulate the heat training performed by real-world elite Ironman triathletes in our recent case study, which also lasted three weeks (13). In our study, the participants trained in our laboratory five times per week for three weeks, with four of these weekly sessions prescribed relative to individual heart rate thresholds, and the remaining session being a ‘best-effort’ interval training work-out. We matched the training interventions in this way based on a small cross-over study we did, in which we put athletes through an incremental test in 18 and 35°C (14). We measured ventilatory and lactate thresholds in the two environments and found that whilst power output at these thresholds declined by ~10-17% with heat – as you might expect – the heart rates at which these thresholds occurred was not significantly different, and in fact very similar on an individual basis, between the two environments. Therefore, we felt that matching the interventions in this way, using individual heart rate thresholds, was appropriate to ensure the two groups were performing similarly demanding training programmes. This last point – that the training interventions were similarly demanding – was borne out in the data. Perceived training load was similar between the two groups, despite the expected reduction in power outputs achieved during training in the group training in 33°C.
So, what happened? Well, firstly, performance in the 30-min time-trial improved significantly in both groups, and numerically in each individual athlete in the study. This is convincing evidence that the training programmes worked. However, the magnitude of the improvement in 30-min time-trial performance was significantly greater in the group training in the heat (30 ± 13 vs. 16 ± 5 W, P = 0.04). It is possible this larger performance improvement in the heat group may have been at least partially explained by the muscle data. Citrate synthase enzyme activity, measured in a sample of muscle taken at rest before and after the intervention period and used to indicate mitochondrial protein content (10), significantly increased in the heat group, whereas this effect was not observed in the temperate group. It should be acknowledged that the interaction between training group and time was not significant, although this may have been an issue of sample size and of the quite variable responses in this parameter within-groups. You can read our study in-full using this open-access link: https://physoc.onlinelibrary.wiley.com/doi/full/10.14814/phy2.14849.
Our study is by no means the last word on this subject. The study was small, as these projects often are, with only 17 athletes included (the data collection period was interrupted by two multi-week COVID-19 lock-downs!). These responses need to be repeated for confidence and need to be studied with more and different mitochondrial markers. It is also necessary to follow-up our work with studies in elite and female athletes, and in different training scenarios. For instance, we need more studies to tell us what the optimum ‘dose’ of heat training might be for stimulating temperate performance adaptations – specific temperature, length of the block, number of heat training sessions per week, etc.
Back to the lab!
How can I learn more about using heat as a training tool?
For a limited time* only, we've pulled out the Endure IQ LDT103 Module: Using Heat as a Training Tool for you to enroll in if you're interested in learning more about how heat can be used as a tool that we can use to advance our performance in temperate as well as hot conditions in long distance triathlon. Check out more info at the link below. *Offer valid May 26th 2021-June 30th 2021 and spaces are limited.
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