Glutamine is the most abundant amino acid in human plasma and in the muscle free amino acid pool (Wagenmakers, 2000). It is a non-essential amino acid, which can be obtained from dietary intake and is constantly produced by the body. Since 50-60% of the glutamine that enters the body via protein in diet is utilized by the intestine, glutamine synthesis by the body is important (Newsholme & Castell, 2000). The major sites of glutamine synthesis in the body have been identified as primarily the skeletal muscle, but also the lungs, liver, brain and possibly the adipose tissue (see Rowbottom et al, 1996 for review). At the same time of synthesis, glutamine is utilized by various organ and tissues of the body, for example, gastrointestinal tract, liver and kidney. Glutamine balance is dependent upon the net balance between glutamine utilization and release.

Glutamine affects the immune system because it is used as an oxidative fuel, together with glucose, for energy production. It is also involved in protein synthesis by providing intermediates for synthesis of purine and pyrimidine nucleotides which are essential for DNA and RNA synthesis. Therefore, it is important, if not essential, for lymphocyte proliferation and macrophage phagocytic activity (Bishop et al., 1999).

Effect of exercise on glutamine concentration

Normally, plasma glutamine level in human is about 600-700μM (Wagenmakers, 2000). Studies have demonstrated that exercise, a physiological stress, alters glutamine level within the body. In athletes, resting plasma glutamine concentration may vary between sports due to different metabolic demands and dietary factors of the different sports (Hiscock & Mackinnon, 1998). Generally, it is observed that different modes, intensity and duration of exercise seem to have different effects on plasma glutamine level. In this paper, exercises are classified into four groups, which are prolonged exercise, shorter duration endurance exercise, high intensity intermittent exercise, and eccentric exercise respectively. Since different methodologies were employed, absolute glutamine concentration values across individual studies cannot be compared directly (Rowbottom et al, 1996).

Prolonged exercise

Studies have demonstrated a decrease in plasma glutamine concentration after prolonged exercise, for example, marathon, triathlon and long distance running. Parry-Billings et al. (1992) reported that plasma glutamine concentration decreased following a marathon. Similarly, Castell et al. (1996) found a decrease in plasma glutamine concentration immediate and one hour after a marathon, which returned to pre-exercise level 16 hours after exercise. Besides running, Rohde et al. (1996) found a decline in serum glutamine concentration after a triathlon (swam 2500m, bicycled 81km and ran 19km). Castell, Poortmans and Newsholme (1996) also reported a decrease in plasma glutamine level after marathon running (20%) in well-trained runners, 15-mile training in mid-distance runners, and 10km race in rowers (12-15% for training and race).

There are experimental studies examining the effect of prolonged exercise on plasma glutamine concentration. Robin et al (1999) found that plasma glutamine fell significantly during the recovery period after cycling at 55% VO₂max for up to 3 hours. In a study of cycling for 60, 45 and 30 minutes at 75% VO₂max separated by 2 hours of rest, the arterial plasma glutamine concentration declined 2 hours after exercise (Rohde, Maclean & Pedersen, 1998). Plasma glutamine concentration was shown to fall below resting values during recovery from cycling at 70% VO₂max for 60 minutes (Gleeson et al., 1998) and after a 60-minute ride at 75% VO₂max (Mitchell et al., 1998). Recently, a study of cycling at 60% VO₂max for 2 hours has demonstrated a significantly lower plasma glutamine concentration measured at 80 minutes post-exercise (Walsh et al., 2000). It should be noted that most studies employed cycling as the exercise mode. Thus, such response of glutamine concentration to prolonged exercise may only correspond to cycling since different sports have different metabolic demands as discussed above (Hiscock & Mackinnon, 1998).

In general, there is a decrease in plasma glutamine concentration after prolonged exercise that lasts for at least 60 minutes. This decline in plasma glutamine concentration seems to last during the recovery period up to 16 hours before returning to pre-exercise level. Yet, not much research has been looking at the whole recovery period of the decreased plasma glutamine concentration.

