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Muscle protein is essential for human health and has a wide range of benefits, including increasing muscle mass, improving bone density, and promoting weight loss. About 40% of the body weight of a healthy human adult weighing about 70 kilograms is muscle, which is composed of about 20% muscle protein. Muscle proteins can be divided into myofibrillar, regulatory, sarcoplasmic, and stromal proteins. Myofibrillar proteins, such as actin and myosin, are the most abundant proteins in muscle tissue and play a crucial role in muscle contraction and relaxation. The anabolic effects of nutrition and exercise are key factors influencing muscle protein synthesis (MPS), which is the process behind the body's adaptive responses to physical activity.
| Characteristics | Values |
|---|---|
| Percentage of muscle in a healthy human adult weighing 70 kg | 40% |
| Percentage of muscle protein in the above case | 20% |
| Weight of muscle protein in the above case | 5 to 6 kg |
| Types of muscle proteins | Myofibrillar, regulatory, sarcoplasmic, stromal |
| Examples of myofibrillar proteins | Actin, myosin, troponin-tropomyosin |
| Examples of sarcoplasmic proteins | Hemoglobin, myoglobin, myogen, myoalbumin, x-globulin, glycolytic enzymes |
| Examples of stromal proteins | Collagen, elastin |
| Muscle protein synthesis | MPS |
| MPS duration | 1.5 hours |
| MPS and exercise | Resistance exercise delays MPS |
| MPS and endurance exercise | Running or cycling associated with increased synthesis of mixed muscle proteins |
| MPS and resistance exercise | Associated with hypertrophy |
| MPS and amino acids | Amino acids from dietary protein sources are transferred and incorporated into skeletal muscle proteins |
| Protein deficiency symptoms | Edema, hair loss, brittle nails, muscle weakness, reduced muscle mass, slower wound healing time |
| Protein benefits | Increased muscle mass, improved bone density, weight loss, better exercise recovery, reduced appetite, etc. |
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What You'll Learn
Muscle protein synthesis (MPS) and exercise
Muscle protein synthesis (MPS) is the driving force behind adaptive responses to exercise. MPS is a widely adopted proxy for gauging the chronic efficacy of acute interventions, such as exercise and nutrition. 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 measurement of 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 describes the degradation of bound muscle proteins into their amino acid precursors, which occurs concurrently with MPS. The difference in the rates of MPS and muscle protein breakdown determines whether muscle protein is gained or lost. MPS is more responsive to exercise and nutritional stimuli than muscle protein breakdown.
Several methods have been used to measure the acute response of MPS to exercise and nutrition in humans. The most common approach is the precursor-product method, which allows for the determination of muscle protein fractional synthesis rate (FSR). This method utilizes stable isotope-labeled amino acids, usually administered intravenously under controlled laboratory conditions, to directly trace the incorporation of free amino acids into newly synthesized bound muscle proteins. This typically occurs over an acute 3-12 hour time period following a single exercise and/or nutrition stimulus.
The anabolic effect of exercise is long-lasting, at least 24 hours, but likely diminishes with increasing time post-exercise. Resistance exercise stimulates a prolonged elevation of MPS that can remain elevated for at least 24 hours. Nutrient-driven increases in MPS are of finite duration (approximately 1.5 hours), switching off despite sustained amino acid availability and intramuscular anabolic signalling.
The response of MPS to exercise varies according to the type of exercise and an individual's genetic makeup. Endurance-type exercises, such as running or cycling, are associated with increased synthesis of mixed muscle proteins, while resistance exercises are associated with muscle hypertrophy. Selectivity over the quantity and type of muscle proteins synthesized underlies the adaptive specificity to distinct exercise training regimens.
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Muscle protein and diet
Muscle protein is essential for building and maintaining muscle mass. Skeletal muscle protein is dynamic and constantly fluctuating between states of growth and breakdown. Consuming an adequate amount of dietary protein is critical for maintaining optimal health and preventing muscle loss during normal growth and ageing.
The human body contains about 5 to 6 kilograms (11 to 13 pounds) of muscle protein, comprising about 20% of muscle mass. Myofibrillar proteins, including actin and myosin, are the most abundant proteins in muscle tissue, directly involved in the muscle's ability to contract and relax. Actin and myosin combine to form actomyosin, a complex molecule that enables muscle contraction and relaxation. Other muscle proteins include sarcoplasmic proteins such as haemoglobin, myoglobin, and various enzymes, as well as stromal or connective tissue proteins like collagen and elastin.
Dietary protein plays a crucial role in muscle health and maintenance. The Recommended Daily Allowance (RDA) for protein is 0.8 grams of protein per kilogram of body weight. However, individual needs may vary, and it is recommended to consult a healthcare professional or use tools like the USDA's online calculator to determine your specific protein requirements. Research suggests that consuming a moderate amount of high-quality protein with each meal, ranging from 25 to 40 grams, is ideal for optimising muscle protein synthesis.
To increase and maintain muscle mass, it is essential to include sufficient protein in your diet. High-quality protein sources include lean meats, fish, dairy, soy, legumes, nuts, and whole grains. For example, chicken, salmon, Greek yoghurt, skim milk, and beans are excellent sources of protein that promote muscle recovery and growth. Additionally, certain plant-based sources like chickpeas, peanuts, and buckwheat can be part of a balanced muscle-building diet, although their protein quality is considered lower than animal sources.
