Metabolism

Our body needs energy for every type of activity. Find out where it comes from and how it is provided in this article...

Metabolism is the sum of the essential biochemical processes for the building, transformation, and breakdown of the organism.

Functions

• Building/maintaining body substance
• Energy production
• Maintaining bodily functions

Our body needs energy for every form of activity. This energy is created in energy metabolism through the splitting of the body’s universal energy storage, ATP (adenosine triphosphate).

In muscle cells, ATP and another high-energy phosphate, creatine phosphate (CrP), are present in very small amounts and ensure a basic energy supply there.

However, during exertion, this energy source is quickly depleted (within a few seconds). The body then needs to obtain energy from other substances. High-energy substrates are burned in the muscle cell to provide the energy needed for muscular exertion (catabolism: breakdown and combustion of food components; releasing energy creates water, CO2, urea, and energy).

The body's main energy sources are phosphates (mainly creatine phosphate), carbohydrates, fats, and proteins. Proteins play only a minor role under physiological conditions during (enduring) muscle exertion. Instead, they are primarily crucial for the body's “building metabolism” (anabolism: creation of new cell and organ structures; the consumed, broken-down food is converted into the body's own substance with energy consumption).

All high-energy substrates are already stored in varying amounts in the body. Fats can be stored almost anywhere in the body and have a high energy value (dietary fat: 9.1 kcal/g, body fat: 7.5-8 kcal/g). Carbohydrates can only be stored as glycogen in the muscles or liver or circulate as dissolved glucose in the blood. They also have a lower caloric equivalent of 4.1 kcal/g. This results in highly varied storage capacities for the main energy carriers depending on body weight and composition.

Example: Energy stores of a man (1.80 m tall, 80 kg, 15% body fat)

For comparison: Energy consumption for a marathon is approximately 3000 to 4000 kcal

Methods of Energy Provision

These stores are depleted during exertion according to the required amount and speed of energy flow. Except for phosphate stores, these can be replenished through food.

The body has three methods to metabolize these energy carriers into ATP (ATP resynthesis):

• Anaerobic alactic energy provision from high-energy phosphates (ATP and CrP)
• Anaerobic lactic energy provision from carbohydrates (glucose or glycogen storage form)
• Aerobic energy provision from carbohydrates and fats (glucose/glycogen and free fatty acids or triglyceride storage form)

The anaerobic alactic pathway occurs without oxygen (anaerobic) in the cytoplasm of the muscle cell. This energy is available immediately and initiates a movement, similar to a car's starting battery. Due to the small stores of usable phosphates (ATP and CrP), this form of energy provision is time-limited. It is mainly used for short-term, intense exertions.

The anaerobic lactic ATP resynthesis also occurs in the cytoplasm without oxygen (anaerobic) and with lactate formation (lactic). In a series of sequential reactions, glucose or glycogen is first broken down to pyruvate in glycolysis. Pyruvate has two possible metabolic pathways, depending on energy needs and oxygen availability: if the cell is low in oxygen or requires quick energy, pyruvate is rapidly converted into lactate (salt of lactic acid) for energy.

If there is sufficient oxygen in the cell and energy demand is low, the formed pyruvate is not metabolized directly but rather transported into the mitochondrion (cell organelle) for aerobic energy production.

Under normal conditions, the body maintains a balance between lactate production and breakdown, as lactate is directly metabolized in other body regions (e.g., heart muscle). If immediate utilization isn’t possible, a lactate buffer occurs via blood buffer systems (e.g., bicarbonate). When lactate levels remain constant in the muscle cell and blood, this is known as the lactate steady-state (LaSS). However, during intense exertion, glycolysis and lactate production increase. Result: More lactate is produced than can be broken down or buffered, and unbuffered lactate leads to the release of hydrogen ions.

This dissolved lactate, as lactic acid, lowers the body’s pH level by releasing hydrogen. This condition is known as metabolic acidosis. As a result, anaerobic lactic energy provision stops, abruptly reducing energy flow. Either the exercise is halted or performance decreases (e.g., speed reduction). This can be observed in 400 m runners losing pace near the finish line, commonly known as “acidic legs.”