Shorter duration endurance exercise

Despite the decreased plasma glutamine concentration after prolonged exercise, plasma glutamine concentration does not seem to fall in response to endurance exercise at similar intensity but shorter duration. Subjects cycling at 80% VO₂max, resulting in fatigue with one hour, did not show a decline in plasma glutamine after exercise (Robson et al., 1999). Neither change in plasma glutamine concentration was found after running 30km nor cycling to exhaustion at 73% VO₂max (Parry-Billings et al., 1992).

High intensity, intermittent exercise

In high intensity, intermittent, or sprinting exercise, there has not been an agreement on how exercise affects plasma glutamine concentration. Data from limited studies have demonstrated inconsistent results. Parry-Billings et al. (1992) reported that plasma glutamine concentration increased following ten 6-s running sprints with 30s recovery between trials. In a later study, Walsh et al. (1998) found that plasma glutamine concentration did not change in the immediate post-exercise period but fell 5 hours after sprinting cycle exercise. More recently, it was shown that plasma glutamine concentration decreased 15 minutes after cessation of cycle blocks sprinting exercise (Hall et al., 2000). The advance in methodology of measuring glutamine level in recent years (Rowbottom et al, 1996) and the different protocol employed in each study may partly account for the varied results.

Eccentric exercise

Some researchers are interested in the effect of eccentric exercise and muscle damage on plasma glutamine concentration. Gleeson et al. (1998) found that exercise-induced muscle damage does not produce changes in plasma glutamine concentration after 20 repetitions of electrically stimulated eccentric muscle actions. In contrast to their finding, Miles et al. (1999) demonstrated a temporary decrease in blood glutamine concentration in response to high-force eccentric exercise. More research is needed before a conclusion could be made.

In summary, it has been shown that plasma glutamine was decreased after prolonged exercise, while the effects of shorter duration endurance exercise, high intensity intermittent exercise and eccentric exercise are not well understood. It is suggested that the increased utilization of glutamine by other organs can account for the decreased plasma glutamine level after exercise (Rowbottom et al, 1996). Gleeson et al. (1998) further summarize four possible factor for the fall in plasma glutamine concentration after exercise: 1) increased renal uptake to buffer hydrogen ion during acidosis and thus maintain acid-base balance; 2) decreased release from skeletal muscle; 3) increased uptake by the liver for hepatic gluconeogenesis in conditions of reduced carbohydrate availability; and 4) increased uptake by the growing number of circulating leukocytes after exercise. Indeed, the mechanism underlying the exercise-induced reduction in plasma glutamine concentration is not fully understood.

Impact of decreased glutamine concentration on immune function

Epidemiological datasuggest that endurance athletes are at increased risk for upperrespiratory tract infection (URTI) during periods of heavy training and1-2 week after major competitions (see Mackinnon, 2000; Gleeson & Bishop, 1999 for review). Although increased incidence of infection in athletes may be partly attributable to increased exposure and facilitated pathogen transmission, the literature generally supports the idea of immunological dysfunction in athletes (Gleeson & Bishop, 1999).

Nieman (1997) reviewed that there is growingevidence that, for several hours subsequent to heavy exertion,several components of both the innate (e.g., natural killer (NK) cellactivity and neutrophil oxidative burst activity) and adaptive(e.g., T- and B-lymphocyte function) immune system exhibit suppressedfunction. At the same time, plasma pro- and anti-inflammatorycytokines are elevated. During this ‘open window’ of altered immunity (which may last between 3 and 72 hours, depending on the immune parameter measured as well as the type, duration and intensity of exercise), viruses and bacteria may gain a foothold, increasing the risk of infection (see Nieman & Pedersen, 1999 for review).

