Bioenergentics: study of the transformation of energy in living organisms
To understand how HIIT is effective, it may be helpful to understand bioenergetics. Bioenergetics is the flow of energy in a biological system; or the conversion of macronutrients (fat, protein, and carbohydrates) into biologically usable forms of energy.
What is metabolism?
Metabolism is the sum of all the catabolic (or exergonic) and anabolic (or endergonic) reactions in a biological system to maintain homeostasis.
What is CAtaBolism?
Catabolism is the breakdown of large molecules into smaller molecules and is associated with the release of energy.
What is ANABOLISM?
Anabolism is the synthesis of larger molecules from smaller molecules; can be accomplished using the energy released from catabolic reactions.
What is a Exergonic Reaction?
Exergonic reactions are energy-releasing chemical reactions that are generally catabolic.
What is a Endergonic REaction?
Endergonic reactions are require energy, and include anabolic processes. Energy released from exergonic reactions drive endergonic reactions.
WHat is ATP?
ATP (Adenosine Triphosphate) is a compound that consists of an adenosine molecule bonded to three phosphate groups, and is present in all living tissue. The breakage of one phosphate linkage (to form ADP [adenosine diphosphate]) releases energy for all physiological processes, such as muscular contraction. ADP can then break another phosphate bond (to form AMP [adenosine monophosphate]) to release more energy. ATP is known as the currency of energy.
"Structure of ATP." Image by Vtvu on Wikimedia Commons
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"Representation of ATP." Image by ALoopingIcon on Wikimedia Commons
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"Ball-and-stick model of the adenosine triphosphate molecule, also known as ATP." Image by Jynto on Wikimedia Commons
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Biological Energy Systems
Exercise begins with skeletal muscle contraction, an active process that requires ATP for energy. A small amount of ATP is stored in the muscle fiber when contraction begins. As this ATP is used for muscle contraction and transformed into ADP, another phosphate compound, phosphocreatine (PCr), transfers energy from its high-energy phosphate bond to ADP. The transfer replenishes the muscle’s supply of ATP.
There are three basic energy systems that exist in cells to replenish ATP:
Anaerobic pathways (phosphagen system and glycolysis) can proceed without oxygen but produce ATP in much smaller quantities, compared to aerobic pathways (oxidative phosphorylation) that require oxygen and yield the most ATP. All systems are constantly being utilized, however, depending on intensity and duration, one system will predominate and create ATP at faster rates than others.
There are three basic energy systems that exist in cells to replenish ATP:
- Phosphagen System
- Glycolysis
- Oxidative Phosphorylation
Anaerobic pathways (phosphagen system and glycolysis) can proceed without oxygen but produce ATP in much smaller quantities, compared to aerobic pathways (oxidative phosphorylation) that require oxygen and yield the most ATP. All systems are constantly being utilized, however, depending on intensity and duration, one system will predominate and create ATP at faster rates than others.
The Phosphagen System
The phosphagen system (also known as ATP-PC) provides ATP primarily for short-term, high-intensity activities (e.g., resistance training and sprinting) and is active at the start of all exercise regardless of intensity. This system is anaerobic, thus it does not require oxygen. This system combines muscle ATP and phosphocreatine, using an enzyme known as creatine kinase, to maintain the concentration of ATP. The rate of ATP production for this system occurs very rapidly, however 2 ATP per glucose molecule (not much) is produced. Anaerobic metabolism has the advantage of speed, producing ATP 2.5 times the rate of aerobic pathways.
This system is adequate to support only about 15 seconds of intense exercise. Therefore, muscle fibers must be able toe synthesize additional ATP from energy stored in macronutrients. Some of these macronutrients are contained within the muscle fiber itself. Others must be transported from the liver and adipose tissue and then transferred to muscles via the circulatory system. The primary substrates (macronutrients) for energy production are carbohydrates (i.e., glucose, glycogen) and fats.
This system is adequate to support only about 15 seconds of intense exercise. Therefore, muscle fibers must be able toe synthesize additional ATP from energy stored in macronutrients. Some of these macronutrients are contained within the muscle fiber itself. Others must be transported from the liver and adipose tissue and then transferred to muscles via the circulatory system. The primary substrates (macronutrients) for energy production are carbohydrates (i.e., glucose, glycogen) and fats.
Glycolysis
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Glycolysis is the breakdown of carbohydrates (blood glucose or muscle glycogen) to synthesize ATP, although it requires some ATP in order to function. At the end of this metabolic pathway, carbohydrates are ultimately converted to a molecule called pyruvate. The carbohydrates (glucose) that supply these pathways are stored in three locations: the blood, glycogen stored within the cells of muscles and liver, and glucose that can be synthesized in the liver via gluconeogenesis. Now, pyruvate can either be converted to lactate (via an enzyme known as lactate dehydrogenase), or shuttled into the mitochondria.
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The catabolic pathways that extract energy from biomolecules and transfer it to ATP are summarized
in this overview figure of chemical pathways.
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Lactate nor lactic acid (different molecules), contrary to popular belief, is not the cause of fatigue; rather it is the concentration of H+, thus the decrease in pH, that causes fatigue. If pyruvate takes the former path (absence of oxygen in cell), also called anaerobic glycolysis (fast glycolysis). If pyruvate takes the latter path (presence of oxygen in cell), it is converted to acetyl-CoA and enters the Krebs (Citric acid) cycle. This cycle makes a never-ending circle, adding carbons from acetyl CoA with each turn of the cycle and producing ATP, high-energy electrons, and carbon dioxide. In the absence of oxygen, the muscle fiber must switch its mode of energy production, metabolism shifts away from fatty acids towards glucose. Muscle and liver glycogen stores can sustain energy for relatively long period of time (<3 minutes). These anaerobic pathways cannot be sustained for an extended period, which explains our need to breathe oxygen.
