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After strenuous workouts or exercises either aerobic or anaerobic ones  adequate rest and recovery period is needed for the utilized particular muscles. The recovery period has great influence on following bouts of exercises. Individuals, such as athletes, who are engaged in these strenuous workouts, require  2-3 day rest period for the exercised muscles to recover fully. Again, athletes such as swimmers, cyclists, weightlifters, and footballers perform successive sessions of strenuous training to enhance and achieve their training goals and, thus with these athletes it is not atypical to include supplements in their diet. There is usually an increasing demand for these athletes to achieve higher levels of exercise and performance and, hence this increases dietary manipulation of diet through nutritional supplement. Over the last few years, dietary supplements have emerged and have turn out to be the basis for boosting performance (Wu, 2009). One common supplement is glutamine. It is one of the most well known supplements, advertised for athletes as it improves muscle strength and recovery.

Glutamine is a five-carbon amino acid, which is of the essence and found in plenty in a human’s body (Roth, 2008). It is very abundant in human plasma with normal plasma concentration  ranging from 500 to 750 μmol/l. There are two main enzymes used in glutamine metabolism: glutamine synthetase and glutaminase. Glutamine synthetase catalyses the production of glutamine from glutamate and ammonia whereas glutamine catalyses the hydrosis of glutamine to ammonia and glutamate (Vance et al., 2001). Excess glutamine can be broken down into glutamate and ammonia by ammonia and enzyme glutaminase. It can also be converted into other amino acids, used in other reactions, and become part of the body’s protein. Nonetheless, there are other inadequate precursors for the body to make glutamine and the demand for it is high; it has to be taken in, otherwise the body becomes glutamine deficient. The enzyme glutamine synthetase converts glutamate into glutamine.

 Although most organs use glutamine as  fuel, the gastrointestinal tract and the immune system mostly use it. Glutamine accounts for more than 60% of the overall muscular free amino acids and given that skeletal muscle is a huge accumulation of tissues, it is quantitatively the most critical location for synthesis of glutamine in spite of the fact that the activity of glutamine synthetase is considerably lower for every unit mass in skeletal muscles (Welbourne, 1987). The body has the ability to make adequate glutamine for its regular stress; however, in some instances the body usually requires more than it can produce and prompts consumption of common supplements. Glutamine is one of the most critical components in forming the proteins that main cellular health and tissue repair. Physical exercises affect glutamine creation and regulate its uptake.

Strenuous training is often linked with less glutamine levels and immune competence (Ivy, 2007). Strenuous training programs and physical exercises lead to the depletion of glutamine because of reduced production and improved absorption by the immune system and liver cells. Thus, glutamine is indispensable and athletes have to ingest supplements to increase muscular strength and boost the recovery process. During and after a prolonged and strenuous training session, the body has increased levels of glucocorticoid, which are catabolic substances that increase proteins and muscle breakdown. Glutamine has muscle protein-sparing effects and counteracts the actions of glucocorticoid  to some degree. It increases the amount of amino acids released from skeletal muscles, reducing protein degradation and increases the rate of muscle glycogen resynthesis. Glutamine assists body recovery from intense exercise at  faster rate by depleting muscle glycogen stores more quickly. During training, glutamine helps in delaying fatigue by buffering skeletal muscle from metabolic acidosis. It does this through its conversion to ketoglutarate and an ammonium ion. This serves to buffer the PH of the skeletal muscle, because decreasing PH leads to metabolic acidosis and reduces the body ability to perform.

During intense training, there is  catabolism of skeletal muscles to use the amino acids elsewhere in the body. During this process, preserving muscle mass is not usually an option. Administering glutamine to athletes decreases the rate of skeletal muscle. This is particularly important because overtraining and prolonged exercises create the environment of extreme stress in the body and can lead to increase in injuries and muscle breakdown. By supplementing with external glutamine, athletes aid their body in the healing process. It also plays a major function in assisting the immune system function and affects lymphocyte function.

Glutamine is important and fuels immune system cells. Lymphocytes, a type of white blood cells, are dependent on glutamine from skeletal muscle for much of their functioning. Macrophages and monocytes are also a type of cells critical to the immune system that require glutamine to function normally. The behavior of monocytes and lymphocytes in response to certain levels of glutamine shows that glutamine has many actions in the cells that allow them to function properly. Evidence suggests that immune depression results from post exercise glutamine depletion (Castell, 2003). On the other hand, restrained training leads to  increased availability of glutamine because of a good stability between peripheral clearance and muscle synthesis. Lack of physical activity leads to lesser glutamine synthesis and uptake whereas after exercise, there is a reduced glutamine availability, which marks overtraining. Increased availability of glutamine contributes to lesser inflammation as well as health benefits allied to optimal training. This indicates that glutamine supplementation may improve immune competence after strenuous training.

Glutamine regulation is especially important while exercising, both to help fight off infections and to prevent breakdown of muscle. Both aerobic and anaerobic exercises have different effects on the amount of glutamine. Aerobic exercises depleted glutamine to  lesser extent than anaerobic exercise. Most of the evidence regarding glutamine and exercise is circumstantial-glutamine levels being at times lower after exercise and most of the problems people experience are regarded as glutamine deficiency. Glutamine also reduces the content of lactic acid, builds muscle and bolsters up immunity.

