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Mixed muscle protein synthesis measures the synthesis of all muscle proteins. It is often studied in relation to resistance exercise and nutrition, with the assumption that there is a direct relationship between the acute response of mixed muscle protein synthesis and muscle hypertrophy. However, studies have shown that while mixed muscle protein synthesis increases with endurance and resistance exercise, increases in myofibrillar protein synthesis are specific to resistance exercise. Furthermore, the ingestion of protein supplements and typical foods or mixed meals may differ substantially in their effects on muscle protein synthesis.
| Characteristics | Values |
|---|---|
| Definition | Mixed muscle protein synthesis measures the synthesis of all muscle proteins |
| Measurement | Mixed muscle protein synthesis can be measured by using highly enriched intrinsically labeled protein |
| Synthesis Rate | Mixed muscle protein fractional synthesis rate (FSR) and fractional breakdown rate (FBR) were examined after an isolated bout of either concentric or eccentric resistance exercise |
| Synthesis Rate Increase | Exercise resulted in a significant increase in muscle FSR at all times: 3 hours = 112%, 24 hours = 65%, 48 hours = 34%% |
| Breakdown Rate Increase | Muscle FBR was also increased by exercise at 3 hours (31%) and 24 hours (18%) post-exercise but returned to resting levels by 48 hours |
| Net Balance | Muscle net balance was significantly increased after exercise at all time points |
| Resistance Training | Resistance training reduces the acute exercise-induced increase in muscle protein turnover |
| Nutrition | The anabolic effects of nutrition are driven by the transfer and incorporation of amino acids from dietary protein sources into skeletal muscle proteins |
| Whole-food Protein Sources | The addition of whole-food protein sources such as chicken does not seem to impact muscle protein synthesis |
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What You'll Learn
Resistance exercise and mixed muscle protein synthesis
The study of how resistance exercise impacts human skeletal muscle protein synthesis and turnover is an evolving area of scientific research. The ingestion of protein following resistance exercise stimulates muscle protein synthesis rates, enhancing the skeletal muscle's adaptive response to prolonged training.
The measurement of muscle protein synthesis (MPS) is typically expressed as the rate of amino acid incorporation into bound muscle protein over a given time period, usually an hour or a day. The metabolic process of muscle protein breakdown (MPB) describes the degradation of bound muscle proteins into their amino acid precursors, occurring concurrently with MPS. The difference in rates of MPS and MPB determines whether muscle protein is gained or lost. MPS is more responsive to exercise and nutritional stimuli than MPB.
Resistance exercise results in a mild stimulation of MPB rates but a greater stimulation of MPS rates. The ingestion of 20g of quickly digestible protein isolate optimizes MPS rates during the first few hours of post-exercise recovery. However, the majority of daily protein intake is consumed as slower-digestible, nutrient-rich, whole-food protein sources as part of mixed meals. The ingestion of 30g of casein protein before sleep did not result in a detectable increase in overnight post-exercise MPS, whereas 40g of casein protein under identical conditions increased overnight MPS rates by around 22%. The ingestion of relatively large amounts of slowly digestible protein may result in a more robust stimulation of post-exercise MPS rates over more prolonged periods.
Resistance exercise coupled with protein ingestion results in the accretion of skeletal muscle protein, referred to as hypertrophy. The ingestion of animal-derived proteins tends to result in a greater increase in MPS rates than the ingestion of plant-derived proteins. However, the ingestion of larger amounts and/or mixing of different plant-derived proteins may compensate for the lower anabolic properties.
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Whole foods and mixed meals
Protein is a building block for muscles, and it is essential for growth, repair, and building cells and tissue. A high-protein diet can help support muscle repair and growth, especially after resistance exercise. Whole foods and mixed meals provide a variety of nutrients and amino acids that are necessary for muscle health and overall well-being.
Some excellent sources of whole food protein include lean meats such as chicken, turkey, beef, and pork. Fish like salmon, tuna, and mackerel are also rich in protein and omega-3 fatty acids, beneficial for heart health. For plant-based proteins, lentils, chickpeas, and black beans are excellent choices, providing fibre, folate, magnesium, potassium, and iron. Nuts and seeds, such as almonds, peanuts, pumpkin seeds, and sunflower seeds, are also high in protein and provide healthy fats, vitamins, and minerals.
When it comes to mixed meals, combining different protein sources can be beneficial. For example, a meal with lean meat or fish, carbohydrates like oats or potatoes, and healthy fats like avocado provides a well-rounded mix of macronutrients. Additionally, including a variety of plant-based proteins in your diet throughout the day ensures a healthy mix of amino acids.
It is important to note that the amount of protein required can vary depending on individual factors such as gender, size, age, and whether one is trying to build muscle or lose fat. Consulting with a healthcare professional or certified fitness instructor can provide guidance on tailoring protein intake and meal plans to specific needs and goals.