Oxidative Phosphorylation
The final and most efficient step in ATP production takes place in presence of oxygen, but transfer of energy between high-energy molecules requires mitochondrial proteins known as the electron transport system (ETS), located in the inner mitochondrial membrane. ETS proteins include enzymes and iron-containing cytochromes. The synthesis of ATP using the ETS is called oxidative phosphorylation because the system requires oxygen to act as the final acceptor of electrons and H+, ultimately creating an average of 30–32 ATP per glucose. The chemiosmotic theory says that potential energy stored by concentrating H+ in the intermembrane space is used to make the high-energy bond of ATP. The production of H+ also contributes to a state of metabolic acidosis (however CO2 production during exercise is another significant source of acid). During oxidative phosphorylation, both glucose and fatty acids can be metabolized to provide ATP. About 30 minutes after aerobic exercise begins, the concentration of free fatty acids in the blood increases significantly, indicating the muscles demand for fat as a long-term fuel source. Fats are the primary fuel for this system, since they can provide more energy per molecule compared to glucose.
Exercise Duration and Intensity and Primary Energy System Utilized
Duration of Exercise |
Intensity of Exercise |
Primary Energy System Utilized |
0-6 seconds |
Extremely high |
Phosphagen |
6-30 seconds |
Very high |
Phosphagen & fast glycolysis |
30 seconds - 2 minutes |
High |
Fast glycolysis |
2-3 minutes |
Moderate |
Fast glycolysis & oxidative system |
>3 minutes |
Low |
Oxidative system |
How is HIIT More Effective At Burning Fat?
HIIT targets both anerobic and aerobic energy systems, while conventional endurance exercise only addresses the aerobic system. HIIT modifies the skeletal muscle mRNA and protein levels of genes involved in fuel utilization and mitochondrial function (Barrès et al., 2012). In other words, the body adapts to HIIT, increasing the ability and making it easier for cells to burn fat.
At low exercise intensities (~20% VO2 max), a high percentage of the energy that is being expended (~66%) is derived from fat (9 calories/gram), however, the total energy expended is low (3 kcal•min-1). In addition, the total fat oxidized (fat burned) is also low (2 kcal•min-1). During high intensity exercise (~60% VO2 max), a low percentage of energy (~33%) comes from fat, but the total energy expended is higher (9 kcal•min-1), and total fat oxidation is higher (3 kcal•min-1). During the shift from low intensity to high intensity, carbohydrates become the primary fuel source (due to the recruitment of fast twitch muscle fibers and increased levels of epinephrine). In other words, while low energy intensities may burn more fat, during HIIT the rate of total energy expended is greater. HIIT burns more carbohydrates (4 calories/gram), but burns them at a faster rate than low intensity exercise. The highest rate of fat oxidation occurs right before lactate threshold. In other words, too low of of intensity then not much energy be expended, too high of intensity and fat utilization will be inhibited. A crude method to estimate this optimal fat burning level level, is the sing-talk method. If you are running at such a low intensity level that you are able to sing, the intensity is too low. If you exercising at such a high intensity that you cannot talk, then the intensity is too high, circulating fatty acids available as fuel will be inhibited. However, total energy expenditure is the key to weight loss, in conjunction with a balanced healthy diet.
At low exercise intensities (~20% VO2 max), a high percentage of the energy that is being expended (~66%) is derived from fat (9 calories/gram), however, the total energy expended is low (3 kcal•min-1). In addition, the total fat oxidized (fat burned) is also low (2 kcal•min-1). During high intensity exercise (~60% VO2 max), a low percentage of energy (~33%) comes from fat, but the total energy expended is higher (9 kcal•min-1), and total fat oxidation is higher (3 kcal•min-1). During the shift from low intensity to high intensity, carbohydrates become the primary fuel source (due to the recruitment of fast twitch muscle fibers and increased levels of epinephrine). In other words, while low energy intensities may burn more fat, during HIIT the rate of total energy expended is greater. HIIT burns more carbohydrates (4 calories/gram), but burns them at a faster rate than low intensity exercise. The highest rate of fat oxidation occurs right before lactate threshold. In other words, too low of of intensity then not much energy be expended, too high of intensity and fat utilization will be inhibited. A crude method to estimate this optimal fat burning level level, is the sing-talk method. If you are running at such a low intensity level that you are able to sing, the intensity is too low. If you exercising at such a high intensity that you cannot talk, then the intensity is too high, circulating fatty acids available as fuel will be inhibited. However, total energy expenditure is the key to weight loss, in conjunction with a balanced healthy diet.
References
Baechle, Thomas R. & Roger Earle (Eds.). (2008) Essentials of Strength Training and Conditioning (3rd edition). Champaign, IL: Human Kinetics
Barrès, R., Yan, J., Egan, B., Treebak, J., Rasmussen, M., & Fritz, T. et al. (2012). Acute Exercise Remodels Promoter Methylation in Human Skeletal Muscle. Cell Metabolism, 15(3), 405-411. http://dx.doi.org/10.1016/j.cmet.2012.01.001
Barrès, R., Yan, J., Egan, B., Treebak, J., Rasmussen, M., & Fritz, T. et al. (2012). Acute Exercise Remodels Promoter Methylation in Human Skeletal Muscle. Cell Metabolism, 15(3), 405-411. http://dx.doi.org/10.1016/j.cmet.2012.01.001