Our bodies are capable of surviving without food for long periods of time, at least 3-4 weeks with hydration!
The reason we can do that is that we can store our dietary fuels, and then break them down when needed to make energy in the form of adenosine triphosphate, or ATP.
Fat is one of the most important ways we store energy and the term “burning fat”, actually refers to fatty acid oxidation.
In fact, if two individuals were stranded in the Andes mountains with no food, the person with more fat content would survive longer - yet another reason to avoid working out.
What makes fat such a great source of energy are fatty acids, which are the simplest form of fats, composed of long chains of carbon and hydrogens.
The transfer of electrons in the form of hydrogen from these fatty acids to certain molecules, can then be used to generate ATP.
Fatty acid oxidation primarily takes place in the mitochondria of heart, skeletal muscle, and liver cells.
Before we can oxidize fat, it needs to be moved from storage sites to the cells that can use it. Fat is stored in adipocytes or fat cells as triglycerides, which are 3 fatty acids attached to a glycerol molecule.
Triglycerides can be broken down by the enzyme hormone sensitive lipase, into free fatty acids and glycerol. So if you’re starving in the Andes, first your blood glucose level falls.
In response, the pancreas secretes a hormone called glucagon which increases the activity of hormone sensitive lipase, and increases the breakdown of triglycerides.
Now, the free fatty acids can leave the fat cell, and enter the bloodstream, where they bind to a protein called albumin.
Albumin carries the fatty acids to target cells, like liver cells, that are capable of fatty acid oxidation. First, the free fatty acid dissociates from albumin and diffuses into the cell.
Once inside the cell, a cytosolic enzyme called fatty acyl-CoA synthetase adds a coenzyme A molecule to the end of the fatty acid, turning it into a metabolically active fatty acyl-CoA.