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Muscle protein balance is the relationship between muscle protein synthesis (MPS) and muscle protein breakdown (MPB). MPS is a fast-occurring process when the body is growing and slows significantly after the age of 20. It is influenced by diet, exercise, and nutritional supplementation. MPB, on the other hand, does not fluctuate much and is less important for muscle gains. To build and maintain muscle mass, a daily protein intake of 1.4–2.0 g protein/kg body weight/day is sufficient for people exercising. The anabolic effect of exercise is also long-lasting, at least 24 hours, but likely diminishes with time post-exercise.
| Characteristics | Values |
|---|---|
| Definition | Muscle protein synthesis (MPS) is the metabolic process that describes the incorporation of amino acids into bound skeletal muscle proteins. |
| Muscle Growth | Muscle hypertrophy (muscle growth) occurs when muscle protein synthesis exceeds muscle protein breakdown. |
| Muscle Maintenance | Muscle protein synthesis helps maintain muscle mass. |
| Muscle Loss | Muscle protein breakdown can accelerate the loss of muscle mass, as experienced in age-related muscle loss, immobilization, inactivity, and other catabolic (or breakdown) conditions. |
| Factors Influencing MPS | Factors such as exercise, nutrition, training status, training paradigms, genetics, and protein quality influence MPS. |
| Recommended Protein Intake | For building and maintaining muscle mass, a daily protein intake of 1.4–2.0 g protein/kg body weight/day is recommended. For athletes, the ideal protein intake per serving is 0.25 g of high-quality protein per kg of body weight or an absolute dose of 20–40 g. |
| Measuring MPS | The precursor–product method is commonly used to determine the muscle protein fractional synthesis rate (FSR). |
| Timing of Protein Intake | Consuming protein after a workout helps distribute protein intake evenly throughout the day, which is linked to faster MPS. |
| Exercise Type | The intensity, type, and duration of exercise affect MPS. High-intensity exercises like bench pressing, deadlifting, interval training, and repeated hill climbs cause stress to the muscle tissue, eliciting a greater MPS response. |
| Exercise and Recovery | Muscle growth occurs during recovery, so allowing time for the body to recover from exercise is crucial to optimizing MPS. |
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What You'll Learn
The role of nutrition
Muscle protein synthesis and breakdown are opposing processes that are always ongoing. Muscle protein synthesis is the process of building muscle mass, while muscle protein breakdown breaks down muscle tissue. The difference in the speed of these two processes determines the net change in muscle protein size.
Nutrition plays a crucial role in muscle protein synthesis and breakdown. The amount, type, and timing of protein consumption all influence muscle protein synthesis. For building and maintaining muscle mass, a daily protein intake of 1.4–2.0 g of protein per kg of body weight is sufficient for people exercising. This amount can be obtained through the consumption of whole foods, and supplementation can ensure adequate protein quality and quantity without excess calories.
Protein intake before and after exercise is important, with 10 grams of essential amino acids or 25 grams of a complete protein being sufficient to maximally stimulate protein synthesis. Essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. These amino acids are either not manufactured in the body or are manufactured in insufficient quantities, so they must be supplied by dietary protein.
Carbohydrate consumption and insulin secretion play an indirect role in skeletal muscle protein synthesis. Carbohydrates may facilitate intracellular signaling through insulin and by elevating and maintaining the cell's energy status. However, significant protein synthesis is unlikely with a carbohydrate-only supplement.
Nutrient-driven increases in muscle protein synthesis are of finite duration, typically lasting around 1.5 hours, and are influenced by the mode of exercise. For example, endurance-type exercises like running or cycling are associated with increased synthesis of mixed muscle proteins, but these acute responses do not lead to significant changes in muscle mass. In contrast, resistance exercises lead to a persistent positive muscle protein synthesis balance, resulting in muscle hypertrophy.
Overall, creating synergy between protein consumption and exercise promotes muscle protein synthesis and can impact patient outcomes, especially in the field of chiropractic health care.
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The impact of exercise
Muscle protein synthesis (MPS) is the process of building muscle mass. On the other hand, muscle protein breakdown (MPB) is the opposing process of breaking down muscle tissue. Both MPS and MPB are always occurring simultaneously, but at different speeds. The difference in speed between these two processes determines the net change in muscle protein size.
Exercise training increases basal muscle protein turnover, with resistance and non-resistance exercises appearing to depress MPS during the exercise. However, MPS is elevated after exercise in the fasted state, when net muscle protein balance remains negative. The anabolic effect of exercise is long-lasting (at least 24 hours) but likely diminishes with increasing time post-exercise.