Glutamine plays an important function in sustaining a healthy immune system, particularly in people who over train. This is especially true in the case of athletes who tend to over train throughout the year and which frequently causes infections as well a considerable dwindling in performance. Studies have also shown that it assists in production and storage of glycogen, which is that of significance to most endurance athletes and a large amount of endurance oriented anaerobic athletes. It is also well known for its anabolic effect of improving muscle mass and strength; it is highly anabolic and considerably reduces protein degradation and catabolism. Most studies propose that it play an imperative role in enhancing a valuable environment for improved growth hormones levels during strenuous exercise levels.

            The body’s supply of glutamine is principally made and stored in skeletal muscle. Athletes in training and competition use most of glutamine and show  less active immune system. Research has shown that consuming glutamine helps in normalizing the levels of glutamine, although it does not necessarily improve the athletes’ immune function. Numerous studies have failed to show  benefit from glutamine supplementation. Candow et al. (2001) examined the influence of ingested glutamine supplements coupled with resistance exercise on young people. They examined the muscular and strength skeletal markers prior to and after  six-week period in the experimental and placebo group and discovered that there was a slight increase in lean muscle mass, a single repetition squat as well as force production in the knee extensor. The numbers appeared slightly higher than the placebo group, although the difference was not significant. Other studies of the immune cells of exercising athletes have shown that while the glutamine levels can be normal, various measures of immune function including activated white blood cells and immunoglobulin do not normalize. A random, double blinded, placebo controlled research followed the clinical course of athletes given glutamine immediately before and after marathon running. The athletes filled out questionnaires about their health during the seven days after the marathon. Of those who took glutamine, 81% did not have any upper respiratory tract infections during that week whereas 49% of those who got placebo had no respiratory infections.

There are claims for glutamine benefit, which include increased absorption of water and maintenance of a normal acid balance as well as preventing the breakdown of muscle protein. Glutamine helps to boost the synthesis of glycogen immediately after exercise. The area in which glutamine may be useful concerns anabolic processes, meaning improving protein synthesis and glycogen synthesis. There is need for better quality studies on the benefits. There are those who believe that glutamine will help preserve muscle mass and increase growth hormone release in athletes. Most researchers are torn between supplementing glutamine and its positive outcomes; however, there are numerous researches, which provide decisive substantiation concerning the usefulness of taking glutamine supplements. In the research conducted by Welbourne et al. (1996), study participants who took only two grams of glutamine had increased the amount of plasma bicarbonate in their body. Increase in the plasma bicarbonate levels improves a person’s ability to safeguard lactic acid, thus increasing performance.

Salehian et al. (2006) conducted a research on a special muscle regulator known as myostatin, which usually produces glucocorticoid -a substance that assists in determining a person genetic potential for gaining muscle strength and mass. In healthy persons, moderate to high myostatin levels cause glucocorticoid muscle waste away. Salehian et al (2006) discovered that glutamine prevents induced muscle wasting; administering glutamine offers a possible method for preventing muscle wasting stimulated by myostatin and glucocorticoid. The subjects who orally ingested glutamine had considerably less wasting on muscles and body weights as well as lesser myostatin expression compared to the control group. Lacey & Wilmore (1990) found that temporary ingestion of glutamine does not have any effect on muscle strength: nonetheless, continuing supplementation is  more efficient application of glutamine when it comes to gaining strength. Moreover, other research have revealed that glutamine has beneficial effects on athletes’ performance because of a number of physiological incidents that comprise high levels of growth hormones, reduced skeletal muscle catabolic outcomes, enhanced anabolic effects and improved protein production and more capacity to maintain soaring intensity exercises because of the high capacity of safeguarding lactic acid.

Research has also proved that glutamine considerably improves the functions of the immune systems in healthy people and athletes who train at high adequate intensities that usually prompt an immune system breakdown (Perriello et al., 1997). From these studies, it is evident that glutamine should be in the dietary regimen of athletes because it plays a major role in sustaining health through benefits offered to the immune system. The upshots of glutamine on the immune system has also been shown by numerous studies, which have examined records on hospital patients taking glutamine to drive away every possible ailments related to hospital stays and surgeries.

In a study by Oquz et al. (2007), half of the patients going through colorectal surgery for cancer ingested glutamine treatment in their dietary regime during their time in the hospital whereas the other half did not receive. The group that received glutamine supplements has fewer problems after surgery and a shorter hospital stay that the group that did not take glutamine supplements. Other studies also show the significance of glutamine on the immune system function. For instance, Fuentes et al. (2004) found that glutamine supplements improve hospitalized patients morbidity by boosting immune system responses and host defenses. Li et al. (2007) found that intravenous glutamine supplementation helps premature infants to spend lesser time in the hospital and decreased the number of hospital related infections.

