Types of Muscles
Discuss about the Nutritional Regulation of Muscle Protein.
The muscular system is made up of several tissues all of which are tasked with performing specific functions. The major muscles of the body consist of the pectoralis major, rectus abdominis, rectus femoris, gastrocnemius, erector spinae, and bicep femoris among others. In most cases, when one talks about the muscular system, then the first structures to come to mind usually are the skeletal muscle tissues. The skeletal muscle tissues are optimized to contract and move the parts of the body.1 They are generally associated with the parts of the muscular system which are under the conscious control of humans.
There are three main types of muscles that include the cardiac muscles, smooth muscles, and the skeletal muscles. The cardiac muscle is solely located in the walls of the heart. The cardiac muscle is usually under the control of the automatic nervous system. It has a high number of mitochondria and a good supply of blood that ensures continuous aerobic respiration and they are therefore resistant to fatigue.1 The smooth muscle, on the other hand, is found in the walls of organs such as the bronchi, stomach, esophagus, and walls of blood vessels. The skeletal muscles attach to the bones and mainly functions to facilitate movement through contractions.
According to Gillies and Lieber,2 the primary functions of the skeletal muscle include movement, support or posture, and the production of heat. The muscles frequently work in groups to ensure the proper functioning of the body. For muscle contraction to be initiated so that the muscles can perform their intended tasks, cellular respiration is necessary, and it involves several metabolic pathways to obtain ATP from the molecules that are rich in energy. In this essay, we will talk about the key factors the mass of the skeletal muscle in the body. We ill additionally talk about the role played by nutrition in regulating the mass of skeletal muscle.
The maintenance of the mass of the skeletal muscle strongly depends on the balance between the rates at which proteins are synthesized and degraded. Different studies in recent times have revealed that cytokines, hormones, growth factors, nutrients, and mechanical loading have the capability of activating the signaling pathways which can help in pushing the balance in favor of the degradation or synthesis of proteins.3 As a result, the will be a loss or gain in the mass of skeletal muscle. The two main signaling pathways are anabolic and catabolic pathways.
Functions of Skeletal Muscle
Anabolic pathways comprise two factors which are translational capacity and translational efficiency. Translational efficiency can be described as the synthesis of proteins per unit amount of Ribonucleic acid. Translational capacity, on the other hand, is described as the total contents of ribosomes per unit tissue. Most studies have normally focused on the description of the pathways that enhances translational efficiency.5 As evidenced by several studies, enhanced translational efficiency takes place over a considerably short frame of time. It is also normally viewed independently of any variations in the cell’s translational capacity. Currently, mammalian target of rapamycin (mTOR) is used as the primary hub for the integration of an array of upstream signaling pathways that result in increased translational efficiency if activated.4
Catabolic pathways are responsible for the atrophy of skeletal muscle that occurs when the rate at which protein is degraded is greater than the rate at which it is synthesized. The loss of the mass skeletal muscles can be witnessed through various factors that may include disuse, aging, and various diseases such as AIDS, sepsis, diabetes, and cancer among others.6 It is also important to note that the proteolytic systems that are involved in the in atrophy of the skeletal muscles respond to several triggers including disuse, hormones, growth factors, oxidative stress, and inflammatory cytokines among others.
It has also been revealed by several studies that factors such as fatty acids which are nutrient modulated may act differently to regulate the mass of the skeletal muscle. This can be shown by exposing C2C12 myotubes to palmitate. Palmitate is the most abundant saturated fatty acid that is in circulation. This exposure reduces the diameter of the myotube thus suppressing insulin signaling.6 The impairment of the signaling may lead to the loss of muscle mass.
Nutrition plays an essential role in the regulation of the skeletal muscle mass. The maintenance of the mass of the skeletal muscle is described by a perfect balance between protein degradation and protein synthesis.7 There is an increase in skeletal muscle if a gain exists during protein synthesis. This gain occurs typically during regular exercises. A loss of the skeletal muscle, on the other hand, occurs when degradation of proteins is more rapid than synthesis.7-8 We will, therefore, focus on the type of nutrients that should be ingested to aid in regulating skeletal, muscle mass, the amount to be ingested, the timing of the ingestion of the nutrients, any other nutritional considerations, and the potential limitations.
Anabolic and Catabolic Pathways
Some of the nutritional determinants of skeletal muscle mass are dietary energy and intake of proteins. According to several studies, it is essential to consume sufficient amounts of dietary energy and proteins to maintain the skeletal muscle mass. The activation of muscle protein synthesis is determined by the quantity of dietary protein that an individual consumes.6 It is worth noting that taking diets that are high in protein content helps in attenuating decrements in the muscle protein synthesis. As a result, the skeletal muscle mass is controlled during a deficit of energy. Researchers have ascertained that controlling the mass of skeletal muscles is very crucial in ensuring that one lives a healthy life throughout their lifespan. Ingesting a dietary protein that is way above the recommended dietary allowance in moments of energy balance enhances the retention of nitrogen which may, in turn, up-regulate muscle protein synthesis.9 This results in the promotion of a favorable balance of protein and accretion of skeletal muscle.
