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THE EFFECTS OF GLUTAMINE SUPPLEMENTATION ON IMMUNE RESPONSE AND FUNCTION, AND ITS IMPLICATION IN THE POST-EXERCISE RECOVERY PROCESS: A META-ANALYSIS

Author: Rosanne E. Chee (June, 2008)


ABSTRACT

The purpose of this meta-analysis was to determine whether supplementation of glutamine increased plasma glutamine concentration following exercise; and whether or not it also increased the number of leucocytes, lymphocytes especially, and if so, was the subsequent immune response and recovery process of the athletes enhanced. Peer reviewed studies between the years of 1995 and 2007 were included in the analysis if they met a predetermined set of criteria, among which were studies using healthy adults with the primary outcome criterion of plasma glutamine concentration. Plasma glutamine concentration, total leucocytes, and total lymphocytes were normalized for meta-analysis by calculating the effect size for each variable. Of the 15 studies examined, only 9 met the criteria for inclusion in the meta-analysis, with a mean quality score of 58.51%. In conclusion, glutamine supplementation was not shown to have a significant effect on plasma glutamine concentration (P < 0.699), with a net decrease of 0.51 and 0.86 in the supplemented and placebo groups respectively (P < 0.05); and plasma glutamine concentration and the dose given were shown to have no correlation (r = 0.018). Although glutamine supplementation produced a large effect size of 2.84 and increased the total number of leucocytes, it was not shown to be significant. Whilst glutamine supplementation produced a moderate effect size in total lymphocytes of 0.365 following exercise, this result was not significant. More research is required to determine the complete effects of glutamine supplementation on the immune response of athletes and the recovery process following intense, prolonged exercise.


Keywords: glutamine supplementation, plasma glutamine concentration, leucocytes, lymphocytes, immune response, recovery, exercise.


INTRODUCTION

Glutamine is the most abundant and versatile free amino acid in the human body (Burke, et. al., 2006; Castell, et al., 2003; Walsh, Blannin, Robson & Gleeson, 1998). More recently it has been considered to be a ‘conditionally’ essential amino acid (Lacey & Wilmore, 1990). Roles of glutamine in the body include providing a fuel source for gut mucosal and immune system cells, regulation of protein synthesis and degradation, acid-base balance in acidosis, transfer of nitrogen between organs, and as a provision of a nitrogen precursor for synthesis of nucleotides (Burke, et. al., 2006; Castell, et al., 2003; Lacey & Wilmore, 1990; Walsh, Blannin, Robson & Gleeson, 1998).

Glutamine compromises 50-60% of the free amino acid pool in skeletal muscle, and has the highest rate of synthesis there than any other amino acid, which in the fed state is approximately 50 mmol/hr (Walsh, et al., 1998). The high rate of synthesis of glutamine is essential, as it is speculated that skeletal muscle provides the majority of the glutamine required and utilized by various other tissues, organs, and cells in the body, including the gut and the immune system (Castell, Poortmans & Newsholme, 1996; Rhode, MacLean, & Pedersen, 1998; Walsh, et al., 1998).

Glutamine is also synthesized in the gut, but it has been suggested that strenuous exercise leads to a decrease in gut glutamine synthesis (Marshall, as cited in Castell, 2003), thereby leading to a decrease in glutamine availability from the gut to the blood, and increasing the requirement for glutamine from plasma to the gut.