Most of these studies on glutamine supplements as well as immune responses are not held on people performing aerobic and anaerobic exercises; however, the findings are of great importance to them. Exercises are in a way being a type of intense stress and causes distress on a human body and this profoundly taxes the immune system. People who undergo extreme training can discover that glutamine has positive effects on the purpose of the immune system. People undertaking strenuous training stand at  risk of infections as mentioned previously; there is evidence that strenuous exercises results partly to the obvious immune-suppression where after such exercise immune system cells have less capacity to produce a defense mechanism. Endurance exercises reduce the plasma glutamine levels, which are an essential fuel for immune system cells. Castell et al. (1996) examined the effects of consuming glutamine after strenuous exercise in elite rowers and runners; they discovered that there was a considerable less illnesses and infections after strenuous training in athletes who ingested glutamine supplements than those who did not.

When it comes to the correct and useful dosage of glutamine supplements, investigations are very diverse. Different researches have shown that success can occur with varied amount of glutamine. Van Gammeren et al. (2002) argued that people should take as low as two grams of glutamine every day. In Welbourne et al. (1996), only two grams of glutamine boosted the amount of plasma bicarbonate in their body.

Other researchers, for example, Candow et al. (2001) provided considerably larger amount of glutamine (45 grams) to their research participants every day. Most of the studies suggest that people take from four to ten grams everyday being adequate for optimum physiological benefits from glutamine supplements. Up to now, other numerous researchers recommend that they take up to twenty grams every day. In research involving hospitalized patients, subjects take more than twenty grams. In Oquz et al research, they received fifty to one hundred and twenty grams every day depending on their body weight. Further evidence would help support the correct and effective amount of dosage.

From the studies, it is evident that glutamine supplements are usually more effective when taken for longer periods rather than shorter ones. Generally the most important benefit includes supporting immune system, lesser wasting of body and muscle, lesser myostatin levels, enhanced levels of hormonal growth, reduced skeletal muscle catabolic upshots, increased protein synthesis, increased anabolic outcomes as well enhanced capacity to maintain high intensity exercise because of increased capability to safeguard lactic acid. Every aspect can potentially cause increased muscle strength, power, and mass.

Objectives of the Study

Numerous studies have shown the importance of glutamine in athletes. However, there are limited studies examining the recovery process as well as the muscle endurance in athletes who supplement with glutamine versus the ones who do not supplement. The objective of the study is

  • To examine the muscle endurance as well as recovery process in two groups of athletes- those who supplement with glutamine versus the ones who do not supplement
  • To conduct a meta-analysis of glutamine and anaerobic and aerobic exercise as the ability of those exercising is limited by slow recovery and repair of muscle, especially after strenuous exercise.

Though there are many factors that contribute to the recovery process, nutrition is the most important; however it is often misunderstood, neglected and surrounded by many mistaken beliefs’. To optimize the performance of the muscles, it is important that appropriate nutrients be taken after exercises. Most of the adaptation for increased muscle recovery and endurance occurs between training sessions. Taking supplements like glutamine during and after exercise bridges the gap between the potential for over exercising and outstanding athletic performance. The muscle cells go through substantial trauma during exercise. The trauma causes discomfort and the need for rebuilding proteins.

Studies have shown that there is  increase of free radicals during exercise, which is also referred to as oxidative stress. The free radicals are mostly responsible for causing damages to the muscle cell membranes. Glutamine plays an important role in the recovery process. It stimulates synthesis of muscle protein and preserves the mass of skeletal muscles. Supplementing the diet with additional glutamine can enhance muscle performance in athletes. However, both aerobic and anaerobic exercises have different effects on the amount of glutamine because aerobic exercises depleted glutamine to  lesser extent than anaerobic exercise. Thus, glutamine is necessary for athletics.

Methods

This study will adopt a descriptive research design that aims at explaining the nature and extent of relationship between the research problem and various variables. This research attempts to obtain a complete and accurate description of the benefit of glutamine. The approach is considered as valuable in assessing and collecting data for this particular study because it is suitable for  in depth study of the benefits of glutamine. This study will be source of information from already published researches on the benefit of glutamine. This will be some information search of all forms of print; the information that is already available on glutamine from peer-reviewed articles will be analyzed to test the objectives of the study. There are considerable studies supporting the helpful aspects of glutamine supplements in reducing muscle loss as well as assisting in the recovery process of athletes who undertake strenuous and prolonged exercises. In recent years, researchers have conducted numerous studies on glutamine supplementation in athletes and they offer a solid rationale for effectiveness of glutamine supplementation in people who perform various athletics.

Results and Discussion

Glutamine is the most essential amino acids in the muscles because it comprises  over 61% of skeletal muscle and 19% nitrogen. Nitrogen is the main transporter of nitrogen amongst the muscle cells. Glutamine is beneficial for its ergogenic effects. It is not an essential amino acid (not required in the diet) since the human body can create it using other amino acids in the body. Skeletal muscles use amino acids from protein catabolism to synthesize glutamine and release it for use elsewhere in the body. Under conditions of trauma, stress, and infection, glutamine is a conditional fundamental amino acid, which supports recovery when consumed in supplemental form.

