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Muscle protein synthesis (MPS) is the process by which the body turns amino acid chains into muscle protein. This process is critical for building muscle mass, as skeletal muscle cannot function without protein. To build muscle mass, the rate of muscle protein synthesis must exceed the rate of muscle protein breakdown. Various factors influence muscle protein synthesis, including exercise, nutrition, and supplementation. Resistance training, in particular, has been shown to stimulate MPS, especially when combined with protein ingestion before or after the workout. Additionally, essential amino acids (EAAs) have been identified as effective stimulators of MPS, with free-form EAA compositions resulting in higher stimulation than intact dietary proteins.
| Characteristics | Values |
|---|---|
| Resistance exercise | Increases muscle net protein balance for 24-48 hours |
| Protein consumption | Stimulates muscle protein synthesis |
| Timing of protein consumption | More effective immediately post-exercise |
| Type of protein | Soy and whey protein supplementation increase lean mass and strength |
| Whole food sources of protein | Contain essential amino acids that stimulate muscle protein synthesis |
| Pre-sleep casein protein intake | Increases in overnight muscle protein synthesis |
| Fasting | Reduces protein synthesis |
| Insulin | May promote protein synthesis |
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What You'll Learn
Resistance training as a stimulus
Resistance training is a powerful stimulus for muscle protein synthesis (MPS). 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.
During resistance training, muscle breaks down and rebuilds protein in response to the stimulus. Both protein anabolism and catabolism increase after exercise, but the increase in anabolism is relatively larger, causing a net muscle protein balance to be positive. This net positive balance leads to muscle hypertrophy.
The amplitude and duration of increases in MPS after resistance training are modulated by an individual's training status. Previous studies have shown that the initial responses of MPS to resistance training and nutrition are not correlated with subsequent hypertrophy. Thus, early acute responses of MPS in the hours after resistance training do not capture how MPS can affect resistance training-induced muscle hypertrophy.
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 can therefore be advantageous in triggering a rise in MPS. Consuming protein before or after resistance training can also stimulate MPS.
Protein supplementation during resistance training augments hypertrophic gains. Protein ingestion provides the building blocks (indispensable amino acids) for, and triggers an increase in, MPS, while also suppressing muscle protein breakdown (MPB). For athletes, a common recommendation is a protein intake of 0.25 g of a high-quality protein per kg of body weight, or an absolute dose of 20–40 g.
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Protein consumption before/after resistance exercise
Muscle protein synthesis (MPS) is the driving force behind adaptive responses to exercise. An acute exercise stimulus, especially resistance exercise, and protein ingestion both stimulate MPS and work synergistically when protein is consumed before or after resistance exercise.
A daily protein intake of 1.4–2.0 g protein/kg body weight/day is sufficient for most exercising individuals to build and maintain muscle mass. However, recent evidence suggests that higher protein intakes (>3.0 g/kg/d) may have positive effects on body composition in resistance-trained individuals, promoting the loss of fat mass.
The timing of protein consumption in relation to exercise is important. Consuming protein within one hour after resistance exercise has a small but significant effect on increasing muscle hypertrophy compared to delaying consumption by at least two hours. This is because muscle protein synthesis is elevated for only a finite period of time after exercise, with studies showing that this duration is approximately 1.5 hours. Therefore, consuming protein immediately after exercise can maximise the anabolic response.
However, the importance of timing may depend on when a pre-workout meal was consumed and its composition. If a pre-workout meal containing protein is consumed, there may be less need for immediate post-exercise protein ingestion as amino acids will be supplied to the muscles during and after exercise. Indeed, one study found a larger anabolic response when a carbohydrate and protein drink was consumed prior to exercise compared to when it was consumed immediately after exercise.
The type of protein consumed may also be a factor in stimulating protein synthesis. Both soy and whey protein supplementation have been shown to increase lean mass and strength compared to a placebo, with whey protein showing greater intracellular signalling.
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Essential amino acids (EAA)
Essential amino acids (EAAs) are the building blocks of proteins, playing a crucial role in various physiological processes, including protein synthesis and muscle growth. There are 20 standard amino acids, of which nine are classified as essential. This classification means that they cannot be endogenously synthesized by the body and must be obtained through dietary sources.
EAAs are intricately linked to muscle growth and hypertrophy. Leucine, in particular, has emerged as a key regulator of the mammalian target of rapamycin (mTOR) signaling pathway, which plays a central role in muscle protein synthesis. The anabolic response of skeletal muscle to resistance exercise is significantly influenced by the availability of EAAs, especially leucine and branched-chain amino acids (BCAAs). Proper EAA supplementation enhances net protein balance, promoting muscle anabolism and mitigating muscle wasting associated with aging and disease. The timing and composition of EAA intake are also important factors in optimizing muscle protein synthesis. For example, immediate post-exercise consumption of EAAs stimulates protein synthesis, whereas delaying consumption by even two hours can blunt the response.
