Aerobic fitness influence on thermoregulation during exercise in the heat- The majority of current literature regarding the thermoregulatory responses to exercise in a hot environment is based on studies conducted on aerobically trained individuals (military recruits and endurance athletes) during prolonged exercise.
My position stand on exercise and fluid replacement… it is advised that fluid and electrolyte replacement should vary depending on the individual degree of heat acclimatization and training status.
Excessive hyperthermia during exercise in the heat induces fatigue and could lead to heat injury. Furthermore, heat exhaustion episodes may stop some individuals from engaging in exercise and thereby prevent them from receiving the associated health benefits.
By understanding the factors that can lead to hyperthermia, recommendations can be made to attenuate their impact.
I say that the thermal adaptations induced by aerobic fitness are not enough to prevent overheating, which will be determined by the interaction between exercise intensity, hydration, and environmental heat load.
During exercise, active muscles perform work, which produces heat that is either dissipated or accumulated in body tissues. The rate of body heat storage is the difference between the rate of metabolic heat production and the rate of heat dissipation. Heat production is determined mainly by the exercise workload with a minor contribution from movement efficiency. Because most energy available to the body depends on the consumption of oxygen, metabolic heat production during prolonged exercise can be estimated by measuring energy expenditure via indirect calorimetry.
Heat dissipation is more complex to determine because it encompasses evaporative and dry heat exchange with the surrounding environment. Humans exchange most heat with the environment through the skin with negligible contribution from heat loss via breathing.
As the environmental temperature increases, the gradient with skin temperature narrows and less dry heat is exchanged with the environment. In a hot environment, most of the heat is dissipated by sweat evaporation with a concomitant increase in skin blood flow to deliver the heat to the skin. Therefore, during exercise in environments where ambient temperatures closely match skin temperatures, heat dissipation will be dictated by sweat evaporation.
Although sweat rate can be measured easily by changes in nude body mass upon corrections for metabolic and respiratory mass loss, it is cumbersome to measure the amount of sweat that truly evaporates versus the amount that simply falls off the body. The measure of un-evaporated sweat requires the use of a Potter platform balance supporting the cycleergometer, subject and a collecting pan containing mineral oil to trap the un-evaporated sweat. Furthermore, several factors can impact the amount of sweat that is evaporated. For example, variations in air flow can lead to a 28% difference in the amount of sweat evaporated when exercising in a hot environment.
During exercise, core temperature initially increases from resting levels, signaling heat loss responses to achieve a new steady-state core temperature. However, the intended thermal equilibrium is not always achieved during intense exercise or when environmental heat stress is high, resulting in unavoidable hyperthermia.
Core temperature originally was believed to be determined by the absolute workload being performed by the subject. However, in 1966, Saltin and Hermansen reported the core temperature response of seven subjects with a wide range of aerobic fitness levels when cycling in a thermoneutral environment (20-C and 55% relative humidity). For a given absolute workload, there was a scatter of rectal temperature responses. However, when rectal temperature was presented for a given percentage of VO2max, the interindividual variation was reduced by 65%.
The dual effect of aerobic training on improving VO2max and heat dissipation may explain why percentage of VO2max is associated tightly with exercise core temperature. When exercising at a similar percent of V˙ O2max, endurance-trained subjects will be exercising at a higher absolute workload than untrained individuals, thereby generating more metabolic heat.
Thermoregulatory benefits of Aerobic Training vs Heat Acclimatization – the thermoregulatory adaptations that emerge from exercise in a hot environment (heat acclimatization) are similar to but extend beyond the adaptations obtained as a result of aerobic training. Heat acclimatization expands plasma volume, improves cardiac function and sweating pattern and acts to conserve sweat sodium. Aerobic training increases sweat rate for a given core temperature, but subsequent heat acclimatization leads to further increases in sweat rate and earlier onset of sweating. The rate at which heat acclimatization occurs is related to the initialV˙ O2max level with untrained subjects requiring more sessions to obtain full adaptation than the trained. Although many thermoregulatory adaptations occur with aerobic training, training in the heat will enhance those adaptations. That likely is why well-trained cyclists can enhance local sweat rate and skin vasodilation upon acclimatization without improvements in VO2max.
The thermoregulatory adaptations that improve heat dissipation during exercise training or heat acclimatization are directed to prevent excessive hyperthermia during exercise. End exercise rectal temperature gives information of the heat dissipation improvements as heat acclimatization progresses. In addition, the effect of aerobic fitness level on thermoregulation can be seen by plotting the individual VO2max levels against the end exercise rectal temperatures.
Oral Rehydration – although training increases the functioning of the heat dissipatory mechanisms. fluid deficit (hypohydration) reduces heat dissipation by lowering the sweat rate and cutaneous blood flow response. Core temperature rises similarly in trained and untrained subjects when exercising 5% hypohydrated. This finding has led to the conclusion that hypohydration will override the positive heat dissipation adaptations that occur with aerobic training.
Trained individuals have larger blood volume and a reduced sensitivity to the inhibitory actions of hyperosmolality on the heat dissipatory mechanisms. This may allow aerobically trained individuals to maintain heat dissipation above the level of their untrained counterparts when both groups are dehydrated mildly. Another interpretation of these data could be that there is a threshold effect where mild dehydration is not detrimental to trained individuals, but levels greater than 2% negate the benefits of training.
Body Characteristics – body composition has been shown to play a role in heat production/accumulation. Trained athletes have higher total body water values by virtue of their higher lean body mass and their lower fat mass. In addition, they have more muscle glycogen and, thus, more of the water associated with it. However, it is unknown if the water liberated as glycogen is metabolized has a role in thermoregulation.
Thus, the larger body mass in the untrained individuals may result in a higher heat storage capacity. However, the increased metabolic rate when transporting a higher body mass during walking or running could override the advantageous mass heat sink effect. In fact, calculations suggest that a small body mass could be a thermoregulatory advantage when running in a hot environment.
Lastly, it currently is unclear if the higher adiposity of the untrained individuals could impose resistance to heat dissipation. Although the sweat gland is above the subcutaneous fat layer and, thus, fatness may not affect evaporative heat loss, heat conduction from the muscles to the periphery may be reduced.
Movement Efficiency – efficiency has been found to increase with prolonged training, which may be mediated by a change of fiber type to the more metabolically efficient Type I fibers. Although the higher heat production in untrained subjects likely is dissipated in a thermoneutral environment, during prolonged exercise in the heat, heat storage may be increased.
During high-intensity exercise in the heat, aerobically trained individuals reach higher core temperatures than their untrained counterparts, leaving them more prone to experience heat-related problems. The reduction in sweat sodium observed with heat acclimatization has been regarded as a mechanism to maintain osmotic forces and thus plasma volume.
Nonetheless, there are data suggesting that aerobic fitness does enhance the effects of fluid ingestion on reducing heat accumulation at least, when exercising at a moderate intensity. Lastly, there are modifying factors (exercise pattern, body composition, air flow, and movement efficiency) that should be taken into account when comparing thermoregulatory responses of trained and untrained populations.
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