Glutamine benefits are numerous for athletes. It plays a key function in anti-catabolism, protein metabolism, and volumizing cells. It is also the most abundant amino acid in the human body and it has an imperative function in the process of gaining strength and rebuilding their muscles.

Relationship between Exercise and Plasma Glutamine Concentrations

Intense bout of exercises  cause decrease in plasma glutamine levels and disparities depending on duration, intensity, and type of exercise. Various researchers have discovered that plasma glutamine is increased after a brief period of less than one hour after high intensity exercise (Babij et al., 1983, Erikson et al., 1985). On the other hand, other researchers have observed that after strenuous and prolonged exercises, for instance, full marathon or exhaustive training sessions, there is a considerable reduction in plasma glutamine during and after exercise. In the research by Castell (2002), plasma glutamine of athletes’ performing prolonged and extensive exercises like marathons is decreased. The plasma glutamine concentration of athletes who trained on a treadmill at 50% maximum oxygen consumption (VO2max) for 3.75 h was increased during the early stage, however, it was decreased while the exercise came to the end; it declined below pre-exercise amounts. The reduction of plasma glutamine was relatively short for those running a marathon and it lasted for from 6 to 9 hours. Hiscock et al. (1998) found that the resting fasting plasma glutamine concentration in athletes undertaking diverse sports varies significantly. Cyclists had more distinctly elevated resting plasma glutamine levels than all the other athletes being studied whereas power lifters had the least ones. Parry-Billings et al (1990) observed that athletes who suffered from overtraining syndrome have considerably reduced levels of plasma glutamine that stayed relatively low even after resting for a few weeks. This study was supported by Row bottom et al (1996).

Athletes are usually susceptible to various infections for a few hours, especially after extensive and prolonged training and this is attributed to decreased availability of plasma glutamine in the plasma, which causes immune cells to be challenged. On the contrary, low intensity training is beneficial for the immune system because plasma glutamine levels remain unchanged at the level of exercise.

In case of prolonged exercises, the skeletal muscles fail to provide adequate glutamine because of exercise stress and this is result in hindered state of recovery and performance. Skeletal muscles provide most of the glutamine because it synthesizes and stores it. The amount of glutamine in skeletal muscles is particularly higher compared to that of other amino acids. According to Parry-Billing et al (1990), the high synthetic rate is important for maintaining glutamine homeostasis because skeletal muscles offer most of the glutamine needed by other tissues in the human body.

Plasma glutamine levels also fall significantly after strenuous exercises-post exercise/training. Rennie et al. (1994) observed closely the levels of plasma glutamine for 4 hours and 50 minutes after cycling at 50% VO2max for 3.75 hours. They registered  reduction from 557 mo1/L while resting to 470 mo1/L immediately after the exercises; this was followed by a further fall to 391 mo1/L. The plasma glutamine levels did not return to the resting rate even after 4 hours and 50 minutes of rest; it was at 482 mo1/L. Parry-Billings et al. (1992) recorded a considerable fall in glutamine levels after a marathon event to 495 mo1/L (after race) from 592 mo1/L (before the race). This shows that similar to the changes that take place during exercise, the return of plasma glutamine levels to pre-exercise values depend on the duration and intensity of the exercises. In Decombaz et al (1979) study, the plasma glutamine level has not returned to pre-exercise rates even after 24 hours recovery period. As  result, researchers have suggested that there is need for significant recovery periods between training, especially the high-intensity exercise to enable complete recovery of plasma glutamine levels. The decrease in plasma glutamine results from an increased absorption by the kidneys to safeguard against metabolic acidosis. Acidosis comes about from increased production of lactic acid coupled with strenuous exercises as well as accumulation of various organic acids such as acetoacetate and free fatty acids. It also results from production of ammonia in the kidneys and its discharge into the distal tubes and secretion of surplus proton in the urine; this safeguards against acidosis.

Therefore, it is important that plasma glutamine levels show stability between synthesis and usage by different body tissues and organs. After very prolonged exercises, the reduction in plasma glutamine level results from  increased absorption and demand of glutamine by the body cells. On the other hand, it can occur because of decreased synthesis and/or changed transport kinetics resulting in decreased release of glutamine by the muscles. The decrease in plasma glutamine may also result from two factors: increased absorption of glutamine and reduced synthesis and changed transportation kinetics. Long and strenuous exercises also a source of  increase in plasma cortisol levels, which fuels both catabolism of proteins and release of glutamine and hepatic, gastro intestinal as well as renal gluconeogenesis. When depletion of liver glycogen and the concentration of blood sugar occur, there is an increased rate of gluconeogenesis in the liver from glutamine, glycerol, and alanine-, which places a considerable drain on the availability of plasma glutamine (Nurjhan et al., 1995). The plasma levels of cortisol, glucagon, and growth hormone increase during intense and prolonged exercises. Cortisol and glucagon lead to increased usage of glutamine along with other amino acids by the liver and it allows enlarged usage of glutamine in gluconeogenesis as well as acute-period synthesis of proteins. On the other hand, growth hormone fuels the absorption of glutamine by the kidneys as well as the gut.

