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Muscle protein breakdown (MPB) is an important metabolic component of muscle remodelling, adaptation to training, and increasing muscle mass. It is a metabolic process that describes the degradation of bound muscle proteins into their amino acid precursors. This occurs concurrently with muscle protein synthesis (MPS), the metabolic process that describes the incorporation of amino acids into bound skeletal muscle proteins. The difference in rates of MPS and MPB determines whether muscle protein is gained or lost. MPB is influenced by factors such as exercise, fasting, and nutrition. For example, resistance exercise increases MPB, but not as much as it increases MPS, resulting in a net gain in muscle protein. Similarly, fasting has been shown to reduce MPS while increasing MPB, leading to a net loss in muscle protein.
| Characteristics | Values |
|---|---|
| Definition | Muscle protein breakdown (MPB) is an important metabolic component of muscle remodelling, adaptation to training, and increasing muscle mass. |
| Process | Degradation of muscle proteins occurs via the integration of three main systems: autophagy, calpain, and ubiquitin-proteasome. |
| Factors Influencing Breakdown | Exercise, fasting, and nutrition can all influence the rate of muscle protein breakdown. |
| Measurement | MPB can be measured using stable isotopic methods, such as labelled amino-acid tracers, and by assessing the appearance of unique metabolites in blood and urine. |
| Net Muscle Protein Balance | A positive net muscle protein balance is achieved when the rate of muscle protein synthesis (MPS) exceeds MPB. Insulin secretion is critical for inhibiting MPB. |
| Muscle Loss | Muscle loss occurs when MPB occurs at a faster rate than MPS, leading to a decline in muscle size and strength. |
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What You'll Learn
Muscle protein breakdown is greater in a fasted state
Muscle protein breakdown (MPB) is an important metabolic component of muscle remodelling, adaptation to training, and increasing muscle mass. It involves the degradation of muscle proteins via the integration of three main systems: autophagy and the calpain and ubiquitin-proteasome systems. While MPB is crucial for muscle health and growth, excessive or uncontrolled breakdown can lead to muscle wasting and other negative health consequences.
During exercise, MPB increases, but this increase is typically smaller than the concurrent increase in muscle protein synthesis (MPS). After exercise, the rate of MPS can surpass that of MPB, particularly when amino acids or protein is ingested, resulting in a positive net muscle protein balance. However, in a fasted state, the rate of MPB tends to be relatively high and can exceed MPS, leading to a negative net muscle protein balance.
Research has shown that resistance exercise stimulates MPS more than it does MPB, and MPS is also more responsive to feeding than MPB. This indicates that changes in MPS play a predominant role in determining muscle protein accrual or loss. In a fasted state, muscle protein breakdown rates are typically higher compared to when amino acids or protein is ingested, further emphasizing the importance of nutritional intake in muscle health and growth.
Furthermore, studies have found that adults with obesity have reduced muscle protein synthesis in the fasted state compared to lean individuals. While obesity appears to alter protein metabolism in muscle, the overall evidence is not consistent, and the responses may depend on the specific protein pool studied. Nevertheless, these findings highlight the complex interplay between metabolic processes, nutritional status, and individual factors in muscle health and growth.
In summary, muscle protein breakdown is a natural and essential process for muscle remodelling and growth. While exercise and nutritional interventions can influence the balance between MPS and MPB, it is important to recognize that a fasted state tends to result in higher MPB rates. Understanding these dynamics is crucial for optimizing exercise routines, nutritional strategies, and overall health outcomes, particularly for individuals with specific health considerations such as obesity.
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Resistance exercise increases muscle protein breakdown
Resistance exercise is a popular way to increase muscle mass and strength. The metabolic process of muscle protein breakdown (MPB) describes the degradation of bound muscle proteins into their amino acid precursors. This process occurs concurrently with muscle protein synthesis (MPS), the metabolic process that describes the incorporation of amino acids into bound skeletal muscle proteins.
It is clear that resistance exercise increases MPB, but not as much as it increases MPS. The ingestion of amino acids or protein after exercise is important as it increases MPS rates, which can surpass MPB rates, resulting in a net positive muscle protein balance. The key amino acid triggering the rise in MPS is leucine, which stimulates the mechanistic target of rapamycin complex-1, a key signalling protein. Ingesting proteins with a high leucine content will therefore trigger a rise in MPS.
The ingestion of 20-25g of protein increases insulin concentrations to 15-20 mU/L, which is sufficient to suppress post-exercise MPB rates. Food intake will therefore substantially reduce MPB rates, with the macronutrient content of the food being of little impact. However, only a moderate rise in insulin concentration is required for maximal inhibition of muscle breakdown rates.
The three main protein breakdown systems are the calpain proteases, the ubiquitin-proteasome system, and the autophagy-lysosome system. These systems work together to remodel skeletal muscle and do not operate independently. Complete degradation of a protein requires some combination of the systems.
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Muscle protein breakdown is required for muscle remodelling
Muscle protein breakdown (MPB) is an important metabolic component of muscle remodelling, adaptation to training, and increasing muscle mass. The degradation of muscle proteins occurs through the integration of three main systems: autophagy, and the calpain and ubiquitin-proteasome systems. These systems do not operate independently, and their regulation is complex. The complete degradation of a protein requires some combination of these systems.
