Exercise can be very healthy. It can also hurt you badly if you overdo it.
"Too much exercise or over training can result not just in exhaustion & injury, but can even slow down your body metabolism, lose muscle, promote fat storage & even prone to infection! Read on to know all about overtraining- what it is, what are the symptoms, how you can prevent it, and how to recover."
Overtraining can cause severe stress resulting in too much cortisol. This can lead to decreased testosterone, increased protein breakdown and limited muscle growth. It can even result in losing muscle and increased fat storage.
Overtraining can impair the thyroid, decreasing metabolism and yet more fat storage. Overtraining can cause pain in back, shoulders, joints and muscles. Fatigue and insomnia, appetite disruption and irregular menstruation can also result.
There are ways to protect yourself from overtraining. Keep workouts to a maximum of 5 days a week. Take a break every 8 weeks. Don't overdo it, and your workouts will work for you.
http://fitho.in/2010/07/16/over-training