Several studies have demonstrated the capacity of acute oral supplementation with glutamine to increase glutaminemia, both in its free form and in dipeptide form. According to Castell and Newsholme (1998), acute oral ingestion of L-glutamine dissolved in water at 0.1 g/kg of body weight or a single dose of 5 g increased plasma glutamine concentration by 100% 30 min after ingestion, considering that glutaminemia returned to its basal levels 2 h after supplementation. Ziegler et al. (1990) verified that acute oral administration of l-glutamine increased plasma glutamine concentration between 30 and 45 min after ingestion, considering that glutaminemia returned close to basal values after 90-120 min of oral ingestion. Klassen et al. (2000) observed that acute supplementation with 20 g of L-alanyl-L-glutamine dipeptide increased the plasma glutamine concentration by 140% in relation to basal concentration 30 min after supplementation, returning to basal value 120 min after ingestion.

Studies with acute oral administration of L-glutamine demonstrate that the dose-dependent increase of plasma glutamine concentration indicates that the main fraction of glutamine administered is supposedly metabolized by enterocytes, although enteral way represents an efficient form of increasing glutamine concentration in peripheral circulation. In vivo, approximately 50% of glutamine absorbed in intestine lumen is subsequently metabolized in the intestine and liver (Ziegler et al. 1990; Dechelotte et al. 1991).

Considering the capacity of oral supplementation of glutamine to promote an increase of plasma concentration of this amino acid, even transitorily, several studies aim to investigate the possible role of this amino acid in relation to immune function of individuals engaged in intense and prolonged exercises. Therefore, Castell et al. (1996) verified the effects of oral supplementation of glutamine on infections in athletes. Ultra-marathoners, marathoners, medium-distance runners (participants of 10 km competitions), and rowers composed the group of athletes studied. The placebo group received a solution of maltodextrin and the supplemented group, a glutamine solution (5 g in 330 mL of water) immediately after exercise and 2 h after finishing competition or intense training. The athletes received questionnaires to report infections during 7 days after finishing the competition. In the glutamine supplemented group (n = 72), only 19% reported some kind of infection in that period. Among athletes that received placebo (n = 79), 51% presented some kind of infection in the same period. Although the incidence of infection has increased in both groups, authors concluded that glutamine supplementation during the first 2 h after exercising reduced the occurrence of infections in the week after the event.

In another study, Castell and Newsholme (1997) verified a significant increase in total leucocyte count immediately after the exhaustive exercise, followed by a reduction in lymphocyte count. Oral administration of solution with 5 g of glutamine in 330 mL of water immediately after the exercise resulted in more ratio of lymphocytes T CD4+ : CD8+ in relation to placebo group 1 h after finishing the exercise. Additionally, Hack et al. (1997) observed that a reduction of plasma glutamine concentration presented strong positive correlation with a reduction in the number of T CD4+ cells after 8 weeks of anaerobic training.

Moriguchi et al. (1995) verified the effect of glutamine chronic supplementation added to food on immune response of rats submitted to treadmill exercise. Plasma glutamine concentration was significantly lower in trained control group immediately after the last day of training (20 m/min, 60 min), unlike the supplemented group that maintained glutaminemia compared to control group at rest. Proliferation of lymphocytes and synthesis of IL-2 reduced significantly in the control group at training, whereas these parameters were maintained in the supplemented group immediately after the exercise. Authors concluded that glutamine supplementation prevented a reduction of proliferation response of lymphocytes induced by exercise, due to an increase of uptake and utilization of glutamine by lymphocytes to energy substrate and to biosynthesis of nucleotides.

However, other studies related to glutamine supplementation demonstrated little or no positive effect over immunocompetence of individuals submitted to exhaustive training or intense and prolonged exercise.

Supplementation with four doses of L-glutamine (100 mg/kg of bodyweight) administered at 0, 30, 60, and 90 min after a marathon maintained the plasma concentration of glutamine near preexercise values; however, no effect over proliferative response of lymphocytes, activity of lympho- kine-activated “killer” cells, and over exercise-induced alterations on concentration and percentage of some subpopulations of leucocytes was observed (Rohde et al. 1998a,b).