The calpain proteases are responsible for disassembling myofibrils into smaller component parts. The ubiquitin-proteasome system then degrades these components into individual amino acids, and can label proteins (membrane receptors, channels and transporters) for destruction by the third system. The third system, the autophagy-lysosome system, breaks down membrane-based proteins.
The plasticity of skeletal muscle is mediated by the constant turnover or remodelling of muscle proteins. Two metabolic processes, muscle protein synthesis (MPS) and MPB, work concurrently in response to various stimuli to repair, replace, and generate new muscle proteins leading to phenotypic adaptations. The rate of MPS and MPB determines whether muscle protein is gained or lost.
While resistance exercise increases MPB, it does not increase it as much as muscle protein synthesis. Both hyperaminoacidemia and hyperinsulinemia inhibit the post-exercise response of MPB.
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Muscle atrophy occurs when protein degradation exceeds synthesis
Muscle atrophy, or muscle wasting, occurs when protein degradation exceeds synthesis. This happens when the body loses muscle mass, which can be caused by ageing or diseases such as cancer, infections, diabetes, organ failure, or inactivity.
Muscle protein breakdown (MPB) is an important metabolic component of muscle remodelling, adaptation to training, and increasing muscle mass. The degradation of muscle proteins occurs via three main systems: autophagy, and the calpain and ubiquitin-proteasome systems. These systems work together to remodel skeletal muscle. The calpain proteases disassemble myofibrils into smaller component parts, which are then degraded into individual amino acids by the ubiquitin-proteasome system. This system can also label proteins for destruction by the autophagy-lysosome system, which predominantly breaks down membrane-based proteins.
Muscle protein synthesis (MPS) is the metabolic process that describes the incorporation of amino acids into bound skeletal muscle proteins. The synthesis of myofibrillar proteins is primarily responsible for changes in skeletal muscle mass following resistance training, whereas mitochondrial proteins are synthesised in response to endurance-type training. MPS and MPB are two metabolic processes that act concurrently to repair, replace, and generate new muscle proteins. The balance between these two processes determines whether muscle protein is gained or lost.
In atrophic muscle, the balance between synthesis and degradation is disrupted, favouring reduced synthesis and/or increased degradation. This can be influenced by environmental factors such as nutrient type, composition, and rate of supply. For example, after exercise, the rate of MPB rises, resulting in a negative net muscle protein balance. However, when amino acids or protein is ingested after exercise, the net muscle protein balance becomes positive as the rate of MPS surpasses MPB.
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Muscle protein breakdown is one of two metabolic processes
Muscle protein breakdown (MPB) is one of two metabolic processes, the other being muscle protein synthesis (MPS). Together, they are responsible for muscle remodelling, adaptation to training, and increasing muscle mass.
MPB refers to the degradation of muscle proteins, which occurs via the integration of three main systems: autophagy and the calpain and ubiquitin-proteasome systems. These systems work together to break down muscle proteins into their amino acid precursors. The ubiquitin-proteasome system, for example, degrades muscle proteins into individual amino acids, while the autophagy-lysosome system primarily breaks down membrane-based proteins.
MPS, on the other hand, is the metabolic process that describes the incorporation of amino acids into bound skeletal muscle proteins. It involves the synthesis of two types of muscle proteins: contractile myofibrillar proteins (e.g. myosin, actin, tropomyosin, troponin) and energy-producing mitochondrial proteins. The synthesis of myofibrillar proteins is primarily responsible for changes in skeletal muscle mass following resistance training, while mitochondrial proteins are synthesized in response to endurance-type training.
The balance between MPB and MPS determines whether muscle protein is gained or lost. When MPS exceeds MPB, muscle protein is gained, and when MPB exceeds MPS, muscle protein is lost. MPS is more responsive to exercise and nutritional stimuli, and research has shown that resistance exercise increases MPS more than MPB.
Factors such as exercise, nutrition, age, and sexual dimorphism can influence the rates of MPB and MPS. For example, fasting has been shown to increase MPB and reduce MPS, resulting in a net muscle protein loss. Conversely, ingesting amino acids or protein after exercise can increase MPS and suppress MPB, leading to a net muscle protein gain.
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Frequently asked questions
Muscle protein breakdown (MPB) is a metabolic process that describes the degradation of bound muscle proteins into their amino acid precursors.
Muscle protein breakdown allows the body to break down damaged muscle proteins and recycle them into new functional muscle proteins. It also plays a role in muscle growth and adaptation.
The three main protein breakdown systems are the calpain proteases, the ubiquitin-proteasome system, and the autophagy-lysosome system.
Exercise, particularly resistance exercise, increases muscle protein breakdown. However, the increase in muscle protein synthesis typically exceeds the increase in muscle protein breakdown, resulting in a net gain in muscle protein.
Insulin inhibits muscle protein breakdown, and amino acid availability can also influence the process. Nutritional interventions can impact the rate of muscle protein breakdown and synthesis.
