When the body is expending more energy than it is consuming (e.g. when exercising), the body's cells rely on internally stored energy sources, such as complex carbohydrates and fats, for energy. The first source to which the body turns is glycogen (by glycogenolysis). Glycogen is a complex carbohydrate, 65% of which is stored in skeletal muscles and the remainder in the liver (totaling about 2,000 kcal in the whole body). It is created from the excess of ingested macronutrients, mainly carbohydrates. When glycogen is nearly depleted, the body begins lipolysis, the mobilization and catabolism of fat stores for energy. In this process, fats, obtained from adipose tissue, or fat cells, are broken down into glycerol and fatty acids, which can be used to generate energy. The primary by-products of metabolism are carbon dioxide and water; carbon dioxide is expelled through the respiratory system.
With intermittent fasting, you narrow the size of your eating window, or you occasionally do fasts of 24 hours. For instance, you can start eating at noon and finish up by 8pm, essentially skipping breakfast. I wrote all about it in our “Beginner’s Guide to Intermittent Fasting,” where I outlined the benefits of teaching your body to consume food more efficiently, and also reduces the total number of calories you are probably eating.