When the body is under stress or engaged in low intensity exercise for a prolonged period, it relies primarily on aerobic energy pathways to fuel activity. Aerobic metabolism involves the breakdown of carbohydrates, fats, and proteins in the presence of oxygen to generate ATP, the energy currency of cells. There are three main aerobic pathways the body uses:
Glycolysis
Glycolysis is the initial step of carbohydrate breakdown that occurs in the cytoplasm of cells. It splits a 6-carbon glucose molecule into two 3-carbon pyruvate molecules, producing a net gain of 2 ATP molecules. Glycolysis does not require oxygen and can occur anaerobically. However, in the presence of oxygen, pyruvate enters the mitochondria where it will continue through the aerobic pathways to produce more ATP.
Beta Oxidation
Beta oxidation is the process of breaking down fatty acids into acetyl CoA. It occurs in the mitochondrial matrix, requiring oxygen. Fatty acids are broken down in a cyclical series of reactions that shortens the fatty acyl chain by two carbons each cycle, releasing acetyl CoA, NADH, FADH2, and ATP. Acetyl CoA can then enter the citric acid cycle.
Citric Acid Cycle
The citric acid cycle, also known as the Krebs cycle or TCA cycle, fully oxidizes acetyl CoA derived from carbohydrates, fats, and proteins. This 8-step process occurs in the mitochondrial matrix and completes the metabolic breakdown of glucose, fatty acids, and amino acids, generating ATP, NADH, and FADH2. These high-energy electron carriers will then donate electrons to the electron transport chain to generate more ATP.
Electron Transport Chain
The electron transport chain is a series of protein complexes and electron carriers within the inner mitochondrial membrane that accepts electrons from NADH and FADH2. As electrons are shuttled down the chain from higher to lower energy states, protons are pumped across the membrane, establishing an electrochemical proton gradient. The proton motive force drives ATP synthase to generate ATP from ADP + phosphate. Oxygen serves as the final electron acceptor at the end of the chain.
During low intensity long duration exercise, aerobic metabolism is preferred because it can generate ATP at a rate that matches the demand. Anaerobic pathways are reserved for burst activity when oxygen demand exceeds supply. The aerobic system provides a steady supply of energy and avoids lactate buildup.
Fatty Acid Oxidation Dominates at Lower Intensities
At rest and during low to moderate intensity exercise, fat provides the largest contribution of ATP. Fatty acids are an abundant fuel source and fat oxidation can meet cellular energy needs. As exercise intensity increases, the percentage of energy from fat oxidation decreases and carbohydrate oxidation increases to supplement the ATP demand. However, fatty acid oxidation remains active even during high intensity activity.
Some key points about fat metabolism during low intensity exercise:
- Lipolysis releases fatty acids from triglyceride stores in adipose tissue.
- Circulating fatty acids are transported to tissues by albumin.
- Carnitine shuttle transports fatty acids into the mitochondrial matrix.
- Each beta oxidation cycle generates ATP, NADH, and FADH2 to support the electron transport chain.
- Fat oxidation can provide up to 60-65% of energy during low to moderate intensity exercise.
Glycolysis Converts Glucose to Pyruvate
At the onset of exercise, glycogen stored in skeletal muscle is broken down to provide glucose for glycolysis. During prolonged low intensity activity, blood glucose and muscle glycogen contribute to maintaining glycolytic flux.
Key points about carbohydrate metabolism during low intensity exercise:
- Glycogenolysis releases glucose from glycogen stores.
- Each glycolytic cycle generates 2 net ATP per glucose molecule.
- Pyruvate is oxidized in the TCA cycle to maximize ATP yield from carbohydrates.
- Glucose oxidation can provide 30-40% of energy during lower intensity exercise.
The Body Conserves Carbohydrates
During low intensity long duration exercise, fatty acid oxidation is prioritized to conserve limited carbohydrate stores in the body. Glycogen reserves in skeletal muscle and liver are relatively small compared to adipose triglyceride stores. Saving glycogen extends how long exercise can be sustained before energy stores are depleted.