Various mechanisms explaining the altered immunityhave been explored, including hormone-induced trafficking of immunecells and the direct influence of stress hormones, prostaglandin-E₂,cytokines, and other factors (see Nieman, 1997 for review). Since cells of the immune system rely on the supply of glutamine in the plasma to meet their metabolic needs, the exercise-induced reduction in plasma glutamine levels have also been suggested to cause impairment of the immune function, and thus an increased risk of infection.

Bailey et al. (1998) carried a study of moderate altitude training in elite distance runners. They reported a 50% increase in the frequency of upper respiratory and gastrointestinal tract infections during the altitude training, and that group mean resting plasma glutamine concentration decreased by 19% after 3 weeks at altitude. The authors concluded that such a decrease in plasma glutamine concentration might have been implicated in the increased incidence of infectious illness.

Despite the finding that glutamine has no effect on the rate of T-lymphocyte proliferation in vitro, Parry-Billings et al (1992) hypothesized that cells of the immune system are not impaired when measured in vitro, but such a decrease in plasma glutamine concentration may contribute to the impairment of immune function in vivo.

There is a positive relationship between lymphokine activated killer (LAK) cell activity and serum glutamine concentration (Rohde et al., 1996). However, the corresponding mechanism and the impact of decreased glutamine concentration on the immune system in vivo is not yet known.

Furthermore, the acute effect of exercise on plasma glutamine concentration may be cumulative, since the plasma glutamine concentration is reported to be lower in overtrained compared with well trained athletes and sedentary individuals (for example, Hiscock & Mackinnon, 1998). It is, therefore, suggested that overtrained athletes and those with chronic fatigue may be in a state of constant glutamine debt, creating a disadvantageous scenario for the function of lymphocytes and macrophages, thus rendering the athlete more susceptible to opportunistic infections (see Bishop et al, 1999 for review). However, there is no direct evidence to date that demonstrates the link between decreased glutamine concentration and immunosuppression (Nieman, 1997; Gleeson et al., 1998; Mitchell et al., 1998; Walsh et al., 1998). On the other hand, the decreased plasma glutamine concentration in overtrained athlete has suggested plasma glutamine as a potential blood marker of overtraining (Mckenzie, 1999).

Effect of glutamine supplementation on immune function

Although the precise role of glutamine in the immunosuppression of athletes is not completely understood, it seems plausible that maintenance of normal plasma glutamine levels is essential for normal function of lymphocytes and macrophages. The 3-4 hours after prolonged, exhaustive exercise creates an opportunity for apparent immunosuppression to occur, yet the increase in number of infections in athletes appears to be modified by glutamine feeding (Newsholme & Castell, 2000). Glutamine supplementation may therefore exert a beneficial effect for individuals engaged in chronic and intense exercise training.

Antonio & Street (1999) suggested a theoretical basis for glutamine supplementation in exercising individuals, which hypothesized that glutamine supplementation may prevent or lessen the severity of illness after an intense bout of exercise, thus enabling the athlete to resume intense training more quickly. They strongly believed that the benefits of exogenous glutamine supplementation shown in clinical situations could also be duplicated in the athletic population.

Castell et al (1996) examined athletes who had consumed glutamine versus a placebo immediately after and 2 hours after a marathon or an ultra-marathon. The glutamine group reported greater percentage of no infection than the placebo group (80.8 ± 4.2% and 48.8 ± 7.4% respectively). The authors explained this effect of glutamine by the fact that its provision after prolonged exercise might restore physiological levels: thus, more glutamine would be available for key cells of the immune system at a critical time for induction of infection.

Parry-Billings et al. (1992) studied the effect of branched chain amino acid (BCAA) supplementation on exercise by providing athletes with drinks containing either BCAA or a placebo during a marathon race. They showed that BCAA supplementation prevents the decrease in plasma glutamine concentration after the race. The authors suggest that BCAA supplementation may be important in maintaining the functions of the immune system following single bouts of prolonged exercise and during periods of hard training or overtraining.

