Updated: Jun 6, 2020
To keep it simple, i am going to ignore the energy stored in the liver cells and focus only on muscles and fat. Also, I am simplifying this for easy understanding while maintaining the accuracy of the process.
Carbohydrate gets converted into energy (glucose/sugar). This glucose gets stored into Fat or Muscles. Glucose is stored by muscle cells as glycogen and stored in fat cells through fatty acids.
Note: Muscles by definition are very active so they need less sugar hence the majority of the sugar gets into the fat and a small part is stored in the muscles.
These glycogens are stored in the muscle fibers. When the body is in need, these glycogens convert back to the glucose and mix in the bloodstream to provide the required energy to the body.
Fuel Intake: What we eat (including all macronutrients), gets finally converted into water, carbon-di-oxide and Sugar [energy or glucose or ATP Adenosine triphosphate]
Fuel Burning: When the body needs energy, here is the sequence
1. It first gets the energy from food (for ex; carbohydrate such as sugar and starch, gets broken into energy for immediate consumption) [~~~~Remember - Carbs are the body's preferred energy source~~~~~]
2. If you need further energy (for example during exercise), the body will convert the glycogen stored in the muscle back to energy. This is why during exercise, the muscle glycogen depletes very fast.
3. Finally, the body will look at FAT :-) as a source of energy and this is what we would want to wish for !. In fact, fat is the most concentrated source of energy and is available in large amounts in most of our bodies. If we can train the body to take fat as a source of energy, we will achieve fat loss (and healthy weight loss), this is exactly what we do in intermediate fasting or keto diet.
Another important note: Muscle is the only organ that gives active and passive weight loss. Active loss is the use of energy during exercise for muscle functioning. Micro-trauma of muscle fibers is a by-product of extra muscle usage. Micro trauma happens due to overstretching and quick relaxation of the muscles. Or due to accumulation of lactic acid due to anaerobic respiration of the muscles. This repair of micro-trauma during rest is the passive loss. [Trivia - for effective regeneration of the muscle tissues, you need protein (remember - the amino acid in protein is required for tissue building)...So lack of protein will waste all the efforts you spend in the gym ]
With this knowledge of body fuel, let's learn more about how fuel cycle works.