Repeated bouts of resistance exercise lead to a persistent positive MPS balance, resulting in muscle hypertrophy. The anabolic response to exercise and nutritional stimuli are not different between young and middle-aged (18–45 years) men and women. However, the response is blunted in older women (65–80 years) compared to men of the same age.
Physically active individuals should aim for a daily protein intake of 1.4–2.0 g protein/kg body weight/day (g/kg/d) to build and maintain muscle mass. Higher protein intakes (>3.0 g/kg/d) may promote fat loss in resistance-trained individuals. Athletes should focus on whole food sources of protein that contain all the essential amino acids (EAAs), which are required for normal health.
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Nitrogen balance
Research has shown that resistance exercise alters the post-exercise response of anabolic and catabolic hormones. In one study, trained individuals exhibited a significantly lower nitrogen balance than untrained individuals, indicating that muscle protein accumulation in response to protein and carbohydrate supplementation is influenced by the timing of intake relative to exercise. Additionally, older patients tend to experience less severe hypercatabolism compared to younger patients, possibly due to lower muscle mass and, consequently, lower urinary nitrogen losses.
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Amino acid tracers
Tracer amino acid-incorporation methods are particularly useful for investigating protein metabolism. These methods have revealed that certain tissues, such as the liver and gut, have significantly higher turnover rates and deposit much more protein than skeletal muscle. There are notable differences in the fractional synthesis rates of individual proteins as well.
Stable isotope tracer methodology offers a range of approaches to assess optimal dietary protein intake in humans. These tracer methods are used to estimate dietary protein and essential amino acid requirements under different physiological conditions. For example, the arterial-venous tracer balance method can be used to measure net muscle protein synthesis, which is an important component of lean body mass.
While amino acid tracers provide valuable insights, it is important to note that their calculations involve multiple variables, which can lead to variability in estimates of protein synthesis. To improve the accuracy of tracer methods, laboratories should focus on measuring low tracer amino acid enrichments in small amounts of protein.
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The importance of skeletal muscle
Muscle protein balance is the relationship between muscle protein synthesis and muscle protein breakdown. While muscle protein breakdown is important, it doesn't fluctuate much, making muscle protein synthesis far more important for muscle gains.
Skeletal muscle is the most common type of muscle in the body, comprising approximately 30% to 40% of total body mass. These muscles are attached to bones by tendons and allow for a wide range of movements and functions. They are under voluntary control, meaning individuals can decide how and when they work.
Skeletal muscle serves many purposes, including producing movement, maintaining body posture and position, controlling body temperature, storing nutrients, and stabilizing joints. It is also an endocrine organ, secreting various proteins, lipids, amino acids, metabolites, and small RNAs under different physiological conditions.
The plasticity of skeletal muscle is mediated by the constant turnover or remodelling of muscle proteins. Muscle protein synthesis (MPS) occurs at a fast rate when the body is growing and slows significantly after age 20. It is influenced by resistance exercise, diet, and nutritional supplementation.
MPS is important for building and maintaining muscle mass. A daily protein intake of 1.4-2.0 g of protein per kg of body weight is sufficient for people exercising, while higher intakes may promote fat loss. Repeated bouts of resistance exercise lead to a persistent positive MPS balance, resulting in muscle hypertrophy.
Skeletal muscle health is vital, as various conditions and disorders can affect these muscles, ranging from mild injuries to serious myopathies. Therefore, maintaining strong and healthy skeletal muscles is crucial for overall health and everyday activities.
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Frequently asked questions
Muscle protein balance is the relationship between muscle protein synthesis (MPS) and muscle protein breakdown (MPB).
Muscle protein balance can be measured through the use of stable isotopically labelled tracers, which can quantify MPS and MPB. Another method is to calculate nitrogen balance, which is the difference between nitrogen intake and nitrogen excretion.
Dietary protein intake influences muscle protein balance. A sufficient protein intake can shift net protein balance from negative (net protein loss) to positive (net protein gain). The ideal protein intake varies but common recommendations are 0.25 g of a high-quality protein per kg of body weight, or an absolute dose of 20–40 g.
Exercise, particularly resistance exercise, stimulates MPS and can work in synergy with protein ingestion. Repeated bouts of resistance exercise can lead to a persistent positive MPS balance, resulting in muscle hypertrophy.