However, not all studies show a positive effect of glutamine supplementation after intense exercise. Shewchuk et al. (1997) studied the effect of exercise and L-glutamine supplementation on lymphocyte metabolism and function in rats. This study demonstrates no effect of the low intensity exercise training or dietary L-glutamine supplementation on plasma glutamine concentration or splenocyte metabolism. Glutamine supplementation even reduced splenocyte NK activity in exercise-trained rats. The physiological significance of these observations is not clear however. The authors suggested that for those who are participating in regular low-intensity exercise, it is unlikely that dietary glutamine supplementation is required to support immune function.

In human, effect of glutamine supplementation on changes in the immune system induced by repeated exercise has been examined (Rohde, Maclean & Pedersen, 1998). The major finding of this study is that glutamine supplementation in vivo, abolishing the post-exercise decline in arterial plasma glutamine concentration, has no influence on the exercise-induced decline in the LAK cell activity, circulating lymphocyte numbers, or phytohemagglutinin-stimulated lymphocyte proliferation. These findings do not support the hypothesis that post-exercise immune changes are caused by decreased plasma glutamine concentration. The authors, therefore, do not recommend ingestion of glutamine for the purpose of avoiding immune changes in relation to exercise.

Recently, effect of oral glutamine supplementation on human neutrophil lipopolysaccharide-stimulated degranulation following prolonged exercise has also been investigated (Walsh et al., 2000). This study indicates that glutamine supplementation during exercise and recovery can maintain the plasma glutamine concentration and thus prevent the post-exercise fall in plasma glutamine concentration. However, the provision of glutamine did not alter the magnitude of the post-exercise leukocytosis or whole-blood neutrophil degranulation response to bacterial simulation. These findings suggest that fall in plasma glutamine concentration does not account for the decrease in neutrophil degranulation response following prolonged exercise.

Based on the present findings, it appears that glutamine supplementation can successfully prevent the post-exercise decline in plasma glutamine concentration. However, this maintenance of plasma glutamine concentration, which can also be achieved by high carbohydrate diet (Gleeson et al., 1998; Mitchell et al., 1998), cannot account for the exercise-induced immunosuppression. It has been suggested that the decreased glutamine level may still be adequate to support lymphocyte proliferation (Mitchell et al., 1998). It might also be that the post-exercise fall in plasma glutamine concentration is not large enough to affect the immune function. Therefore, it seems that glutamine supplementation do not affect the immune function perturbations that have been examined to date (see Gleeson & Bishop, 2000 for review).

The immune response to heavy exertionis transient, and further research on the mechanisms underlyingthe immune response to prolonged and intensive endurance exerciseis necessary before meaningful clinical applications can be drawn (Neiman, 1997). Therefore, before further evidence is available to demonstrate a positive link between glutamine concentration and immune function, glutamine supplementation for athletes is not recommended. As long as future research may be able to clarify the role of glutamine in exercise stress, Rowbottom et al. (1996) suggest that it is more beneficial to athletes in long term to enhance glutamine intake naturally from diet, and that the use of additional supplement should be avoided.

Conclusion

Glutamine is an important fuel for the immune system, particularly for lymphocyte proliferation and macrophages activity. Prolonged exercise has shown to cause a decline in plasma glutamine concentration. This exercise-induced reduction in plasma glutamine concentration is suggested to impair immune function. Consequently, some attempts have been made to attenuate immune changes following intensive exercise. It is believed that using oral glutamine supplementation to maintain plasma glutamine concentration can reduce exercise-induced immunosuppression. Although substantial evidence has proved that glutamine supplementation can prevent the decline of plasma glutamine concentration after prolonged exercise, it appears that the maintenance of plasma glutamine concentration do not affect the immune function perturbations. While the link between decreased plasma glutamine concentration and impaired immune function is still unsettled, athletes should not try to take glutamine supplements to minimize the effects of heavy exercise on immune function.

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