Glutamine supplementation (500 mL of solution of either 3.5 g of glutamine or 3.5 g of malto- dextrin and subsequent four doses of the beverage were ingested at intervals of 45 min) effect over the reduction of lymphocyte function induced by exhaustive exercise was also investigated in athletes after ergometer cycle exercise (2 h at 75% VO2max) (Krzywrowski et al. 2001). Glutamine oral supplementation during and 2 h after the exercise prevented the decline of postexercise plasma glutamine concentration; however, it did not affect the activity of NK cells and lymphokine-activated killer cells, the proliferation of T lymphocytes, and catecholamine concentration, growth hormone, insulin, and glucose. Despite these results, it was observed that exercise-induced neutrophil was less pronounced in the group supplemented with glutamine; notwithstanding, it is possible that this result does not have any significant clinical meaning.

Rohde et al. (1998a,b) verified the effect of glutamine supplementation on exercise-induced alterations in the immune system. Eight healthy individuals performed three series of exercises in the ergometer cycle during 60, 45, and 30 min at 75% VO2max, within a 2-h interval. The individuals were supplemented with glutamine (100 mg of glutamine/kg of body mass) 30 min before finishing exercise, immediately after and 2 h after finishing each exercise. Arterial plasma glutamine concentration reduced from 508 ± 35 pM (pre-exercise) to 402 ± 38 pM (2 h after last series of exercises) in the placebo group, whereas this concentration increased above pre-exercise values in L-glutamine supplemented group. Lymphocyte circulating number and lymphocyte proliferation response reduced 2 h after the first and second series, respectively, whereas activity of lymphokine- activated killer cells declined 2 h after finishing the third series. Glutamine supplementation in vivo did not influence these alterations in postexercise immune response, despite the maintenance of glutaminemia above pre-exercise values.

Some researchers propose one possible relation between glutaminemia reduction and IgA salivary concentration after intense and prolonged exercise. Krzywkowski et al. (2001a, 2001b) investigated this relation in athletes submitted to a 2-h ergometer cycle exercise (75% VO2max) and supplemented during and 2 h after the exercise with l-glutamine (17.5 g), protein (68.5 g), or placebo. Plasma glutamine concentration reduced by 15% 2 h after finishing the exercise in the placebo group, whereas this reduction was prevented in supplemented groups with glutamine and protein. However, none of the supplements were efficient to prevent reduction of concentration and release of IgA salivary induced by exercise.

Studies prove that neutrophils actively consume glutamine and, consequently, Walsh et al. (2000) investigated the influence of glutamine oral supplementation on degranulation and oxidative burst of neutrophils stimulated after a 2-h exercise (60% VO2max) in trained individuals. Glutamine supplementation was administered during and after finishing the exercise, although none of the parameters of neutrophil function have been altered through this nutritional intervention.

Aside from glutamine supplementation, other nutrients have been used for maintenance of plasma glutamine concentration and immunocompetence of athletes submitted to exhaustive exercises. Accordingly, Bacurau et al. (2002) verified the carbohydrate supplementation effect (solution at 10% with 95% of glucose polymers and 5% of fructose), 1 g/kg/h, on plasma glutamine concentration and immunocompetence in cyclists who cycled at a speed corresponding to 90% of that obtained at the anaerobic threshold. The athletes cycled for 20 min and rested for 20 min, and this protocol was repeated six times. Exercise induced a reduction in peripheral blood mononuclear cell proliferation as well as in the production of cytokines by cultured cells (IL-1, IL-2, TNF-a, and IFN-y). All of these changes were prevented by the ingestion of a carbohydrate drink by the athletes, except that in IFN-y production, which was equally decreased (17%) after the second trial. Also, carbohydrate supplementation resulted in the maintenance of plasma glutamine concentration.

