Most of our fat is stored as trigylcerides, which consist of 3 fatty acids (of varying length) bound to a glycerol backbone. We mobilize these, the fatty acids are released, and then the fatty acids are oxidized. They are highly reduced to begin with and go through many rounds of oxidation, in the end generating ATP because the acetyl-CoA which is produced from each round enters the TCA cycle from which ATP is generated. This process of “burning” fatty acids (which is oxidation of them to eventually form CO2 and water with the energy of the their bonds converted into the energy in ATP bonds, a form the body can use) is called “beta oxidation”. So if you want to learn more on the biochemistry of fat utilization, looking that up is a good place to start.
If you want to know more about where the energy comes from, look up “redox reactions”. This Khan Academy page is a good summary:
There, it says, “[E]lectrons are at a higher energy level when they are associated with less electronegative atoms (such as C or H) and at a lower energy level when they are associated with a more electronegative atom (such as O). So, in a reaction like the breakdown of glucose above, energy is released because the electrons are moving to a lower-energy, more “comfortable” state as they travel from glucose to oxygen.
The energy that’s released as electrons move to a lower-energy state can be captured and used to do work. In cellular respiration, electrons from glucose move gradually through the electron transport chain towards oxygen, passing to lower and lower energy states and releasing energy at each step. The goal of cellular respiration is to capture this energy in the form of ATP.”
That process is oxidative, meaning it requires oxygen, which eventually is going to accept those lower energy electrons. In other words, it is aerobic metabolism. It is also slower than anaerobic metabolism. When oxygen is limiting, we still need to generate ATP and do so using anaerobic processes, which are faster, in which carbohydrate is used in glycolysis (“sugar breakdown”) to generate ATP without using oxygen. The end product, pyruvate, can enter the TCA/aerobic cycle to form acetyl-CoA and ATP from that but only when oxygen is available. If not, the pyruvate is converted to lactate so that the anaerobic process can continue. (High levels of the products interfere with the enzymes trying to drive the reactions forward, so pyruvate is converted to lactate and the lactate is “handled” by shuttling to the blood and other cells. Eventually, high levels of products affect the system, and the athlete has to slow down because ATP production becomes limiting.)
Thus, the mantra here (my current understanding of it, anyway) is to train in order to optimize your ability to mobilize fats and oxidize them during exercise. By training in that state where your body can use these processes, you stimulate adaptations to improve their use and efficiency. If you train in states where your body needs to mobilize other resources (such as high intensities requiring much glycolysis to supply the ATP needs), the body will adapt by increasing that anaerobic side of the house. The aerobic side gets relatively neglected because the stimulus saying, “We need more of this” is reduced compared to the stimulus for anaerobic adaptations. Generating all these enzyme systems is somewhat “expensive” to the body in maintenance terms, so the body has evolved to respond to stimuli with upregulation of those enzyme systems which are needed and downregulation of those which are not. Thus, the neglect is not just passive, in which you would keep whatever level of enzymes you previously had, but you will lose some of the existing capacity if you do not provide a stimulus to maintain it. What a bummer in our current environment where we are not energy-limited, but we are never that far from famine, so probably a good thing overall.
A key insight to have, in my view, is the one about what “training harder” means. One wants to train each state “hard”. But when someone interprets “training harder” as that overall feeling of training “hard”, that person exits the aerobic state and starts training the anaerobic state hard. The aerobic state is actually getting a much lower training stimulus than is needed to make (or even maintain) the aerobic adaptations. So train hard, but know which system you are training. Preaching to the choir, but there you go.
(I don’t know how statins interact in these processes, sorry to say!)