Resting levels of glutamine in healthy humans in the fasted state range from 500 to 750 µmol/L (Walsh, et al., 1998). In conditions of severe stress, such as exhaustive, prolonged endurance exercise, starvation, or trauma, plasma glutamine levels can be significantly decreased, with levels as low as 200 µmol/L having been recorded (Parry-Billings, et al., 1992). Plasma glutamine concentrations have been reported as being lowest in overtrained athletes, compared to either healthy athletes or sedentary individuals, with Parry-Billings, et al. (1992) reporting values of 503 µmol/L in overtrained and 550 µmol/L in healthy control athletes; and Rowbottom, et al., (as cited in Walsh, et al., 1998) reported mean plasma glutamine concentrations of 703 µmol/L in overtrained athletes, compared with 1179 µmol/L in healthy control athletes and 1179 µmol/L in sedentary controls. Studies done on endurance trained athletes such as middle distance, ultra-endurance and marathon runners, and elite rowers, in and out of competition have shown decreases of 9% in plasma glutamine concentration immediately post-exercise, with an overall decrease of 21% at 2 hours post-exercise (Castell & Newsholme, 1997). Other studies have reported similar effects on plasma glutamine concentration (Castell, et al., 1997), with levels returning to baseline within 16 to 24 hours of finishing a marathon (Castell, 2003). The decrease in plasma glutamine concentration is thought to be due to a higher demand of glutamine by skeletal muscle and other tissues (Rhode, et al., 1998). In athletes this decrease is simultaneous with the transient immunosuppression experienced after exercise (Castell, 2003).

The decrease in plasma glutamine concentration has been suggested to be a possible mechanism for the post-exercise immunosuppression seen in athletes (Castell, et al., 1996; Parry-Billings, et al., 1992), especially as lower plasma glutamine levels have been reported in athletes in a variety of modes, all whom have high-load training programmes, endurance athlete, and athletes classed as ‘overtrained’. It has been proposed that the reason for this decrease in plasma glutamine concentration whilst the immune system is being challenged is due to the uptake of glutamine by other cells and tissues, especially the gut, which reduces glutamine availability for the cells of the immune system (Castell, 2003). Studies have shown that during times of severe stress, glutamine synthesis is exceeded by glutamine utilization (Lacey & Wilmore, 1990) of the various tissues and cells of the body, resulting in a depletion of glutamine stores in the body, thus meaning that there is insufficient glutamine for plasma uptake and subsequent delivery to the immune cells. The resulting disruption of the immune system then leads to susceptibility to illness and infection (Walsh, et al., 1998), such as upper respiratory tract infections commonly seen in endurance and overtrained athletes (Castell, et al., 1996; Castell, et al., 1997).

Events essential to the normal response of the immune system are sensitive to the levels of glutamine present in the surrounding medium. Glutamine’s roles as a precursor for purine and pyrimidine synthesis are essential for leucocytosis. The immune cells that predominantly utilize glutamine as a fuel source at high rates are lymphocytes and macrophages.

Lymphocytes are the immune cells primary responsible for the specificity of the immune response, as they are the only leucocytes capable of recognizing antigens in the body. After heavy or endurance training lymphocyte numbers increase during the recovery period, but then decrease within 15 to 30 minutes post-exercise, as lymphocytes only remain in the blood for approximately 30 minutes each time that they pass through (Castell, 2003). A decrease in plasma glutamine concentration below that of the healthy human decreases lymphocyte proliferation and thus decreases the response of the immune system (Parry-Billings, et al., 1992). Decreases less than 600 µmol/L have been hypothesized by Parry-Billings, et al. (1992) as having severe detrimental effects on the immune system of those involved in heavy and/or prolonged training.

Clinical studies have shown beneficial effects of intravenous and oral glutamine supplementation in athletes, including a decreased incidence of infections of 80.8% (SD + 4.2) in marathon runners (n = 151) seven days after running a marathon, compared to the placebo group, of which only 48.8% + 7.4 were reported as being infection free (Castell & Newsholme, 1997).

Studies conducted where plasma glutamine concentration was observed following continuous and/or intermittent high-intensity and prolonged light to moderate-intensity exercise have shown both increases and decreases in plasma glutamine levels for both types of exercise intensity and duration (Parry-Billings, et al., 1992; Walsh, et al., 1998).

There have been few studies on the effects of exercise on glutamine metabolism and the immune response, without the use of supplementation. It has also yet to be determined whether or not the decrease in plasma glutamine concentration during and following prolonged, exhaustive exercise decreases the post-exercise recovery time, and thus causes athletes involved in heavy training programme to enter an overtrained state, where their immune function is impaired.

