I can remember a time when people used to talk about how muscle 'turned into fat' if you didn't use it. That, along with 'spot reduction' (the idea that you can lose fat from a region of the body by exercising that region) has generally been consigned to the 'myth' category. Most people now know that fat and muscle are separate entities. And millions of abdominal crunches will burn calories, but not specifically from your tummy.
However, there is still a huge amount of ignorance about how the human body stores, uses and loses fat.
Human body fat is stored inside fat cells in a compound called triaglycerol. An average young male stores about 100,000 calories of energy in this way, and assuming he consumes the same amount of energy as he expends, that's the way it will stay.
If the little men in your body notice that you're eating less calories than you are burning, they instruct hormones and enzymes (just stupid, technical names for even littler men) that start a process called hydrolysis (or lipolysis).
This process splits the molecule of triaglycerol into glycerol and free fatty acids (FFAs). An enzyme called hormone sensitive lipase is the catalyst for this reaction.
The stored fat gets released into the bloodstream as FFAs and they are shuttled to the muscles where the energy is needed. An enzyme called lipoprotein lipase (LPL) then helps the FFAs get inside the mitochondria of the muscle cell, where the FFAs are employed in a complex sequence of energy-releasing chemical reactions.
Fat cells never disappear. When FFA’s are released from a fat cell, the fat cell shrinks, but remains there waiting to be replenished. Women carry more fat cells than men. And recent research has demonstrated that fat cells can increase both in size and in number.
They are more likely to increase during late childhood and early puberty, pregnancy and when extreme amounts of weight are gained.
An infant has about 6 billion fat cells. This number increases during early childhood and puberty. A healthy adult with normal body composition has about 30 billion fat cells. In the case of severe obesity, this number can be as high as 300 billion.
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