Several factors contribute to carbohydrate sparing during aerobic activity:
- Increase in free fatty acid availability provides abundant lipid fuel.
- Decreased glycolytic flux due to less ATP demand at lower intensities.
- Pyruvate dehydrogenase activity is low, slowing entry of carbohydrates into the TCA cycle.
- Insulin levels are low while glucagon levels rise, favoring fat metabolism.
The Aerobic System Provides ATP Over Long Durations
Aerobic metabolism is the dominant energy system used for low intensity exercise lasting more than 2 minutes. It can sustain activity ranging from 30% to 80% of VO2max for up to several hours in trained athletes. There are several key advantages of aerobic metabolism during endurance exercise:
- Aerobic pathways generate up to 36 ATP per glucose molecule.
- Fatty acids yield more ATP per gram than carbohydrates.
- Oxidative pathways maintain a steady supply of ATP.
- Aerobic metabolism will not lead to lactate accumulation.
- Enough oxygen is available to support mitochondrial respiration.
The ratio of fat to carbohydrate oxidation will shift over time based on exercise duration, intensity, and training status. But the aerobic system provides a sustainable source of energy for low to moderate intensity activity lasting more than a few minutes.
The Anaerobic System is Downregulated
During low intensity exercise, anaerobic glycolysis is minimized in favor of aerobic metabolism. Anaerobic glycolysis converts glucose to lactate in the absence of oxygen, generating 2 ATP per glucose molecule. This pathway is essential to provide burst energy for high intensity efforts lasting up to 2 minutes. However, it is a much less efficient source of ATP than aerobic respiration.
Reasons anaerobic metabolism is suppressed during low intensity activity include:
- ATP is resynthesized at a rate sufficient to match demand.
- Oxygen delivery meets the needs of working muscles.
- Lactate clearance remains minimal due to low production.
- Cellular concentrations of ADP, Pi, and AMP do not rise enough to activate glycolysis.
While anaerobic metabolism is not shut off completely, its contribution during low intensity exercise is small. Energy is supplied mainly through aerobic pathways that can support activity for prolonged durations.
The Body Responds to Perceived Threats
In situations where the body perceives an imminent threat or stressor, it triggers a “fight-or-flight” response. This activates the sympathetic nervous system and stimulates release of catecholamines like adrenaline that rapidly prepare the body to respond to danger.
Some key effects include:
- Increased heart rate and blood pressure
- Dilated bronchioles to maximize airflow
- Activation of fat and glycogen breakdown
- Inhibition of digestive processes
- Increased blood flow to muscles
- Improved oxygen delivery and ventilation
This coordinated response is driven by survival instinct and provides an immediate boost in energy availability. Glycogenolysis and lipolysis are stimulated to deliver glucose and fatty acids to tissues. Although aerobic pathways are still functioning, the acute nature of fight-or-flight requires rapid ATP turnover that also relies on anaerobic metabolism.
The Stress Response Mobilizes Fuel Stores
The increase in circulating catecholamines during an acute stress response activates glycogenolysis and lipolysis to liberate glucose and fatty acids. Breakdown of glycogen and triglycerides is stimulated in tissues like muscle, liver, and adipose tissue to provide an immediate energy substrate.
Key effects on energy mobilization include:
- Increased glycogen phosphorylase activity releases glucose-1-phosphate from glycogen.
- Hormone sensitive lipase is activated, hydrolyzing triglycerides into free fatty acids.
- Glucose and fatty acids are released into circulation at higher rates.
- Blood glucose levels may rise 1-2 mmol/L from baseline.
- FFA levels can increase 3- to 5-fold.
This rapid preparation for fight-or-flight comes at the expense of energy efficiency and metabolic homeostasis. However, the immediate priority is having sufficient ATP to respond to a perceived threat.
Anaerobic Glycolysis is Enhanced
When the sympathetic nervous system is activated, fast glycolytic motor units are recruited to mobilize the muscles. Although aerobic metabolism remains active, the accelerated rate of ATP turnover exceeds mitochondrial oxidative capacity.