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Measuring mixed muscle protein synthesis
Mixed muscle protein synthesis involves measuring the synthesis of all muscle proteins. This can be done by measuring the mixed muscle protein fractional synthesis rate (FSR). The precursor-product method is the most common approach to measuring FSR. This method utilises stable isotope-labelled amino acids, such as 13C6 phenylalanine or 1-13C leucine, which are administered intravenously under controlled laboratory conditions. This allows for the direct tracing of the incorporation of free amino acids into newly synthesised bound muscle proteins.
The choice of amino acid tracer can affect the measured rate of muscle protein synthesis. For example, studies have shown that the use of leucine and phenylalanine tracers can result in a discrepancy in the measured muscle protein synthesis rate. Leucine exhibits functional compartmentalisation within the muscle, with separate pools directed toward oxidation and protein synthesis. On the other hand, phenylalanine is not oxidised within skeletal muscle, and thus its enrichment in muscle tissue fluid may provide a better estimate of its tRNA charged pool.
The measurement of mixed muscle protein synthesis is often done in conjunction with resistance exercise and nutrition interventions to inform sport and exercise nutrition recommendations, particularly for athletes and other exercisers. Acute measurements of mixed muscle protein synthesis can be predictive of chronic muscle adaptation, correlating basal rates of mixed muscle protein synthesis with measurements of muscle mass, strength, and muscle quality.
Additionally, the ingestion of protein following resistance-type exercise stimulates muscle protein synthesis rates and enhances the skeletal muscle adaptive response to prolonged training. The amount of protein required to maximise mixed muscle protein synthesis rates may be higher following whole-body resistance-type exercise compared to exercise involving fewer muscle groups. Ingesting 20 grams of quickly digestible protein results in a near-maximal mixed muscle protein synthesis response, with a further 10-20% increase when the ingested amount is doubled to 40 grams.
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The role of anabolic hormones
Mixed muscle protein synthesis and breakdown are affected by resistance exercise in humans. Research has been conducted to determine the effects of nutrition and post-exercise nutrition on skeletal muscle protein turnover.
Anabolic hormones play a crucial role in muscle growth and development. This group of hormones includes insulin-like growth factor-1 (IGF-1), testosterone, and growth hormone (GH). These hormones are responsible for stimulating tissue growth, particularly muscle. Anabolic-androgenic steroids (AAS) are synthetic forms of testosterone that can be used to treat medical conditions such as low testosterone levels and muscle atrophy. While they can effectively stimulate muscle growth, their misuse can lead to harmful side effects.
Resistance training (RT) and weight lifting have been shown to increase the body's levels of anabolic hormones. This increase in hormones does not always lead to enhanced muscle strength or size. The local presence of anabolic hormones in the exercised muscle seems to be more critical than whole-body increases. For example, performing squats before arm curls will increase anabolic hormones, but it won't make the arm muscles stronger or bigger than if the exercises were done separately.
Research suggests that chronic or long-term increases in anabolic hormones do lead to increased muscle mass and hypertrophy. The timing of these hormonal increases may be a factor, as short-term elevations may not directly contribute to muscle growth. However, the relationship between resistance exercise, anabolic hormones, and muscle growth is complex and remains a subject of ongoing study.
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The effect of age and gender
Mixed muscle protein synthesis and breakdown refer to the body's metabolic response to resistance exercise. The ingestion of protein before or after resistance exercise stimulates muscle protein synthesis rates, enhancing the skeletal muscle's adaptive response to training.
Age and gender do appear to have an effect on muscle protein synthesis and breakdown. Studies have shown that while there are no differences in muscle protein turnover in young and middle-aged men and women, there are significant differences in the rate of muscle protein synthesis between older adult men and women. This suggests that ageing affects muscle protein turnover differently in men and women.
The basal muscle protein fractional synthesis rate (FSR) was found to be similar in young and old men, but there was a notable difference between the two groups of women. The basal FSR was higher in young women than in old women. However, when combined with insulin, glucose, and amino acid infusions, the muscle protein FSR increased in both young and old men, but the increase was more pronounced in young men.
In terms of gender differences, one study found that obese older women had surprisingly higher basal muscle protein synthesis rates than men. Another study, which was part of an androgen replacement trial, found that women had higher rates of basal muscle protein synthesis than men, regardless of age. However, the subjects in this study were selected based on their low androgen levels, which stimulate muscle protein synthesis, so this may have influenced the results.
Additionally, the muscle protein synthetic response to feeding is influenced not only by acute nutrient intake but also by habitual energy and nutrient intake, as well as non-dietary factors such as habitual physical activity, body composition, age, and/or sex.
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Frequently asked questions
Mixed muscle protein synthesis measures the synthesis of all muscle proteins.
Mixed muscle protein synthesis can be measured by using highly enriched intrinsically labeled protein. It can also be measured using deuterium oxide (D2O) or amino acid tracers.
Mixed muscle protein synthesis is beneficial for muscle growth and training performance. It helps to build muscle mass and strength, and can also improve endurance and metabolic health.
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