There are many things related to metabolism to learn about, but don’t get overwhlemed; just remember, the primary purpose of all metabolic systems is to produce ATP.
Every living organism has a set of enzymes that converts macromolecules from one form to another and, in the process, makes ATP. As odd as it may sound, “biological death” begins when an organism fails to make ATP at a rate required by the enzymes which make ATP! Without sufficient ATP, a cell cannot maintain, repair, or reproduce itself – and a cell that cannot respond to the dynamic world around it is as good as dead.
Fortunately, the human body can generate ATP from the three major macromolecule categories:
Carbohydrates are only made up of carbon, hydrogen, and oxygen, and the cost of carb metabolism is the lowest because the body can fully metabolize carbohydrates to H2O and CO2. Fatty acids, too, are only made up of carbon, hydrogen, and oxygen, but their metabolism results in the production of ketoacids. Unlike CO2, ketoacids cannot be eliminated via exhalation; their accumulation increases the acidity of the blood and can cause body-wide problems. Proteins are most costly because their degradation means a loss of structural integrity as well as the generation of ketoacids and ammonium ions.
Because of these associated costs, the body has evolved to greatly favor glucose for fuel. When glucose is low, it relies on fatty acids and then only shifts to proteins in dire conditions.