Key effects promoting anaerobic glycolysis:
- Increased reliance on fast-twitch muscle fibers that are highly glycolytic.
- Oxygen delivery cannot keep pace with the sudden energy demand.
- Glycolytic flux is accelerated to supplement ATP production.
- Lactate accumulation rises as pyruvate outpaces mitochondria.
- Phosphocreatine reserves help buffer ATP/ADP ratios.
This burst activation of glycolysis provides a rapid stopgap source of energy until aerobic metabolism can catch up. However, it comes at the expense of metabolic efficiency and reduced endurance capacity.
The Body Prioritizes Immediate Energy
The fight-or-flight response is an integrated effort to deliver immediate energy substrates so muscles can respond and protect against a threat. In this state, the priority is ATP production over metabolic efficiency or homeostasis. The body is prepared to burn through glycogen and fat stores quickly if needed.
Some of the metabolic consequences include:
- Incomplete breakdown of glucose and fatty acids due to rapid flux.
- Oxygen deficit as energy demand exceeds cardiovascular capacity.
- Increased lactate production and acidosis.
- Depleted glycogen reserves.
- Impaired capability for sustained power.
However, these effects are a tradeoff the body is willing to make for short-term survival. If the threat passes quickly, then normal metabolic homeostasis can be restored.
The Body Adapts to Regular Exercise
Regular physical training induces positive biochemical and physiological adaptations that improve the body’s capability to sustain aerobic activity. Some of these effects include:
- Enhanced fat oxidation: Increased mitochondria and enzyme content.
- Greater glycogen reserves: Expanded storage capacity in muscle.
- Reduced lactate production: Improved buffering and clearance.
- Increased capillary density: More blood vessels in trained muscle.
- Higher VO2max: Increased maximal oxygen uptake.
Together, these training adaptations allow muscles to utilize fat and carbohydrates more efficiently. This extends endurance capacity during aerobic activity like long duration low intensity exercise.
Mitochondrial Biogenesis
Exercise training triggers mitochondrial biogenesis – the growth and proliferation of new mitochondria in muscle cells. This expands the reticulum network and increases the oxidative capacity. More mitochondrial mass allows greater fatty acid oxidation at a given submaximal intensity.
Enhanced Enzyme Activity
Aerobic training also increases the activity of oxidative enzymes like beta-oxidation enzymes, citrate synthase, succinate dehydrogenase, and cytochrome c oxidase. This expands the capacity to digest fats and process pyruvate through the TCA cycle and electron transport chain.
Improved Blood Flow
Endurance training enhances vascularity in trained muscles. Capillary density increases along with arteriole diameters. This improves blood flow delivery and expands the maximum cardiac output during activity. More blood circulation enhances oxygen and nutrient transport while removing waste products.
Table Comparing Energy Pathway Contributions
This table summarizes the relative contribution of each energy system during low intensity, long duration aerobic activity compared to higher intensity anaerobic efforts.
| Energy Pathway | Low Intensity Activity | High Intensity Activity |
|---|---|---|
| Aerobic Glycolysis | ++++ | +++ |
| Beta Oxidation of Fatty Acids | +++++ | ++ |
| TCA Cycle | ++++ | ++ |
| Electron Transport Chain | +++++ | +++ |
| Anaerobic Glycolysis | + | +++++ |
Key:
+++++ = Primary energy source
++++ = Major contribution
+++ = Moderate contribution
++ = Minor role
+ = Minimal activity
Conclusion
During situations of physiological stress or low to moderate intensity exercise lasting more than 2 minutes, aerobic energy pathways dominate ATP production. Fatty acid beta oxidation provides the largest fuel source, supplemented by glucose oxidation through glycolysis and the TCA cycle. This provides a steady supply of energy and avoids lactate buildup.
Anaerobic glycolysis is minimized but not shut off completely. The body responds to sudden threats by releasing glucose and fatty acids while also enhancing anaerobic metabolism as a stopgap measure. But the priority for sustaining activity is aerobic metabolism of fats and carbohydrates. This system provides stable ATP turnover that can support low intensity activity for prolonged durations.