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Exercise Essentials: A Better Understanding of Our Aerobic Energy Pathway

A fundamental attribute of fitness has always been our relentless pursuit of new ideas when it comes to programming. Whether evolutionary or incremental in nature; trend or fad, we appear to thrive on challenging the status quo in our quest for better, bigger, stronger or faster. Outside of wearable technology, perhaps the biggest fitness trend over the past few years has been the rising popularity of shorter, more time-efficient, interval-type workouts performed at higher intensities or work-rates (e.g., Tabata, HIIT). Although this training format is by no means new, dating back to the early-to-mid 1990s in sports conditioning, it is its recent introduction to the general public that has escalated the significance in this exercise modality.

What might have started with our obsession with weight loss; tracking activity and calories; and our lack of available time to exercise; has now shifted our entire philosophical approach to exercise programming and casts greater attention on metabolism, the energy pathways and fuel utilization. Furthermore, this has also spurred interest in a variety of non-traditional practices such as fasted cardio and ketones. Unfortunately, the complexity of these biological systems has also generated much misinformation and misunderstanding on these topics as well. And, as fitness practitioners, we are held to a standard of providing evidence-based education and programming to the public, and therefore need to understand fuel utilization and the respective roles within the energy pathways. It is the intent of this  article to present key essentials of our energy pathways and discuss the popular practice of fasted cardio.

The Energy Pathways:

Energy to fuel biological work is produced aerobically in the presence of oxygen, or anaerobically in the absence of oxygen (1). As illustrated in Figure 1-1, the aerobic pathway produces large amounts of energy, albeit it more slowly, and can utilize all three macronutrients as a fuel source. By contrast, the anaerobic pathways, comprising both the Phosphagen (immediate) energy system and the fast glycolytic pathway produce energy quickly, but in limited quantities, and can only utilize glucose as a fuel source (2). Given the scope of this article however, we will only review the aerobic pathway as it reflects the largest source of our calories in a day and involves all three macronutrients consumed in our diet.

Figure 1-1: Overview of the bioenergetics pathways

Figure 1-1: Overview of the bioenergetics pathways

Aerobic Respiration: Mitochondrial Respiration

Aerobic respiration occurs within the mitochondria, the organelles located inside cells that produce energy. All three fuels however, undergo some primary preparation in order to prepare them for entry into the mitochondria. The pathways for all three macronutrients are illustrated in Figure 1-2, but before discussing each, let’s first define the fuels:

  • Triglycerides (TG) – a simple fat that represents the primary storage form of fat within the body. Triglycerides are comprised of a glycerol molecule which forms the backbone of the molecule that is joined to three free fatty acids (FFA). It is these FFA that many of us are familiar with as they can be categorized by their length as short-chain, medium-chain or long-chain; and are classified by their structure as saturated, monounsaturated or polyunsaturated. Before entering the respiratory pathway, TGs must be separated into the building blocks glycerol and FFA, after which FFAs are further prepared for entry into the Krebs cycle via beta-oxidation which breaks down the longer carbon chains into 2-carbon fragments (3).
  • Carbohydrates exist within the body as either a stored form called glycogen or as glucose when absorbed and used immediately as a fuel. All carbohydrates undergo glycolysis (anaerobic carbohydrate metabolism), a process that takes places outside the mitochondria. The end product of glycolysis is the formation of pyruvate which either crosses into the mitochondria to continue into the aerobic pathway, or is converted to lactate (anaerobic) – both can occur simultaneously. It is the availability of oxygen being delivered to the mitochondria that determines how much pyruvate will pass to the mitochondria (i.e., more oxygen equals greater entry of pyruvate into the mitochondria). Any excess cannot remain as pyruvate in the cell as this slows glycolysis – thus it is converted to lactate by joining with hydrogen ions, which are also produced during glycolysis. Normally, hydrogen ions are also shuttled to the mitochondria to complete respiration, but an excess is problematic as it lowers the pH of the tissue, which impairs muscle function and the energy pathways. Essentially, the combination of pyruvate with hydrogen ions to form lactate enables the muscle to continue working longer than normal. This is illustrated by the number 1 in Figure 1-2.
  • Proteins differ from fats and carbohydrates in that they contain the element nitrogen, which does not serve a function in respiration. Subsequently, it must be removed (deamination), producing ammonia which is potentially harmful to the body (4). Ammonia is quickly converted to urea in the liver and then excreted primarily through urine via the kidneys. This is illustrated by the number 6 in Figure 1-2.

Figure 1-2: The metabolic mill – fuel utilization within the energy pathways

Figure 1-2: The metabolic mill - fuel utilization with the energy pathways

On first impressions, the metabolic mill looks complicated and confusing, but we will use an analogy that we are more familiar with to help understand its intricacies. Let’s start by looking at the Krebs cycle – it is the point where all 3 macronutrients converge in the mitochondria. Think of the Krebs cycle as a bar or night club, a location where everyone wants to meet.