Consuming protein-rich meals and snacks before and after strength-training sessions can maximise muscle growth and boost muscle recovery. However, it is important to note that a well-rounded diet should also include healthy fats and carbohydrates to provide fuel for physical activity. While protein supplements, such as protein shakes and powders, can help individuals struggling to meet their protein goals, they should be used in conjunction with a balanced diet that includes whole foods.
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Types of muscle protein
Muscle proteins are the most important component of striated skeletal muscle. They are the basic material of tissue structure. Muscle fibres are composed of myofibrils, which are made up of myosin and actin filaments. These filaments are oriented parallel to each other and to the long axis of the muscle. During contraction, the S filaments that link the actin filaments shorten, causing the actin filaments to slide towards each other and past the myosin filaments, resulting in muscle shortening. Myosin constitutes as much as 35% of the total protein and is a contractile protein that also occurs in blood platelets. Actin, on the other hand, can exist in two forms: G-actin, which is globular, and F-actin, which is fibrous. Together, they form actomyosin, a complex molecule.
Myofibrillar proteins, including actin and myosin, make up 50-55% of the total meat protein content. They are the most abundant proteins in muscle and are directly involved in the muscle's ability to contract and relax.
Sarcoplasmic proteins, such as glycolytic enzymes, hemoglobin, and myoglobin, comprise approximately 30-34% of muscle protein content. Hemoglobin carries oxygen from the lungs to the tissues, including muscle. Myoglobin, on the other hand, stores the oxygen transported to the muscle via the blood by hemoglobin until it is utilised in metabolism. It also contributes to the red colour of muscle.
Stromal or connective tissue proteins like collagen and elastin account for the final 10-15% of proteins in meat. Collagen is the main structural component of connective tissues, while elastin needs to be broken down by the enzyme elastase for digestion.
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Muscle protein and weight loss
Muscle proteins are the most important component of striated skeletal muscle, with about 40% of a healthy human adult's body weight being muscle, and about 20% of that muscle being muscle protein. In other words, the human body contains about 5 to 6 kilograms of muscle protein.
Protein is essential for building and maintaining muscles and bones, and it also helps to regulate a host of cellular processes, from immune function to the transportation of oxygen through the bloodstream.
Protein can aid in weight loss by stimulating the release of satiety signals in the small intestine, helping you feel full. It also takes longer to digest than carbohydrates, helping to regulate blood sugar and slow down the pace at which food leaves the stomach. Additionally, protein suppresses the hunger hormone, ghrelin, while boosting appetite-reducing hormones like GLP1 and leptin.
The recommended daily allowance of protein is a modest 0.8 grams per kilogram of body weight per day. However, this is only the minimum required to prevent a protein deficiency, not what is needed for muscle gain, satiety, weight management, and glycemic control. For weight loss, a recommended intake of protein is 1-1.2 grams per kilogram of body weight. This can be achieved by eating protein-rich foods such as chicken, salmon, pork, shellfish, whitefish, lamb, soybeans, tofu, chickpeas, lentils, nuts, peas, beans, and seitan.
It is important to note that a well-rounded diet should include a mix of proteins, carbohydrates, fats, vitamins, and minerals.
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Muscle protein and ageing
Muscle protein is essential for muscle contraction and relaxation, and it constitutes about 20% of muscle mass. Ageing is associated with a progressive loss of skeletal muscle mass and strength, which can lead to the loss of functional capacity and an increased risk of chronic metabolic diseases. This loss of muscle mass is attributed to a disruption in the regulation of skeletal muscle protein turnover, resulting in an imbalance between muscle protein synthesis and degradation.
Research suggests that the muscle protein synthetic response to food intake decreases with age, which is a key factor in the decline of skeletal muscle mass. This response can be stimulated by physical activity and/or exercise, especially resistance-type exercise training, which can help augment skeletal muscle mass and improve functional performance in older adults. The anabolic effect of exercise is long-lasting, at least 24 hours, and likely diminishes with increasing time post-exercise.
To maintain muscle mass and function, it is recommended that older adults consume a moderate amount of high-quality protein (25-30 grams) with each meal and incorporate habitual exercise in close temporal proximity to protein-containing meals. This combined approach may help delay the onset of sarcopenia, slow its progression, and reduce its functional consequences.
While both animal and plant-based proteins provide essential amino acids, animal proteins generally have a higher proportion of the amino acid leucine, which plays a key role in stimulating muscle protein anabolism. However, it is important to note that the benefits and challenges of optimising dietary protein intake in older adults are still being studied, and there may be health and environmental considerations to take into account when increasing animal-based protein recommendations.
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Frequently asked questions
Muscle protein refers to the proteins that make up the muscles in the body. About 40% of the body weight of a healthy human adult weighing about 70 kilograms is muscle, which is composed of about 20% muscle protein.
Muscle proteins can be divided into myofibrillar, regulatory, sarcoplasmic, and stromal proteins. Myofibrillar proteins include actin and myosin, which are the most abundant proteins in muscle and are directly involved in the ability of muscles to contract and relax. Sarcoplasmic proteins include hemoglobin, myoglobin, and a variety of enzymes. Stromal proteins include connective tissue proteins like collagen and elastin.
Muscle protein is important for muscle growth, repair, and regeneration. It can help increase muscle mass, improve bone density, and promote weight loss. It also plays a key role in supporting cells, organs, and tissues, as well as managing hunger and cravings.
You can increase your muscle protein through a combination of diet and exercise. Consuming a sufficient amount of protein in your diet is important, with current dietary guidelines recommending that adults consume 10% to 35% of their calories from protein. Resistance or endurance exercises can also stimulate muscle protein synthesis, leading to increased muscle growth and repair.