The consumption of high protein diets that range between 1.6-2.4 g/kg on a daily basis or meals based on protein during energy deficit periods is reported to attenuate intracellular proteolysis. Additionally, it restores the synthesis of muscle protein and mitigates the loss of skeletal muscle mass. The United States recommended dietary allowance suggests that an individual should consume 0.8g/kg of protein daily to maintain the skeletal muscle mass.10 It is, however, worth noting that the protein requirements increase above the recommended dietary allowance during increased energy demands. These requirements increase to sustain the retention of protein and maintain the mass of the skeletal muscle.11 The recommendations also suggest that the physically active individuals such as athletes and those who take part in aerobics should consume 1.2-1.7 g protein/kg on a daily basis. The military personnel, on the other hand, need to consume a diet that provides around 1.5-2.0 g/kg of protein every day. This group of people is none to take part in combat operations and very challenging metabolic training and thus needs this amount of protein daily to aid in the repair the damaged protein and the synthesis of new muscle proteins thus maintaining the skeletal muscle mass.12 This is one of the reasons why high protein diets are gaining new popularity among the physically active individuals.
As had been previously stated, the quantity of dietary protein ingested determines the activation of muscle protein synthesis.6 Studies reveal an existing dose-dependent relationship between dietary protein and muscle protein synthesis. Therefore, consuming a meal that contains around 0.25-0.30 g/kg maximally stimulates postprandial muscle protein synthesis for around 2 hours.12 Consuming high doses of proteins leads to an increase in the oxidation of proteins with no additional anabolic stimulus.
Protein Synthesis and Degradation
The timing of nutrients intake is a very important factor in muscle hypertrophy. A person who exercises frequently needs to practice nutrient timing to ensure that they can appropriately regulate their skeletal muscle mass. Studies reveal that timing of the nutrients ingestion can shift hormonal profile, up-regulate metabolism, or alter the composition of the body.13 The manipulation of nutrient intake also helps an individual to take advantage of some anabolic hormones like insulin. It is important to note that nutrient timing helps the body to prioritize in muscle gain rather than fat gain.13
Pieces of evidence on the timing of milk consumption reveal that consuming milk moments after work-outs is essential in enhancing muscle hypertrophy. Milk proteins are very fundamental in the development of skeletal muscles.14 The timing of essential amino acids is also crucial in protein synthesis that helps in the regulation of skeletal muscle mass. Studies indicate that, consuming essential amino acids moments before a resistance exercise enhances protein synthesis. It is therefore important to have an appropriate personal schedule that helps in reminding one about the appropriate time of the consumption of nutrients.14 Such a schedule helps in ensuring effective absorption of the nutrients by the skeletal muscles.
It is important to maintain energy balance in the body to regulate the skeletal muscle mass. Ingesting inadequate energy is known to accelerate the loss of muscles during disuse massively. Additionally, excess energy results in the deposition of fats that may lead to unnecessary weight gain.15 Other nutritional considerations that can be used to regulate the skeletal muscle mass includes the ingestion of carbohydrates. Special attention is given to glucose which is widely reported to be a precursor for glycogen re-synthesis. Ingestion of carbohydrates increases the storage of glycogen above that of water. Reports also reveal that ingesting 6 to 12 g carbohydrates/ kg daily, is enough to restore the glycogen reserves provided that the recovery time is more significant than 24 hours.
It is widely known that glucose significantly stimulates the pancreas to release insulin. However, protein ingestion by healthy individuals also stimulates insulin secretion. An evidence-based research report that the level of muscle glycogen re-synthesis rises when there is a protein co-ingestion with carbohydrates.15 This co-ingestion will have positive effects on the regulation of skeletal muscle mass.
The proposals and recommendations as mentioned above, however, have some limitations, especially in injured individuals. Managing the balance between the conservation of skeletal muscle mass and the prevention of body fat accrual in injured athletes are some of the main challenges faced by general practitioners.16 Injuries lead to a reduction in physical activities and the general energy requirements which interferes with the regulation of skeletal muscle mass. This is because, during injuries, the athlete experiences disuse and thus inadequate intake of dietary energy may lead to the loss of skeletal muscle mass. However, the consumption of excess energy also leads to aft deposition that may lead to weight gain. These recommendations, therefore, pose a challenge to the injured athletes due to the difficulty of balancing between the conservation of muscle mass and fat accrual.
Nutrition and Skeletal Muscle Mass
The skeletal muscles have tissues that are optimized to contract and enhance the movement of the body parts. Its main functions include movement, support, and heat production. The maintenance of the mass of the skeletal muscle strongly depends on the balance between the rates at which proteins are synthesized and degraded. Some of the nutritional determinants of skeletal muscle mass are dietary energy and intake of proteins. It is essential to consume sufficient amounts of dietary energy and proteins to maintain the skeletal muscle mass. It is important to note that the quantity of dietary protein ingested determines the activation of muscle protein synthesis. The timing of nutrients intake is key in regulating the skeletal muscle mass.
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