The runner tethered to the treadmill and the couch potato gripping a bag of chips may seem like polar opposites, but new research suggests that a single alteration in the brain’s reward system could cause both obsessions.

More than one third of regular gym-goers show signs of exercise depen­dence, continuing to exercise even when sick or injured or arranging their lives around working out. Nearly half of all people diagnosed with an eating disorder report excessive levels of exercise to control body shape and weight and to relieve stress and improve mood. Researchers have developed two opposing hypotheses to explain how someone could become addicted to exercise, eating or any other behavior. The first hypothesis states that these people’s brains grow more sensitive to reward; they find exercise more plea­surable, so they seek it more. Alter­natively, these individuals may grow less sensitive to reward; over time they begin to require more exercise to achieve the same level of pleasure.

A new study, led by Wendy Mathes of the University of North Carolina at Chapel Hill, found that two strains of mice, bred for either excessive exercise or obesity, had the same malfunction in the brain’s reward pathways. Genetic analysis of the two kinds of mice showed that they had lower-than-normal levels of activity in a gene that helps brain cells register the presence of dopamine, the neurotransmitter involved in reward. The finding suggests that excessively exercising mice and obese mice may be less sensitive to the rewards of physical activity and food, respectively. Researchers do not know yet how the same underlying problem could lead to such radically different outcomes, but Mathes suggests that other neurotransmitters may act on the dopamine-deprived brain cells to push the mice in one direction or the other.