Previous studies where the effects of glutamine supplementation on immune response and function have been observed, the majority of the studies involved acute treatment, whereas a longer period may be required to gauge an accurate response of glutamine supplementation on the immune system. Whether or not glutamine supplementation enhances the recovery process of athletes engaged in intense, prolonged exercise has yet to be determined.

Links between the decrease in plasma glutamine concentration during and post-exercise and the increased risk of illness and infection in the transient immunosuppression that follows, would suggest that glutamine supplementation in athletes undertaking heavy and/or high load endurance training programmes may be useful for restoring plasma glutamine concentrations to physiological levels (Castell & Newsholme, 1997; Castell, 2003; Newsholme, et al., 1988, as cited in Castell, et al., 1997), and in turn increasing the effectiveness of the immune system. Although dosage and timing is important (Castell, et al., 2003), glutamine supplementation could potentially maximize the immune response of athletes and thus enhance the recovery process, by providing the necessary glutamine required by all the tissues and cells that need it during and following exercise. Thus, glutamine supplementation has become common in athletic populations, especially among endurance athletes, in an attempt to enhance immune response and function (Castell, et al., 1996; Castell & Newsholme, 1997; Castell, et al., 1997), as well as glycogen resynthesis (Bowtell, et al., 1999; Varnier, Leese, Thompson & Rennie, 1995) and provide positive net protein balance in the body (Tarnopolsky, MacDougall & Atkinson, 1988, as cited in Lehmkul, 2003). The data from previous studies supports glutamine dosage is set at 4-5 grams per day of pure glutamine (Castell & Newsholme, 1997).

The primary objective of the present meta-analysis was to determine whether supplementation of glutamine increased plasma glutamine concentration following exercise. A secondary objective was to determine whether glutamine supplementation increased the number of leucocytes (immune cells), in particular lymphocytes post-exercise, and if it did, was there an improvement in the immune response, and thus recovery, following exercise in athletes.

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METHODOLOGY


Data Sources

The search for literature was limited to the English language, articles published between 1995 and 2008, and citations published between 1998 and 2008. In the GoogleScholar and PubMed search engines terms glutamine supplementation, effects, exercise, athletes, resistance training, performance, immune system, and glycogen storage were used. The results then included both in vivo and in vitro studies.

A hand search was made of certain relevant peer-reviewed journals, which were not indexed by either GoogleScholar or PubMed.

The abstracts of potential articles were then examined for the following criteria: 1) studies had to be published in English; 2) studies had to be published in peer-reviewed journals; 3) a placebo control had to have been administered; 4) the experimental group had to have been supplemented with glutamine; 5) if possible there had to have been an intervention; 6) pre and post data for all measures taken had to have been available; 7) studies had to have measured either glycogen storage, immune response, or exercise performance when subjects were supplemented with glutamine; and 8) the article had to be a primary resource – i.e. the original research/study article If any of these criteria were unclear on analysis of the title or the extract, the article was discarded. Citations were rejected if they were found to be a thesis, an abstract, a review article, a roundtable discussion, a letter, or a comment. Communiqués, research notes, and symposium proceedings were accepted if they were by the original authors of a study on that particular study. The first 15 articles that met the inclusion criteria were selected for further examination for their final inclusion/exclusion from the meta-analysis.


Data Extraction

Information collected. The full text of each of the 15 articles was then examined, with characteristics of each study recorded in a table. The following characteristics of each study were recorded: author(s), title, publication type, publication source, year published, original journal, study type, duration of study/treatment, duration of intervention (if applicable), glutamine dose per day, state in which glutamine was given, the number of doses of glutamine given, timing of doses of glutamine given, the means of dose ingestion, whether or not the glutamine was ingested alone or with other substances, substance used as placebo, quantity of placebo used, the type of exercise performed, the mode of exercise performed, the protocols used, what outcome measures were taken, how each outcome measure was taken, number of subjects, number of subjects in the placebo and experimental groups, gender of subjects, average or age range of subjects, training status of subjects, sport/s that the subjects were involved in. Where necessary, means and standard deviations were approximated from figures contained in the actual manuscript, and in studies where mean values were presented with standard errors instead of standard deviations, the standard deviations were estimated from the rearrangement of the calculation SE =SD/√n (Thomas & Nelson, 2001), where SE is the standard error, SD is standard deviation, and n is the number of observations (i.e. subjects) included in the standard deviation.