  • Think of TGs representing single ladies coming to the Krebs cycle bar from the TG part of town. They arrive dressed to dance and socialize as acetyl-CoA (the compound that enters the Krebs cycle) – consider their preparation for the evening as beta-oxidation. This is illustrated by the number 3 in Figure 1-2. Unfortunately, the Krebs cycle bar has a couples-only policy, therefore all the single ladies need a partner to enter. Ordinarily, they count on meeting single men who are also converging on the Krebs cycle bar from a different part of town (i.e., carbohydrates). What this essentially means is that for fats to be completely metabolized, they need to have carbohydrates present.
  • Now consider the fate of carbohydrates. As mentioned previously, glycolysis produces pyruvate which is unique in that it can produce either single ladies (acetyl-CoA) or single men (oxaloacetate). This is illustrated by the number 2 in Figure 1-2. Referring back to our bar analogy, considering how the TG part of town provides ample single ladies, the carbohydrate part of town generally provides the single men (i.e., oxaloacetate). Collectively, the single ladies from the TG part of town join with the single men from the carbohydrate part of town and enter the Kerbs cycle bar.

Throughout our existence, humans have had to endure periods of famine, where food was scarce or absent. In the event that our available carbohydrates stores become depleted or scarce (think of carbohydrate restricted diets today), we limit the availability of single men arriving from the carbohydrate part of town. This forces a state of metabolic survival where the body is forced to adapt to survive and not perish.

  • Referring back to our bar analogy, think of the bar not receiving sufficient business on account of a lack of single men from the carbohydrate part of town. In an attempt to drum up business, the manager opts to call his friend who manages a sports bar in the protein part of town (consider business and profitability the production of energy). He makes two offers to the patrons at the sports bar in protein land – join single ladies to socialize in the club or be secretly snuck in the back door to enjoy the drink specials (i.e., make energy).
  • Those that wish to socialize with the single ladies (glucogenic amino acids) are shuttled through the carbohydrate part of town and arrive as single men. The term glucogenic means to create glucose. In other words, we are converting proteins to carbohydrates in order to continue metabolizing fats and, unfortunately, 99% of usable protein in the human body is living tissue called muscle. This is illustrated by the number 5 in Figure 1-2 of part one).
  • However, this conversion may not be sufficient to meet the body’s energy demands, so additional protein may also need to be used to produce energy. This represents those men at the sports bar who are interested in visiting the Krebs cycle bar, but only for the drink specials (i.e., to make energy). They are snuck in the back door of the bar and are called ketogenic amino acids as they primarily produce energy. However, they can also produce ketones which we will discuss next. Remember, these proteins also originate from muscle tissue.

So what becomes of the extra single ladies accumulating outside the bar who cannot enter on account of insufficient single men? The manager, in following city laws, informs these ladies that they cannot loiter outside the bar and must move away. To allow mitochondrial respiration to continue, the extra single ladies (acetyl-CoA) are converted to ketones which can then be removed from the mitochondria and placed to circulation via the blood. This is illustrated by the number 4 in Figure 1-2 in part one. Ketones represent compounds manufactured as a result of incompletely metabolized fats (and possibly ketogenic proteins). The two primary ketones manufactured are acetoacetone and β-hydroxybutyrate which can be used as a fuel by almost every cell in the body except for the liver and red blood cells that need glucose (1, 2). Any ketones not utilized by the body are quickly converted to acetone for removal from the body because ketones lower blood pH (acidosis) which can lead to a life-threatening condition called ketoacidosis when they accumulate in large quantities. Acetone is the same compound found in nail polish remover and sometimes when ketone levels in the blood become elevated, a sweet ‘acetone’ scent might be noticeable in urine, breath or sweat, indicating a metabolic survival state.

Takeaway – all 3 macronutrients are indispensable in the production of energy, but the deletion or restriction of carbohydrates from the diet, especially for sustained periods can alter our normal metabolic pathways which can have significant consequences (i.e., promoting the concept of skinny fat by attacking muscle protein which in turn, will also slow metabolic rates in the body).


  1. Kenny WL, Wilmore H, and Costill DL. (2015). Physiology of sport and exercise (6th edition). Champaign, IL. Human Kinetics.
  2. Pocari J, Bryant CX, and Comana F. (2015). Exercise Physiology, F.A. Davis Company, Philadelphia, PA.
  3. Juekendrup AE. (2002). Regulation of fat metabolism in skeletal muscle. Annals of New York Academy of Sciences, 967: 217 – 235.
  4. Brooks GA. (1987). Amino acid and protein metabolism during exercise and recovery. Medicine and Science in Sports and Exercise, 19: S150-S156.


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The Author

Fabio Comana

Fabio Comana

Fabio Comana, M.A., M.S., is a faculty instructor at San Diego State University, and University of California, San Diego and the National Academy of Sports Medicine (NASM), and president of Genesis Wellness Group. Previously as an American Council on Exercise (ACE) exercise physiologist, he was the original creator of ACE’s IFT™ model and ACE’s live Personal Trainer educational workshops. Prior experiences include collegiate head coaching, university strength and conditioning coaching; and opening/managing clubs for Club One. An international presenter at multiple health and fitness events, he is also a spokesperson featured in multiple media outlets and an accomplished chapter and book author.