Beta oxidation is in large part the same process as slow glycolysis, but instead of starting with glycogen or glucose, triglycerides (the common blood circulated form of fat) are broken down into their constituent three fatty acid chains and the glycerol that binds them (an alcohol), and from there these fatty acid chains are broken down into acetyl coenzyme a, which from there enters the kreb cycle and the electron transport chain. The difference is, while slow glycolysis yields about 36 ATP, each triglyceride yields around 463 ATP! This degree of efficiency is why we live aerobically from our carbohydrates and fats unless we are exerting ourselves enough to need more rapid energy supply from one of the anaerobic energy systems. It should be noted that protein can be oxidized as well, but as protein is a lot less efficient to metabolize for energy, it is usually kept for use as a structural component, only making up some 8% or so of materials to fulfill energy needs.
Also, something many athletes may have wondered about, the period between slow glycolysis dominance and beta oxidation is often referred to as the second wind, as our body desperately wants to minimize its energy expenditure, especially from its precious fat stores, and seeks to spare them and mentally tries to deter us from continuing such exertion without a situation where we are running for our lives or such similarly important activities. This is because while oxidation of fats is by far the more efficient energy system, working at high intensities while burning through our fat stores cuts into our caloric reserves at a severely faster rate than during periods of fasting such as while we sleep. To our evolutionary brain this means we are cutting into important starvation prevention reserves and so tries to get us to slow down unless we are, as said, running for our lives. Evolutionarily none of our ancient ancestors in hunting and gathering times would have ‘exercised’ because this would only mean a greater chance of starvation, and as our biological period of more available food is only about 200 years old (0.0001% of our time as humans) this hasn’t changed our feelings on exercising at times! As our body and mind passes this grueling period we get a new steady stream of energy from the highly efficient process of oxidation, especially fat oxidation, and the body accepts that it’s happening and we have our second wind.
You might ask, as I’m sure many have before, what actually stops each energy system from continuing and what would happen if they did anyway? Over time when performance intensity raises, as we have described before, aerobic energy systems can no longer make energy fast enough to deal with energy needs, even though by oxidizing fat in particular twice as much energy per unit mass is generated (remember nutrition, fats are about 9 calories per gram compared to carbohydrates approximate 4!). Over time as the intensity level is maintained the anaerobic energy pathways reach their capacity and aerobic energy systems take over once more, with a decrease in performance to match whatever energy output the aerobic system can muster.
The big reason aerobic systems are dependent on oxygen is to take the waste hydrogen away from the mitochondria and the cells in general, bound in water, among other waste products. The hydrogen is acidic and would alter the pH of the system, inhibiting enzymes and other components from carrying out reactions. This is a functionally similar reason to why we have storage glycogen in the liver instead of large amounts of glucose stored in each cell for it’s quick use, as too much glucose stored in a cell eventually crystallizes the cell like it were filled with cement. The cell simply cannot cope with a buildup of H+, or at least this is the pervading theory, but as mentioned previously ongoing research suggests a different mechanism is our cause. With the anaerobic energy systems the culprit is simply a faster use of substrate materials (such as creatine phosphate) than can be regenerated. As fast as the anaerobic systems are at generating new ATP it can only keep up with so much. Aerobic systems do also require materials like glucose or triglycerides to function, we just simply often have many hours or even weeks supplies of them on hand, compared to moments in an anaerobic environment.
For both aerobic and anaerobic systems, both ultimately can hit a wall in situations where the energy demand brought on by activity is greater and more urgent than their energy production rates can meet, as well as their total capacity. Passing out and more severe situations such as anaphylactic shock can occur when there is so little metabolic components to maintain the brain and vital organs, though this is rarely a present occurrence (but it is not unheard of at all) in sports. This is regardless, however, why a quick source of calories such as simple sugar with electrolytes should always be available for athletes and active people at all times, regardless of degree of health and fitness.
Perfection in motion. BS CPT - NPTI NASM
"We must not forget that even in the most perverted and cruel human being, as long as he is human, a small grain of love and compassion exists that will make him, one day, a Buddha." -Dalai Lama