Overtraining Syndrome

Overtraining refers to the condition whereby there is stress of the adaptive mechanisms of athletes, which diminishes their capacity to retain a balance between exercise and recovery. Excessive stress with inadequate recovery period is the main cause of overtraining. It usually occurs from sudden increase in training volume coupled with shorter recovery time  between training session. Athletes usually engage in hard training tirelessly to attain better performance without knowing that glutamine levels go back to normal while resting after strenuous training. There is failure to maintain balance between training and recovery. The role of glutamine in overtraining has received much attention from practitioners and researchers over the last twenty years because of its serious threat to athletic performance and health. Numerous researchers have noted considerable reductions in plasma glutamine after strenuous or prolonged training and exercises. The over training syndrome causes under performance and prolonged fatigue in athletes, especially after periods of strenuous training. Kingsbury et al (1998) observed that the plasma glutamine levels are lower in over trained athlete than sedentary individuals and well-trained athletes. Smith & Norris (2000) hypothesized that glutamine concentrations go down when an athlete’s amount of work go beyond his or her ability to tolerate work. Researchers have put forward numerous reasons for reduction of glutamine concentration over time; they include increased glucocorticoids levels, reduced nutritional absorption of proteins, mitochondrial lesions in the skeletal muscles as well as  increase rate of use of glutamine by other tissues.

Glutamine and the Immune System

Glutamine is very abundant in human plasma and muscles. Human beings have a normal glutamine plasma level of 500-to750 µmol/L, which relies on the net stability between its synthesis and absorption by tissues and organs. Rowbottom et al (1996) outline the numerous benefits associate with glutamine, such as transport of nitrogen amongst organs and ammonia detoxification, preservation of acid-base balance when acidosis occurs, serving as a nitrogen precursor during nucleotides production, regulating protein synthesis and degradation, and fueling the immune system and mucosal cells. Regarding the last role, it is widely known that macrophages and lymphocytes use glutamines at very high rates. Glutamine provides energy via its fractional oxidation in glutaminolysis process and offers nitrogen and carbon as precursors of DNA, RNA, and protein synthesis (Wallace & Keast, 1992). Glutamine availability controls critical aspects of the immune function, especially through regulating the biosynthesis of purine and pyrimidine nucleotides. The function of glutamine in acting as a precursor of pyrimidine as well as purine is important for lymphocytes and other cells of the immune system. The importance of glutamine in providing energy and synthesis of nucleotide, which led Parry Billings et al (1990) to hypothesize that  decline in plasma glutamine levels lower than 600 µmol/L, has deadly consequences on the immune system functions. Low glutamine level leads to low synthesis of RNA, low IL-2 production and immunoglobulin synthesis as well as low proliferative responses to lymphocytes mitogens and reduced rate of macrophages phagocytosis. Glutamine exerts its immunological effects through direct action on immune system cells or indirectly through maintaining gut barrier function or preserving antioxidant glutathione (O'Riordain, 1996).

Exhaustive exercises or training has adverse effects on the immune function. These effects consist of reduced natural killer cells cytolytic activity, reduced circulating amount of T lymphocytes for three to four hours after intense training, reduced proportion of CD4 to CD8 cells, and reduced proliferation capacity of lymphocytes and neutrophil activity, weakened antibody synthesis and reduced immunoglobulin levels in saliva and blood. In most athletes, the responses are usually reasonably temporary and last only for a short period although exhaustive and prolonged exercise affects various parameters for a day or two. This means that undertaking strenuous exercise or training sessions within 2 days, such as running a marathon, offers inadequate time for various factors of the immune system to recuperate adequately to function in a normal manner. Various researchers have observed this after prolonged high intensity exercises (Rowbottom et al., 1996) and after a short period of exhaustive training carried out after eight weeks of endurance runners training who were preparing for a major competition (Castell et al., 2000). Exhaustive and prolonged training boosts the amount of white blood cells (leucocytosis). There is also a slow increase in the number of lymphocytes in the circulation when the rest period begins. Nonetheless, their amount is usually decreased afterward to less than pre-exercise levels in 15-30 minutes following exhaustive training. The white blood cells become dehydrated after an exhaustive exercise.

Glutamine in Skeletal Muscle

The human skeletal muscles usually have about 20 mmol/kg wet weight of glutamine. The pace of glutamine synthesis is relatively high compared to that of the other amino acids. It is usually at 50 mmol/h and this high synthetic rate is important in maintaining glutamine store in the muscles and plasma glutamine homeostasis. The skeletal muscle offers most of the glutamine, which other body tissues utilize (Parry-Billings et al., 1990). Glucocorticoid  influences on the synthesis as well as the transport of glutamine. For instance, muscle glutamine synthetase activity is increased after treatment by glucocorticoid. This increased activity mostly occurs during catabolic states.