In addition to leucine, other amino acids such as threonine and tryptophan may also stimulate muscle protein synthesis. The type of protein consumed also matters, with soy and whey protein supplementation shown to increase lean mass and strength compared to a placebo or carbohydrate-only supplement.
Overall, EAAs are essential for normal health and play a pivotal role in muscle protein synthesis. They are required to maintain and repair muscle tissue, and their deficiency or imbalance can disrupt these processes, leading to impaired muscle function and suboptimal growth.
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Fasting and muscle breakdown
Muscle protein synthesis (MPS) is the driving force behind adaptive responses to exercise. It is a widely adopted proxy for gauging the chronic efficacy of acute interventions, such as exercise and nutrition.
Intermittent fasting, on the other hand, has been shown to be effective in muscle preservation. Studies have found that participants who underwent intermittent fasting lost weight, and the weight loss was attributed to fat loss rather than muscle loss. Additionally, intermittent fasting has been linked to improved biomarkers related to the risks of diabetes and coronary heart disease.
To prevent muscle breakdown during fasting, it is crucial to maintain adequate protein intake. This can be achieved by evenly distributing protein consumption throughout the day, ensuring a sufficient overall daily protein intake. For individuals exercising, a daily protein intake of 1.4–2.0 g protein/kg body weight/day is generally sufficient. For resistance-trained individuals, higher protein intakes (>3.0 g/kg/day) may promote fat loss and positively impact body composition.
Furthermore, combining intermittent fasting with exercise can be beneficial. Resistance exercise, in particular, has been shown to increase muscle net protein balance for 24-48 hours. The immediate post-exercise consumption of protein stimulates protein synthesis, with elevated synthesis observed when consuming essential amino acids post-workout.
In summary, while fasting may raise concerns about muscle breakdown, intermittent fasting coupled with adequate protein intake and exercise can help preserve and even enhance muscle mass while providing additional health benefits.
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Insulin and muscle synthesis
Insulin is a potent anabolic stimulus for muscle proteins. Insulin deficiency leads to a protein catabolic state and loss of muscle mass, which can only be reversed with insulin therapy. Insulin promotes muscle anabolism, but its stimulatory effect on muscle protein synthesis in humans is still unclear. Insulin has been shown to increase muscle protein synthesis in various tissues, including skeletal muscle.
Insulin can acutely stimulate muscle protein synthesis by increasing the initiation of mRNA translation. However, if the physiological increase in insulin secretion is suppressed during feeding in rats, the stimulation of translation initiation is abolished, and muscle protein synthesis is suppressed. Insulin can also reduce protein breakdown by stabilizing lysosomes and reducing the activity of the ubiquitin-proteasome pathway.
The effect of insulin on muscle protein synthesis is modulated by insulin-induced changes in muscle blood flow and amino acid availability. Insulin increases muscle protein synthesis when total amino acid levels, or at least essential amino acid levels, are at or above their postabsorptive concentrations. Insulin does not stimulate muscle protein synthesis in the presence of increased circulating levels of plasma branched-chain amino acids (BCAA) alone. However, it does decrease whole-body proteolysis.
The relationship between diet and protein balance is complex. Even with increased protein intake, muscle protein synthesis is triggered for only a finite period of time. This is because the body can only utilize a certain amount of the essential amino acids it receives. The ideal protein intake per serving for athletes to maximize muscle protein synthesis varies but is typically between 0.25 g of a high-quality protein per kg of body weight or an absolute dose of 20–40 g.
Resistance training is a powerful stimulus for muscle protein synthesis. It has been shown to increase muscle net protein balance for 24–48 hours. Both protein anabolism and catabolism increase after resistance training, but the increase in anabolism is relatively larger, resulting in a positive muscle protein balance.
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Frequently asked questions
Muscle protein synthesis (MPS) is the process by which the body turns amino acid chains into muscle protein. Skeletal muscle cannot function without protein as it provides the energy needed.
The body is constantly breaking down and rebuilding muscle protein. When the rate of muscle protein synthesis is higher than the rate of muscle breakdown, the muscle is able to grow.
Resistance training is a well-known stimulus for protein synthesis. Consuming protein supplements, especially those containing essential amino acids (EAAs), after resistance exercise also promotes increased muscle protein synthesis.
EAAs are required for normal health and are either not manufactured or manufactured in insufficient quantities by the body. They include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
The type of protein consumed, biological factors such as DNA and sex, nutrition, and training variables can all impact muscle protein synthesis. For example, soy and whey protein have been shown to increase lean mass and strength compared to a placebo.
