Doctors are issuing a warning about extreme cross training: overdoing it may have a dangerous effect on your kidneys, WPTV reports. According to Palm Beach Gardens nephrologist Dr. Ron Goldin, intense workouts like Crossfit - a high-energy exercise program that has gained popularity - may cause some people to experience a serious kidney condition called rhabdomyolysis. This condition causes muscles to break down and release their contents into the blood system, Goldin said, and is caused by overexertion in intense exercise. Headaches, fatigue, and swollen joints may result, as they did for Lake Park resident Amanda Edwards, who participated in a high-intensity boot camp for 10 months. Some emergency room doctors are reporting an increase in patients being admitted to the hospital with kidney problems related to the exercise fad.