Metabolic Implications of Environmental Heat Stress for Endurance Athletes
Carbohydrates and fats are the primary substrates used to fuel energy metabolism during exercise, and endogenous carbohydrate availability may be an influential physiological characteristic in some endurance events. Substrate metabolism during exercise is regulated by various factors such as exercise intensity and duration, nutrient availability, and training status. Importantly in the context of the many endurance competitions and training camps that take place under environmental heat stress, substrate metabolism during acute exercise, as well as metabolic adaptation to endurance training, may also be impacted by environmental heat stress. Specifically, previous research has observed increased carbohydrate metabolism to support a given bout of exercise performed under heat stress compared to temperate conditions, which may have specific implications for acute exercise responses and metabolic training adaptations. However, the majority of studies investigating the acute effect of heat stress on substrate metabolism during endurance exercise have employed relatively extreme combinations of exercise and heat stress, thus making it difficult to make inferences regarding the specific conditions under which acute heat stress is likely to influence substrate metabolism during endurance exercise. Furthermore, despite significant discussion, systematic evaluation of the effect of heat stress on metabolic training adaptations has so-far not been conducted. Therefore, the purpose of this thesis was to improve the understanding of the metabolic implications of environmental heat stress for endurance athletes, with respect to both acute heat exposure, and the adaptive metabolic response to serial training sessions performed under heat stress. Accordingly, in the present thesis two main research questions are investigated: (i) in what situations might environmental heat stress be expected to impact substrate metabolism during acute endurance exercise? and (ii) does environmental heat stress impact the adaptive metabolic response to endurance exercise training?
From an acute perspective, studies in thesis provide evidence that exercise intensity appears to regulate the effect of heat stress on substrate oxidation rates, with greater shifts towards carbohydrate metabolism seen at higher absolute workloads in 35°C (Acute Study 1). However, when matched for temperature-specific ventilatory thresholds, the reduced absolute workloads achieved under heat stress at individual physiological thresholds may actually necessitate reduced carbohydrate use, at least at low-intensities (Acute Study 1). The magnitude of the environmental heat stress also appears to be important for the metabolic perturbation, with higher temperatures evoking greater stimulatory effects on carbohydrate metabolism at given absolute workloads, and lowering the absolute workload at which metabolic perturbations are observed (Acute Study 2). From a training adaptation perspective, a case study of elite Ironman triathletes undertaking a three-week heat stress training camp in Kailua-Kona, Hawaii demonstrated that elite endurance athletes appear capable of undertaking successful heat stress training camps despite the apparent physiological strain experienced when exercising under environmental heat stress (Training Study 1). In a laboratory setting, heart rates observed at given physiological thresholds appear to be consistent between 18 and 35°C, and therefore may provide a useful for tool for regulating training load and intensity when beginning a heat stress training camp (Training Study 2). These findings were used in the design of a three-week, laboratory-based training intervention in which endurance training performed in 33°C evoked greater improvements in pre-loaded 30-min time-trial (TT) performance than a matched training programme performed in 18°C, with some of this effect explained by positive effects of maximal vastus lateralis citrate synthase activity, and therefore mitochondrial protein content (Training Study 3).
Overall, this thesis adds to existing literature by providing evidence for the specific combinations of exercise-heat stress likely to impact acute substrate oxidation rates, and the metabolic and performance adaptations observed when training under environmental heat stress. Specifically, the data presented in this thesis suggests practitioners working with endurance athletes performing acute bouts of exercise under heat stress can expect minimal heat stress-effects on substrate oxidation rates unless the exercise intensity or environmental temperature is high (~40°C), and that executing a well-controlled heat stress training camp may provide a useful stimulus for endurance performance-related adaptations.