Anaerobic lactic energy provision supplies a high amount of energy per time unit, but is time-limited due to lactate buildup and metabolic acidosis protection. Thus, it plays a crucial role during high-intensity, limited-duration exertions.

The aerobic energy provision, also known as "internal respiration," occurs in the mitochondria of the muscle cell. It involves the complete combustion of glucose (full glycolysis) and free fatty acids (lipolysis), which can only occur aerobically, with the help of oxygen.

Fats can only be used for energy provision when there is enough oxygen in the cell, while carbohydrates can be metabolized both aerobically and anaerobically.

Through aerobic metabolism, the body can access large energy amounts due to substantial stores. So why don’t we always use this large storage? The complete combustion of carbohydrates and fats takes significantly more time than anaerobic energy provision methods. This is because split carbohydrates and fats must go through the citric acid cycle and electron transport chain for complete combustion.

Due to their molecular structure, fats take more time to break down and enter metabolic cycles, making aerobic fat combustion slower than aerobic carbohydrate combustion. Glycogen stores are limited, while even very lean individuals have practically unlimited fat reserves.

Low-energy-flow activities can be sustained over a long period thanks to large fat reserves. As exertion increases, energy demand can only be partially met by slower fat combustion, requiring more glucose for energy, leading to glycogen depletion. Result: Reduced energy flow, and high-intensity exertion becomes unsustainable.

Once glycogen stores are depleted, insufficient glucose is available for energy, lowering blood sugar. If blood sugar isn't quickly restored with food, typical low-blood-sugar symptoms like hunger, dizziness, and/or cold sweats may occur.

This “hitting the wall” phenomenon is especially familiar to endurance athletes.

The aerobic carbohydrate reserve is particularly significant for moderate-duration, moderate-intensity exertions due to its faster availability, while aerobic fatty acid oxidation plays a key role in long-duration, low-intensity exertions.

Conclusion

No matter the exertion form, energy is always generated through all three resynthesis methods based on the type, duration, and intensity of exertion and the metabolic condition (training level), but with varying contributions. Purely aerobic or anaerobic exertions don’t exist. For so-called aerobic endurance, effective use of the fat metabolism is crucial.

Those with good aerobic endurance can provide much of their energy through fat combustion even at higher intensities, preventing early fatigue from depleted glycogen stores or lactate acidosis protection. Thus, they can sustain given exertions (e.g., running speeds) for longer or handle shorter, higher-intensity exertions more effectively. Since glycogen depletion or even full depletion greatly prolongs recovery, aerobic endurance also leads to faster recovery.

Excursus: Fat Metabolism Training

Endurance training is the only way to target energy metabolism utilization. As always, it depends on individual goals: for example, one can train to a certain extent to tolerate lactate or acidosis. Through high-intensity lactate tolerance training, the body's buffering capacity is increased, prolonging the time to “acidification” and exertion fatigue during high intensities.

This training is particularly found in competitive sports. In health-oriented training, “fat metabolism training” is preferred. This low-intensity endurance training increases the flow rate and speed of aerobic fat metabolism (higher mitochondrial density, improved enzyme activity), improving aerobic fat burning efficiency and conserving limited carbohydrates.

“Fat burner” courses in some gyms are more like pseudo-metabolism training with intense, not exclusively aerobic exertion. These courses aim to reduce body fat through higher energy expenditure from intense, slightly anaerobic endurance training, targeted muscle growth, and delayed post-workout energy intake (utilizing the “afterburn” effect).

Real “HIIT” (High-Intensity Interval Training) only results in desired adaptations (on fat burning) when the stimuli are intense enough. The key factor is contraction energy flow (= training intensity). Real HIIT, for example, would be 5 x 30 seconds of maximum effort on a bike ergometer. In gyms, HIIT sessions are typically at the aerobic-anaerobic threshold.