How Calories Work
Calories are fuel for your body. The body has a backup storage system that it uses to save energy for lean times. The excess energy is stored as fat.
One pound of fat equals 3500 calories. Because you have to eat more calories than you burn, you would have to eat 3500 extra calories over a seven-day period before gaining one lb. of fat. That equals 500 extra calories a day.
The Chemical Reaction
Calories are delivered to our bodies via carbohydrates in food. When the body has enough energy, it takes the excess carbs and sends them to the liver, which converts them to fatty acid and stores them in adipose cells. The chemical process works like this: In the cytosol, intermediates attach to the Acyl Carrier Protein, or ACP. Two carbon units are donated by malonyl CoA. This is powered by a CO2 release. A synthesis is created by enzymes in a complex containing ACP, resulting in a fatty acid.