The human body has two systems for using and storing the food we eat. One system deals with the fats we eat, the other deals with carbohydrates. The fat system stores the fat we eat and has a seemingly No Nonsense Fat Melting System unlimited capacity to do so, but must get a series of green lights from other systems before it will use this fat as fuel. On the other hand, the system that deals with Carbohydrates will use them as fuel first then attempt to store them. I say attempt because the body's ability to store carbohydrates, in the form of glycogen, is limited to only a few pounds.
When we eat carbohydrates they are turned into glucose and go directly into the blood stream. Depending on the type of carbohydrates, and the other foods eaten with the carbohydrates, determines how fast glucose is pumped in to the blood stream. As glucose enters the blood stream, insulin is released from the pancreas that tells the cells throughout the body, and the liver, to grab this glucose and use it for fuel and the production of glycogen that will be stored for later use. Glycogen is the form in which carbohydrates are stored in muscles and other organs of the body, such as the liver, for later use as fuel when blood glucose levels are low. The release of insulin also signals the fat system to stop burning fat. A fast large spike in insulin, that which you get from refined processed foods, causes a greater response from systems affected by insulin, such as the halt of fat burning and the forced feeding of cells throughout the body.