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Human studies suggest the existence of an exercise dependency syndrome and a link between drug intake and intense physical activity. Our aim was to assess whether a link actually existed between running activity and cocaine intake in mice. Thirty male Swiss mice were used. Ten mice were used as controls, individually housed in cages without a wheel, and 20 mice were in cages with free access to a running wheel. Cocaine preference was estimated as the ratio (as percent) of cocaine solution intake over total fuid intake in the course of free oral access to cocaine solution versus water. High cocaine scores were only found with high wheel activity. The lowest activity scores were found with low cocaine preference. A group of “high runners” impervious to cocaine appetence and to the effects of exercise withdrawal were found, which may suggest that shared mechanisms could be involved in both dependence on sport and drug taking. Findings suggest that moderate activity seems to be associated with low cocaine preference, and cocaine intake could increase in cases of intense activity. The urge for physical activity (as seen with top-level professional athletes) may theoretically combine with different degrees of vulnerability to cocaine. The use of substances by those engaging in intense physical activity, for performance enhancement or recreational purposes, could potentially trigger a pattern of consumption and addiction. This pattern corresponds with the theory that there may be an addictive element in physical activity. Animal models could prove useful for identifying biological or behavioral predictors of such vulnerability and identifying persons either at risk or possessing resistance.

This study used animal models to examine potential similarities between dependence on physical activity (i.e., exercise) and dependence on morphine. Using C57BL/6 mice, the study also tested the hypothesis that physical exercise (e.g., long-term wheel running) may enhance vulnerability to the development of morphine dependence. The existence of an endorphin-related dependence induced by physical activity was also assessed. Naloxone was used to precipitate morphine withdrawal in mice accustomed to morphine. Specifically, the study sought to assess the intensity of addiction provoked by injection of morphine in mice that engaged in wheel-running activity as opposed to inactive mice. After 25 days of free access to activity wheel, mice that engaged in wheel-running demonstrated increased vulnerability to naloxone-induced withdrawal symptoms, which may be linked to activation of peripheral, as opposed to central, opioid receptors. These results indicate a behavioral interaction in which engaging in wheel running appears to potentiate the effects of morphine addiction. Implications of these findings for understanding human behavior and exercise addiction are also discussed.

Background: Genetic variants within the FTO gene have been associated with increased adiposity and metabolic markers; however, there is limited evidence regarding the association of FTO gene variants with physical activity-related variables. The authors aimed to investigate the association of the rs17817449 single-nucleotide polymorphism of FTO with physical activity, sedentary time, and cardiorespiratory fitness in Chilean adults. Methods: A total of 409 participants from the GENADIO study were included and genotyped for the rs17817449 single-nucleotide polymorphism of FTO in this cross-sectional study. Physical activity and sedentary time were measured with ActiGraph accelerometers. Cardiorespiratory fitness was assessed using the Chester step test. The associations were assessed by using multivariate regression analyses. Results: No associations were found for FTO variant with physical activity levels and cardiorespiratory fitness. The risk allele (G) of the FTO was found to be associated with sedentary time in the minimally adjusted model (β = 19.7 min/d; 95% confidence interval, 4.0 to 35.5, per each copy of the risk allele; P = .006), but the association was no longer significant when body mass index was included as a confounder (P = .211). Conclusion: The rs17817449 single-nucleotide polymorphism of the FTO gene was not associated with the level of physical activity, cardiorespiratory fitness, and sedentary behaviors in Chilean adults.