New research points to the potent addictive effects of excessive exercise. When exercise-addicted rats were given naloxone, a chemical used to block the effect of opiates, they experienced withdrawl effects, such as "trembling, writhing, teeth chattering, and drooping eyelids," whereas inactive mice did not.
From a press release:
The active rats who had access to food for only one hour a day both ran the most and displayed the most severe withdrawal symptoms. Like people with anorexia athletica, they ran so much that they lost significant amounts of weight. Additionally, the more a given rat had run, the worse its withdrawal symptoms after naloxone. In contrast, regardless of how much they ate, inactive rats responded very little to the drug.
Because of the way the active rats responded to naloxone, they seemed to have undergone the same changes in the brain's reward system as rats addicted to drugs. "Exercise, like drugs of abuse, leads to the release of neurotransmitters such as endorphins and dopamine, which are involved with a sense of reward," noted [lead researcher Robin] Kanarek.
Insights into behaviors that trigger the release of the brain's "reward" chemicals may lead to addiction treatments that incorporate moderate exercise, according to the researchers. The findings also suggest that active rats given limited food may make a good experimental model for studying and developing treatments for anorexia athletica, added Kanarek.
The paper itself (free and full-text!), "Running and Addiction: Precipitated Withdrawal in a Rat Model of Activity-Based Anorexia," had some very interesting findings. There, the authors directly linked the commonalities between drug and exercise dependence. They found that "excessive running shares similarities with drug-taking behavior."
Similarities between the effects of exercise and drugs of abuse extend beyond opiate drugs. Research demonstrating that rats will perform operant responses to obtain access to either drugs of abuse or a running wheel provides evidence of the rewarding properties of both drugs of abuse and running. Moreover, under certain circumstances, such as food deprivation, both drug selfadministration and running escalate and become maladaptive behaviors. These findings suggest that running may be able to substitute for drug-taking behavior. In support of this suggestion, rats running in activity wheels self-administered smaller quantities of opiates, alcohol, and psychomotor stimulants (e.g., amphetamine and cocaine) than rats housed in standard cages.
The finding that symptoms resembling those of opioid withdrawal occur in food-restricted active rats may have correlates in clinical populations. Excessive exercise is a common symptom of eating disorders, particularly anorexia nervosa Initially, physical activity is used as a means of weight control, but with time it can become an end in itself. In the extreme, individuals with eating disorders can have difficulty refraining from exercise despite adverse physical consequences (e.g., an unhealthy decrease in body weight; decreased bone density; stress fractures). Additionally, symptoms reminiscent of drug withdrawal, including anxiety, depression, and irritability, often develop when these individuals are unable to exercise. The high comorbidity of drug abuse and eating disorders provides further evidence of a common neurobiological basis for these disorders.
The authors also found that female rats showed much higher rates of excessive running than male rats under similar conditions. Furthermore, the authors noted that they were unable to determine whether the withdrawl was specifically due to the increased exercise, or whether it was confounded by differences in food intake and body weight in the active rats.
I would also be curious to see if people with exercise addiction have higher or lower rates of opiate abuse. I could see it go both ways: higher levels of abuse because your system is "primed" for the flood of endorphins, or lower levels because you're getting those endorphins via exercise. It would be interesting to look at how other addictive behaviors overlap in EDs, and whether relatives of those with exercise addiction have higher rates of other addictive behaviors. The authors also suggested that moderate exercise might be a good prescription for those addicted to opiates as it is a non-maladaptive way to get that nice endorphin rush, though I would be hesitant given the known neurochemical overlap between these two addictions.
To me, exercise was a very safe and egosyntonic addiction. Whereas random strangers would have been alarmed if I started shooting heroin (which, given my OCD-related AIDS fears would seriously never ever happen), but they told me how "good" and "dedicated" I was when they learned about just a few of my exercise habits. If exercising X hours each day makes me "good," I figured, then the actual Y hours that I do must make me "fantastic!" I wasn't going to be arrested for having gym shoes on me, or a collection of rank sports bras in my closet. I am almost pathologically risk-averse, so exercise is probably the ultimate addiction for my personality, and I fell in to the trap head-first.