Branched chain amino acids can act as precursors of glutamine synthesis in muscle tissue. These amino acids supply amino groups in transamination reactions, which result in the formation of glutamate that, afterward, at a reaction catalyzed by the enzyme glutamine synthetase, participates in the glutamine synthesis (Gleeson et al. 1998; Wagenmakers 1998). In this context, some studies have evaluated the effectiveness of supplementation of branched chain amino acids to maintain plasma glutamine concentration and modify the immune response to exhaustive endurance exercise.

Concerning the study about the effect of branched chain amino acid supplementation during exhaustive exercise on plasma glutamine concentration, Parry-Billings et al. (1992) evaluated the effect of branched amino acid supplementation (ingestion of four beverages, containing 4 g of branched chain amino acid diluted in 100 mL of each beverage, in a total of 16 g of branched chain amino acids), which was offered to healthy individuals after 10.5, 20.5, 32.5, and 37.5 km throughout a marathon (42.2 km). Branched chain amino acid supplementation promoted an increase of plasma branched chain amino acid concentration, at the same time it maintained plasma glutamine concentration at the end of the marathon. On the other hand, the placebo group had a significant reduction of plasma glutamine concentration (16%) and of branched chain amino acids (18%).

Bassit et al. (2000) evaluated the effect of branched chain amino acid supplementation on immune response and plasma glutamine concentration in triathletes who performed an Olympic triathlon (swim 1.5 km, bike 40 km, and run 10 km). Individuals were distributed in a placebo group or in a group supplemented with branched chain amino acids 30 days prior to triathlon. Branched chain amino acid supplementation (6 g/day; leucine 60%, valine 20%, and isoleucine 20%) was ingested 30 days before the triathlon competition. A branched chain amino acid dose of 3 g was ingested 30 min before the triathlon competition, and 7 days subsequent to the completion of the triathlon. The authors verified that the plasma glutamine concentration after the triathlon was maintained in relation to basal values in the branched chain amino acid supplemented group, whereas there was a significant reduction of plasma glutamine concentration in the placebo group after the triathlon. Regarding immune response, supplemented group presented a higher in vitro IL-1, IL-2, TNF-a, and IFN-y synthesis from mononuclear cells of peripheral blood at posttriathlon in relation to placebo group. Furthermore, branched chain amino acid supplementation promoted a higher capacity of peripheral blood lymphocyte proliferation when stimulated with mitogens in relation to the placebo group either before or after the triathlon competition. Simultaneously to these effects, this study also demonstrated a reduction of infection symptom rate (34%) reported by athletes supplemented with branched chain amino acids throughout supplementation period—30 days before and the week after the triathlon.

In another study (BASSIT et al. 2002), the effect of branched chain amino acid supplementation on immune response of marathoners submitted to a 30 km run was evaluated. The supplementation protocol was identical to that in the above described study of Bassit et al. (2000). Placebo group marathoners presented a reduction of plasma glutamine concentration of 24% by the end of the competition, whereas the supplementation of branched chain amino acids prevented this reduction. Supplemented group presented higher proliferation response of peripheral blood lymphocytes in relation to placebo group. Cytokine synthesis—IL-1, IL-4, TNF-a, IFN-y—from mononuclear cells of peripheral blood was reduced after exercising in comparison to pre-exercise values in the placebo group, whereas branched chain amino acid supplementation restored the synthesis of TNF-a and IL-1 and increased the synthesis of IFN-y and IL-2 and Th1 response.

Accordingly, the maintenance of plasma glutamine concentration through branched chain amino acid supplementation presents beneficial effects over the immunocompetence of athletes; however, studies with glutamine supplementation during and after endurance exercises indicate that this nutritional intervention does not prevent a reduction of immunocompetence induced by exercise. Under these circumstances, it is not clear which mechanism of branched chain amino acid supplementation acts over immunocompetence, that is to say, if it is an effect resultant of maintenance of plasma glutamine concentration, or if it is a direct effect of branched chain amino acids.

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