First came the bruises all over her legs and the chronic “tired-to-the-bone” feeling. Her periods stopped coming. She kept losing weight.

For two years, Katherine Kross’s doctors puzzled over what could be wrong with this athletic, 39-year-old go-getter from Estes Park, Colo. Perhaps she had a tumor on her adrenal gland, they thought — until tests proved otherwise.

When she was referred to a psychotherapist, Kross was forced to recognize what had been wrong all along.

“I’m totally addicted to exercise,” says Kross, who, for several years, has worked out up to three hours a day, seven days a week.

“I make plans based on whether or not I am going to be able to exercise. I base everything that I eat on how much exercise I am going to get. My whole life revolves around it. I can’t take a day off and feel good about it.”

At a time when obesity is at the top of the minds of health practitioners, the suggestion that exercise could be addictive and hazardous is often met with skepticism, experts say. There is no mention of exercise addiction in the Diagnostic and Statistical Manual of Mental Disorders, a practitioners’ guide to mental illnesses. Treatment for it is seldom covered by insurance. And many hard-core athletes joke that, as addictions go, it’s a good one to have.

But a growing body of scientific evidence suggests the same neurological underpinnings that lead people to grow addicted to drugs and alcohol may lead others to crave and abuse exercise. Whether they do it to lose weight or to fend off depression and keep a sense of control, the results can be equally devastating, say some therapists.

“People don’t take it seriously enough and people don’t know how widespread it is,” says Taylor White, a Boulder, Colo., social worker who specializes in eating disorders and compulsive exercise. “In a place like Boulder, it is probably a huge problem for a lot of people.”

Proof of addiction

A recent study published in the journal Behavioral Neuroscience may explain why.

For the experiment, 48 mice were let loose on individual running wheels to sprint for as long as they wanted to. Half had been selectively bred to love running. The other half could take it or leave it.

For six days, the rodent ultrarunners sprinted furiously around the clock, logging roughly three times the distance as their counterparts. On the seventh day, a wooden slat was placed in front of the wheel, forcing all the mice to take a reprieve from training. When University of Wisconsin researchers measured their brain activity, they found the brains of the ultrarunning mice went wild on the seventh day, with the regions that control desire and reward showing much higher activity levels than those in the control group.

“If you give a rat morphine or alcohol or something addictive for several days and then take it away, you see the exact same response,” says study author Stephen Gammie, an assistant professor of zoology at University of Wisconsin in Madison. “We felt it was pretty good evidence that there is an addictive component to exercise.”

Justin Rhodes, a behavioral neuroscientist and lead researcher for the study, says previous animal research already had shown that mice with a tendency to love running seemed to also respond differently “at the receptor level” to drugs that promote feel-good brain chemicals like dopamine. While it is hard to say how that translates to humans, researchers say, it suggests that some people may be genetically hard-wired to get a stronger buzz from exercise.

“There may be a genetically predisposed group of people out there that may be more prone to addiction, and in extreme cases it can have negative consequences,” Rhodes says.

Help for legitimate problem

Carolyn Costin knows those consequences well.

As someone who jumped from anorexia to compulsive exercise, and eventually recovered, she founded the MonteNido Residential Treatment Center in Malibu, Calif., eight years ago, creating one of the first and only places in the country with a program devoted specifically to exercise addiction.

“People years ago used to say, ‘Is there really such a thing as an eating disorder, or is it just dieting gone bad?'” Costin says. Today, she notes, eating disorders are widely accepted as a legitimate mental health problem. And she believes the same will happen with compulsive exercise. “It can be just as bad and debilitating of an illness.”

Through the years, her center has treated runners, snowboarders, tennis players, lacrosse and soccer players and a few ex-soldiers who got accustomed to the rigorous workouts of the military and felt lost when back in the civilian world. Some patients haven’t socialized in years, instead packing their schedules with exercise. Others have been getting up in the middle of the night to work out, because they don’t want friends and family to see them. Many arrive at the center with serious health problems.

While weight and body image are the driving factors for some, she says, many are driven more by an internal compulsion to “keep moving.”

“We actually had to have someone watch one of our patients 24 hours a day to stop her from doing sit-ups on her bathroom floor or doing jumping jacks,” Costin says. “If you weren’t watching her, she would be sneaking push-ups, or sitting there tightening and relaxing her muscles during group.”