Many people do not realize that the body uses calories simply through digesting and processing food. This is described as the thermic effect of food.
The simple act of eating less causes lower energy output.
Once the body senses a loss of body fat, it will begin to lower thyroid levels and diminish nervous system output in an effort to stop the weight loss. Once further calorie cuts are made and cardio is increased, fat loss will resume again, and the body further lowers thyroid levels and nervous system output. It also lowers testosterone levels and raises cortisol levels, both of which eventually lead to muscle loss. Since muscle is a metabolically active tissue—it consumes calories simply to exist—
the metabolism will drop even further.
Let me run you though a familiar scenario. A hypothetical male bodybuilder is prepping for an upcoming show. He has put on a bit of fat in the offseason, so being ready on time is going to be difficult. But he is a can-do person, a guy who does whatever it takes.
Time is of the essence, so our competitor begins with aggressive cuts to his diet. He was maintaining his body weight with 3,000 calories per day in the offseason, so he begins by cutting to 1,600 calories and doing an hour of cardio each day. Boom: He loses several pounds in the first few weeks.
A few more weeks pass, and his fat loss stalls. Our competitor, who is already eating very little, decides to cut out nearly all carbs while lowering his fat intake to 20-30 g per day.
This gets things moving again—but not nearly as quickly as before. After another few weeks, fat loss stops again. Since he can't eat much less, our competitor has no choice. He adds another hour of cardio per day.
Fat loss barely crawls along for the next few weeks before it inevitably stops altogether. Our competitor is exhausted, has no energy to train, is eating zero carbs and little fat, and doing 2-3 hours of cardio per day. But the scale does not budge. He still needs to lose more fat, but he is out of luck. His metabolism stalled. His body won't surrender any more fat.
This is exactly the type of situation that leads to a huge metabolic slowdown and
makes it nearly impossible to lose any fat.~
http://www.bodybuilding.com/fun/metabolism-massacre-7-ways-to-avoid-undermining-fat-loss.html