Renaldo Wilson

Metabolic Efficiency


Our bodies use three types of energy systems: Phosphagen, Glycolysis (fast & slow), and Oxidative. The phosphagen system is primarily used for quick, high-intensity workouts and activities. This system utilizes phosphagens, creatine and primarily type II (fast-twitch) muscle fibers. The glycolysis system uses glucose or glycogen (stored glucose molecules) and can either be used without adequate oxygen (anaerobic activities) leading to lactic acid build up, or with adequate oxygen (aerobic activities) via mitochondria leading eventually to continuous energy in a process called the Krebs Cycle. Finally, the oxidative system uses primarily remaining carbohydrates, fats and type I (slow-twitch) muscle fibers, for long term aerobic activities.


Energy produced from glycolysis yields up to 10 ATP molecules, whereas 40 ATP come from the Krebs cycle. Amazingly however, 463 total ATP are yielded from the oxidation of one (18-Carbon) triglyceride molecule (18 from glycerol and 441 per fatty acid/triglyceride molecule)[1].



Because fatty acids are so useful, they are stored (carbohydrates, proteins and fats can all be stored in adipose tissue) and reserved for later use, whereas glucose is the body’s preferred substance used for energy production. In order to release and use these fatty acids, exercise can help but the specific type of exercise being done will determine when and how much fat is utilized for energy production.



One option that can be utilized to burn fatty acids is to engage in low-intensity aerobic activities since it will allow the body to use either slow glycolysis (where fatty acids are converted into acetyl coenzyme A via Beta Oxidation, the Krebs Cycle or the Electron Transport Chain. Training at this level will allow the body to burn both glycogen and fatty acids, but the higher the intensity, the more the body uses glycogen as its primary energy source. Some athletes engage in aerobic exercise during the early morning when glycogen stores are low, forcing the body to utilize mainly fatty acids for energy, but this can be risky for those unconditioned for it.



Instead, another option to use fatty acids is to work through glycogen reserves before initiating beta-oxidation or gluconeogenesis (converting fats or proteins into glucose for energy) options. The higher the intensity, the more the body uses glycogen, so if an athlete sprints or engages in resistance training until fatigue sets in, they can then switch to an aerobic activity and burn fatty acids instead. This will release stored substances from adipose tissue, using it instead as the primary energy source.



When long distance running or other long-term low-intensity workouts are used alone, many times glycogen as well as amino acids is seeped from muscles, jeopardizing lean muscle instead of body fat[2]. This stress to the body also releases cortisol, a catabolic hormone, which is usually counterbalanced with testosterone or insulin, anabolic hormones, leading to more lean mass loss, and more fat gain[3].



Your Resting Metabolic Rate (RMR) determines how many calories your body needs for homeostasis (normal inner functioning) and beyond that number; you can add calories depending on the intensity of the exercise. If weight/fat loss is the objective of the workout however, then it is important to remember that the body has already stored foods in adipose tissue for later use. Because of this, extra calories are not needed and more importantly “Carb Loading” is not necessary because these calories will be used instead of fat for energy.



In the end, knowing how the body works, you can now pick what macronutrient (carbohydrates, proteins or fats) that your body uses for energy. If you are simply looking to maintain homeostasis with no body mass change, then carb loading can help. It is important however, to make sure not to eat too much, since this many times leads to G.I. and stomach issues. If your goal is to burn fat while running and exercising, remember that excess carbs are not the answer.



[1] Brooks, G.A., and T.D. Fahey, 1984, Exercise Physiology: Human Bioenergetics and its Applications, New York: Wiley

[2] Saltin, B., and Karlsson, Muscle glycogen utilization during work of different intensities, In: Muscle Metabolism During Exercise, B. Pernow and B. Saltin, eds, New York: Plenum Press, 1971, pp. 289-300

[3] Florini, J.R., Hormonal control of muscle growth, Muscle Nerve, 10:577-598, 1987


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