Glucocorticoid  boosts the production of glutamine from the skeletal muscles (Parry- Billings et al., 1990); it reduces intracellular glutamine accumulation and changes transportation kinetics, enabling maximum glutamine at reduced intracellular glutamine rates. These effects make certain that there is enhanced availability of glutamine from the muscles when catabolism occurs (Row-Bottom et al., 1996). During exhaustive exercises, there is  reduction of muscle glutamine levels. This forces the muscles to enter in the catabolic state in which muscle proteins degradation occurs to offer free glutamine to the other body tissues and organs. Given that skeletal muscles are the key resource for glutamine, prolonged and strenuous exercises lead to deficits in glutamine and this leads to a considerable loss of skeletal muscle mass and proteins. Researchers have also put forward the fact that skeletal muscles play a major role in proper immune functions (Parry-Billings et al., 1990) and, therefore, a failure of the muscle to offer adequate glutamine results in the impairment of immune system functions. Muscular activity affects the rate of glutamine release and, thus exercises directly influence on the immune system. Thus, skeletal muscles play a critical function in glutamine metabolism as well as immune functions.

Glutamine Supplementation

In recent years, researchers have carried out various researches on glutamine supplementation in athletes. A strong rationale exists for the effectiveness of glutamine supplementation in athletes, for instance, researches have shown that glutamine function in immune system support prevents infection after exhaustive sessions of physical exercises, which tends to decrease the amount glutamine in the plasma (Smith & Norris, 2000: Castell & Newsholme, 1997). Glutamine supplements also have a major part in working against stress hormones like cortisol, which causes muscle wasting, or catabolism and which prolonged and strenuous exercise increases. Glutamine supplements help in reducing protein or muscle tissue breakdown, enhanced lymphocyte function, and reduced infections (Newsholme, 2001).

The role of glutamine in fueling glycogen synthase, the enzyme that manages the production and storing of glycogen fuel storage in the liver and muscles, offers a means via which glutamine supplementation promotes increased fuel storage. Glutamine also increases cells volume and stimulates enzyme activities, which take part in glycogen storage and those concerned in anabolic activities like protein synthesis in the liver and muscles. Researchers have also hypothesized that glutamine supplementation increases the amount of growth hormones, which stimulates synthesis of proteins and encourages growth in strength and mass.

Glutamine supplementation has a beneficial effect for individuals engaged in intense and chronic exercises. Exercise training boosts the requirement for glutamine necessitating external self-administration for ultimate performance and recovery (Antonio & Street, 1999). Glutamine plays an imperative part in the immune system functioning and its cells, the supplements, thus lessen or prevent the severity of infection or illness after an intense bout of exercise; this enables athletes to carry on intense training. In addition, the supplements also offset catabolic effects of increased glucocorticoid levels generated during intense training/ exercise. In addition, supplementation fuels other organs such as immune cells, kidney, and liver and this spares the potential loss of glutamine because of insufficient dietary intake; this spares muscle proteins (Antonio & Street, 1999).

Supplementation also influences on the acid-base balance in the body through generating a safeguarding effect. Glutamine changes the acid-base balance through increasing the retention of plasma bicarbonate (HCO3) in the kidneys. This process takes place through deamination, when glutamine enters the kidneys epithelial cells. The ammonium formed from glutamine deamination then binds with  H to form an ammonium ion. 

Antonio et al. (2002) carried out an investigation to determine whether ingesting glutamine affects weight lifting performance. They used a placebo-controlled, double blind cross over study whereby six resistances trained men completed weightlifting exercises after ingesting glutamine supplements together with placebo- a fruit juice that was calorie free. The research subjects performed four full exercise sets to temporary failure one hour after ingestion. Their study findings showed that short-term consumption of glutamine did not improve the resistance-trained men weightlifting performance. This study is comparable to Haub et al (1998) study, which investigated the possibility of glutamine supplements to change the blood acid-base balance and as  result boost the time to exhaustion when undertaking strenuous exercises. This research had the basis on the theory that glutamine changes the acid-base through boosting retention of plasma bicarbonate in the kidneys. The research subjects performed five sessions of exercises on a cycle ergometer at 100% VO2peak. The first four sessions lasted one minute whereas the last session continued to fatigue. The exercise sessions started one and a half hours after taking 0.03-g.kg body mass of either placebo or glutamine. There were not any considerable variation in plasma bicarbonate, PH, and lactate concentration between pre-exercise, pre-ingestion, bout five and bout four. The time to fatigue was also not considerably different between conditions.

Generally, the data being obtained showed that acute glutamine ingestion did not improve either exercise performance or buffering potential in the research subjects. Nonetheless, researchers have suggested that an environment should be acidic to set off glutaminase and eventually utilize glutamine to boost plasma bicarbonate. Piatolly et al (2004) researched the effects of glutamine supplement on recovery from intense exercises among elite cyclists. They discovered that the cyclists in the glutamine group took long before they got exhausted after ingesting the supplements for six days and they recovered from exhaustive exercise before the placebo group-cyclists who did not take glutamine. Castell et al. (1996) examined a likely prophylactic effect or oral glutamine supplements on infection occurrence. They found out that providing two-glutamine drink in the first two hours after the race reduced infection incidences in the week that following the event. Similarly, Rohde et al (1998) discovered that a glutamine solution given at particular time intervals (i.e. 0, 30, 60, and 90 min) after the marathon race prevents reduction in plasma glutamine concentration. 

