Amphetamine enhances endurance by increasing heat dissipation

Abstract

Athletes use amphetamines to improve their performance through largely unknown mechanisms. Considering that body temperature is one of the major determinants of exhaustion during exercise, we investigated the influence of amphetamine on the thermoregulation. To explore this, we measured core body temperature and oxygen consumption of control and amphetamine‐trea ted rats running on a treadmill with an incrementally increasing load (both speed and incline). Experimental results showed that rats treated with amphetamine (2 mg/kg) were able to run significantly longer than control rats. Due to a progressively increasing workload, which was matched by oxygen consumption, the control group exhibited a steady increase in the body temperature. The administration of amphetamine slowed down the temperature rise (thus decreasing core body temperature) in the beginning of the run without affecting oxygen consumption. In contrast, a lower dose of amphetamine (1 mg/kg) had no effect on measured parameters. Using a mathematical model describing temperature dynamics in two compartments (the core and the muscles), we were able to infer what physiological parameters were affected by amphetamine. Modeling revealed that amphetamine administration increases heat dissipation in the core. Furthermore, the model predicted that the muscle temperature at the end of the run in the amphetamine‐treated group was significantly higher than in the control group. Therefore, we conclude that amphetamine may mask or delay fatigue by slowing down exercise‐induced core body temperature growth by increasing heat dissipation. However, this affects the integrity of thermoregulatory system and may result in potentially dangerous overheating of the muscles.

Tissue oxidative metabolism can increase the difference between local temperature and arterial blood temperature by up to 1.3oC: Implications for brain, brown adipose tissue, and muscle physiology

“…Therefore, at rest, muscles have temperatures close to that of the core or slightly lower, due to heat dissipation to the environment. Our calculations show that, regardless of exercise intensity, the difference between muscle and core temperatures cannot exceed 1.3 C. Consistent with this, the muscle-core temperature difference at the highest workload was found to be »1.0 C [31,56]. In the study by Cotter et al [57], core temperature (as an average between esophageal and rectal temperatures) and muscle temperature (in the m. vastus lateralis) were measured simultaneously in healthy adult volunteers during exercise.…”

“…As shown above, at equilibrium, if there is no heat exchange between the organ and its environment (other than through the circulation), the organ temperature cannot be more than 1.3 C higher than the arterial blood temperature. Furthermore, the core temperature in rats fluctuates between »37 and 38 C, with a potential to decrease to »36 C during sleep [25,26] and to increase to »39 C during intense exercise [27][28][29][30][31][32] or pharmacological stimulation [33][34][35]. As such, the physiological range of the core temperature is almost three times wider than the maximum effect of local metabolism.…”

Muscle fatigue: general understanding and treatment

“…Although amphetamine usage is prohibited during competitions, it may be used in some situations, such as in combat, to improve performance by delaying exhaustion. 111 …”

ADHD Prescription Medications and Their Effect on Athletic Performance: A Systematic Review and Meta-analysis

“…The belief that these medications provide a physical advantage has been purported but yet to be conclusively validated in humans. However, in animal models, rats treated with amphetamine were able to run significantly longer than non-treated controls [ 14 ]. More specifically, rats treated with amphetamines demonstrated increased time to exhaustion, increased time to reach VO 2 max with concomitant increase in VO 2 max values, and higher core temperatures at exhaustion, suggesting that amphetamines decrease the sensation of fatigue [ 15 ].…”