Jake Gyllenhaal transformed his body for the movie "Prince of Persia," spending so many hours at the gym that he says he became addicted to working out, eventually injuring himself.
Reality TV star Heidi Montag recently said she goes to the gym 14 hours a day.
Gyllenhaal and Montag are high profile examples of extreme exercisers – people who work out several hours a day to keep themselves fit.
Brooke Mora became an extreme exerciser after a going through a bad break-up.
"You know, after days of crying, weeks of crying myself to sleep … I used the gym as a refuge, I used it as therapy, I used it as an escape," the 31-year-old Los Angeles native told "Good Morning America."
Soon, her habit developed into a compulsion. She now works out nearly 30 hours a week.
Asked whether the driving force behind her extreme exercising was aesthetics or health, Mora replied: "I think it's a combination of both … I love the way I look."
"GMA" followed her on her workout routine recently. That day -- a Sunday -- she started at 9 a.m. with a 4-mile hike.
She repeats the hike three or four times per week. And when she's not hiking, she's in the gym.
After her Sunday hike was done, she headed to The Sports Club/LA. She worked out for two hours there, had a light lunch, then headed back for even more cardio and weights.
During the work week, Mora's workouts are just as intense. She starts with one hour of cardio at the gym before work, then comes home and makes herself egg whites and oatmeal for breakfast.
She even squeezes in a one-hour yoga class in the middle of her full work day. After work, she heads back to the gym for two hours or more of weights and cardio.
The U.S. Department of Health and Human Services recommends 2 ½ hours of moderate aerobic activity a week for adults. Mora's weekly activity exceeds the recommendations by 12 times.
Experts warn that extreme exercise can lead to anorexia athletica – a condition in which someone exercises compulsively.
She is participating in fitness competitions, but says she has it under control and is not addicted.
"First of all I don't have an addictive personality, I've never been addicted to anything in my life. I think when it becomes an obsession it's unhealthy," she said. "I listen to my body if I need a rest, if I need a day off, if I need to sleep in a extra few hours in the morning, I'll listen to my body and do that."