Benefits of glutamine to athletes

Glutamine is particularly important in the muscle growth process. It preserves the already built muscle tissues rather than directly promoting the growth of new muscle tissues. Muscle breakdown occurs frequently during strenuous training and in case when the body does not receive adequate protein and while resting. The process of muscle breakdown is very natural. Glutamine assists by reducing the rate of muscle breakdown and this result in increased overall net gains in muscle mass.

During prolonged or strenuous exercise, plasma glutamine levels are decreased by as much as 34-50% (Don Santos et al., 2009). Don Santos et al used rats to determine the effects of exhaustive exercises on glutamine production and transportation in skeletal muscles after a 24-hour exercise. They discovered that after the 24 hr exercise, lower glutamine synthetase contributed to the decrease in muscle glutamine concentration. This is comparable to Borgenvik et al study. Borgenvik et al (2012) evaluated alterations in muscle and plasma intensities of free amino acids,  particularly in endurance of exercises and after recovery. Nine athletes took part in one-day standard endurance trial together with controlled energy consumption and they took part in twelve sessions of kayaking, cycling, and running. The researchers took blood samples prior, during, after the exercise, and after twenty-eight hours of rest. They also took muscle biopsies before, after the exercises, and after the recovery period. They discovered that ultra-endurance exercises cause considerable change increases in muscle and plasma amount of phenylalanine and tyrosine; this implied an increased net muscle protein breakdown at the time of the exercise. During the exercise, there were reduced plasma concentration and glutamine levels whereas after the exercise, there were not changes in the muscle concentration.

 Various immune system cells such as white blood cells, macrophages, and lymphocytes rely on glutamine as the main source of fuel. These cells usually attempt to draw their glutamine from muscle tissue but the supply is normally poorer than the demand. This leads to a temporary suppression of the immune system. Most athletes experience an increased rate of infections, especially those related to the upper respiratory tracts like flu and common cold. In case that an athlete does not replenish enough glutamine after the workout, it leads to the suppression of the immune system function for a longer period and this leads to more incidences of infections as well as wound healing

During intense exercises, muscle tissues require a lot of glutamine to prevent muscle breakdown. For this reason, the skeletal muscle activities control the immune system directly. In case of overtraining, for example, when the intensity and frequency of training is disproportionately increased and not balanced with enough recover periods, there is extreme glutamine depletion together with weakening of the immune system. The athlete who over train do not replenish the glutamine levels to the pre-training levels and further exercises lower glutamine levels in the body leading to muscle loss as well as a weakened immune system. Increase in the catabolic cortisol hormone or stress hormone, fatigue, poor performance, depression, nausea and more prolonged illnesses is characterized by the over training syndrome. Thus, this warrants glutamine supplementation for athletes who take part in prolonged and intense activities. This allows them to maintain and improve performance and health, increase fitness and train intensely without down periods of sickness.

Glutamine has a powerful anti-catabolic effect which prevents muscle breakdown; the effect powerfully reduces muscle breakdown; it exerts this in various ways. Primarily by neutralizing and attenuating catabolic effects of cortisol. The levels of cortisol hormones are increased considerably during stressful periods. Cortisol levels are risen greatly as the workout progresses and this simultaneously increases the stress on the working muscles; they are particularly high in the end of the strenuous workout. Hill et al (2008) discovered that high intensity exercises provoke increase in circulating cortisol levels. Cortisol increases muscle breakdown for acquiring glutamine as well as other amino acids for energy use evidenced by  increase in the glutamine synthetase enzyme needed for glutamine synthesis. Glutamine supplement increases glutamine concentration in the muscle cells inhibiting the glutamine synthetase enzyme and efficiently neutralizing or reducing muscle breakdown induced by cortisol.

Glutamine also spares muscle breakdown. Its supplements do this in two ways: maintaining glutamine concentration in the skeletal muscles, thus, inhibiting production of glutamine synthetase and preventing muscle breakdown for glutamine. The second way entails increasing the levels of glutamine in the blood, which helps to meet the demand for glutamine by other cells and tissue such as immune system and small intestine, and again prevents muscle breakdown for releasing glutamine. Studies show that glutamine supplements minimize the breakdown of muscles and improve metabolism of proteins. In their research, Bonetto et al (2010) demonstrated that glutamine supplementation efficiently stops induced loss of proteins and reinstates the normal myostatin amounts. In addition, the supplements exert a protective effect that represents a possible strategy for improving muscle mass. Enhanced protein breakdown from activation of proteolytic systems causes depletion of skeletal muscle protein.

Glutamine also acts as an anti-catabolic by preventing down-regulation of synthesis of the myosin heavy chain, one of the muscle contractile proteins and sparing them from breakdown preventing muscle atrophy and muscle wasting. The anabolic effect allows enhanced protein synthesis by cell hydration. Glutamine supplements contribute to an anabolic effect. It acts as a cell hydrating/cell volumizing agent through drawing water into the muscle cells and increasing their cell capacity. This serves as a metabolic signal for cellular anabolism, promoting  increase in protein and glycogen synthesis. Alternatively, loss of water from the muscle calls causes cell shrinkage, which signals cellular catabolism. This shows the importance of maintaining good hydration status for greater catabolism.

Depletion of glutamine levels during intense training decreases stamina, strength, and recovery. It takes days for glutamine levels to normalize in cases where athletes do not take any supplements. The supplements help in glucose regulation and glycogen formation. They serve as an antecedent of glucose because its carbon skeleton synthesizes glucose. Normally, the insulin hormone serves to decrease blood glucose while the glucagon hormone increases blood glucose. Thus, the body requires a balance of the two hormones to maintain normal blood glucose levels. Nonetheless, glutamine helps to maintain blood glucose levels by glucose synthesis (gluconeogenesis) irrespective of the insulin-glucagon ratio. . Studies have shown that glucogenesis usually increases seven times after glutamine supplementation and there is an increase in muscle glycogen after consuming glutamine after the exercise. It also promotes glycogen synthesis in the skeletal muscle partly because of its cell hydrating effect. Therefore, the general role of glutamine in glucose formation and regulation is increasing glycogen breakdown in the liver independent of any signal from glucagon, increasing blood glucose because of the glycogen breakdown and glucose synthesis and increasing stores of muscle glycogen even in situations where there are low insulin levels. According to Roth (2008), administering glucose has a positive effect on glucose metabolism when it comes to insulin resistance. As  result, it assists athletes recover quickly  between workouts and prevents them from getting sick.

Glutamine also has elevated growth hormone; it increases the levels of growth hormone which then  increases the growth of muscles. It also increases glutathione (GSH), a strong antioxidant in the body. Glutathione is a strong antioxidant defense in the body and comprises  three amino acids: glutamate, glycine and cysteine (Izaki et al., 2008). Glutamine increases the synthesis of glutathione, which protects tissues from free radical attach and further contributed to reducing catabolism. Glutathione also assists in enhancing the immune function (Izaki et al., 2008).

Glutamine also has an active part in healing and recovery. It not only acts in accelerating recovery of muscles after a strenuous training but also plays an active role in recovery from trauma and healing of wounds. It also increases the athletes’ capacity to produce human growth hormones, which assist in metabolizing body fat and supporting growth of new muscles. Athletes with muscle wasting are at times not capable of creating their own glutamine supplies and, thus, benefit from glutamine supplement consumed together with other amino acids. Glutamine is versatile and has a range of associated functions making it one of the most vital amino acids in a human body. Ensuring enough glutamine stores in the body is crucial for athlete for muscle gain results. Thus, supplementation is essential.

Since exercise is a form of increased metabolic stress, it depleted glutamine from an athlete’s body. The depletion rate is dependent on the intensity and length of exercise. Low glutamine levels cause damaging effects on the body, however, supplementation wards of the effects in numerous ways as discussed above. In addition, glutamine has an anti-inflammatory effect, which reduces muscle tissue inflammation after strenuous exercise. This assists in reducing swelling and delaying onset muscle soreness after a workout and helps in proper recovery.

In summary, glutamine is the amino acid that is critical for numerous homeostatic roles and for the operation of various tissues in the human body. Various catabolic states such as infection and acidosis glutamine homeostasis come under stress and cause depletion of glutamine reserves. Strenuous and prolonged exercises result in  decrease, increase or no variation in the levels of plasma glutamine during and after exercise. The resultant changes depend on the type, intensity, and durations of exercises being performed. Based on the previous human research studies performed on glutamine supplements, some researchers believe that glutamine has a possible effectiveness as a dietary supplement for athletes’ performing prolonged and high intensity exercises. This arises from the fact that glutamine has numerous benefits primarily due to its positive influence on the acid-base balance in the body. The aim of this research was to establish the outcomes of glutamine supplements on athletes’ maximum performance as well as their recovery from prolonged and high intensity training. The supplements help in preserving muscle mass, reducing catabolism after exercise (breakdown of muscle tissues), and hastening recovery from exhaustive training. High strenuous training leads to a well-described reduction in plasma glutamine levels. Some of the studies being reviewed implicate persistently low levels of glutamine as  likely contributing aspect in transient immune suppression and greater than before risk of illnesses, which usually affect athletes during intensive competitions and training. Under metabolic stress conditions, the body requirement for glutamine becomes conditionally important as the body does not create adequate levels; dietary supplements are essential in preventing skeletal muscle catabolism, skeletal muscles being the main source of stored up glutamine in the human body. For athletes who have increased stress levels, such as the ones recuperating from exhaustive exercise, glutamine supplementation is a good way of promoting repairing of tissues, reducing muscle catabolism, as well as helping in prevention of infections. The scientific literature reviewed supports the valuable outcomes of glutamine supplementation in preserving muscle mass and assisting the recovery process of the athletes who are engaged in prolonged and strenuous exercises. 

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