Quality scoring. The quality of the 15 studies was then subjected to a quality assessment through a variety of coding scores (See Appendix: Coding Scoring). Studies were scored on their compliance with a set of 21 aspects of research methodology, among which included (possible points to award) 1) publication type (0-4), 2) source (0-1), 3) year published (0-4), 4) type of study (0-7), 5) duration of treatment (0-5), 6) duration of intervention (0-5), 7) amount of glutamine taken (0-5), 8) number of doses (0-1), 9) state in which glutamine taken (0-2), 10) timing of doses (0-8), 11) means of dose ingestion (0-3), 12) glutamine ingested with (0-4), 13) exercise performed (0-4), 14) exercise modality (0-3), 15) results measured by (0-4), 16) internal validity (0-4), 17) external validity (0-3), 18) number of subjects (0-3), 19) gender of subjects (0-3), 20) age of subjects (0-6), and 21) training status of subjects (0-4) (See Appendix: Table 3). The potential scores derived from the assessment procedure range from 0 to 79 for each study. The assessment results are presented as a percentage of the maximal score in each case.


Study Inclusion/Exclusion for the final articles included in meta-analysis

Included Studies. Following rigorous evaluation of the 15 studies, only 9 papers (Bowtell, et. al., 1999; Castell, et. al., 1997; Castell, & Newsholme, 1997; Krieger, Crowe, & Blank, 2004; Krzywkowski, et. al., 2001a; Krzywkowski, et. al., 2001b; Rohde, et al., 1998; Varnier, et al., 1995; Wilkinson, Kim, Armstrong, & Phillips, 2006) met the inclusion criteria for analysis. Characteristics of the included studies are summarized in Table 1.

Subjects. Only studies using healthy adults (>17 years of age) were included for analysis. There was no discrimination of gender, and no restrictions were placed as to the exercise, supplementation, and nutritional history of the subjects, although (where applicable) training and diet was recorded as a variable.

Experimental Design. Only randomized, placebo-controlled studies published in peer reviewed articles were selected. Both double-blind and non-double-blind studies were included. Studies were included regardless of statistical significance of the results.

Outcome measures. The primary outcome criterion was that of plasma glutamine concentration. Measurements of plasma glutamine concentration could be made in any measure – i.e. µmol/L, mmol/L, etc. Any physical measure of plasma glutamine concentration was accepted as long as the same method was used to obtain the values pre- and post-treatment. In studies where plasma glutamine concentrations were presented in graphic form, an estimate was made from the graphic form of data presentation.

Total leucocytes and total lymphocytes were the secondary and tertiary outcome criterions, but articles were not rejected if they failed to report either measure.

Excluded Studies. 5 studies (Antonio, Sanders, Kalman, Woodgate & Street, 2002; Candow, Chilibeck, Burke, Davison, & Smith-Palmer, 2000; Carvalho-Peixoto, Alves, & Cameron, 2007; Falk, Heelan, Thyfault, & Koch, 2003; Lehmkuhl, et. al. 2003) did not meet the outcome criterion of plasma glutamine concentration. Of those 5 studies, 4 looked at the effects of glutamine supplementation on weightlifting and resistance training performance (Antonio et. al., 2002; Candow, et. al., 2000; Falk, et. al., 2003; Lehmkuhl, et. al., 2003), as opposed to the effects of glutamine supplementation on immune factors or glycogen storage. Although Bassit, Sawada, Bacurau, Nararro & Costa Rosa (2000) met the outcome criterion of plasma glutamine concentration, on closer examination of the full text, it was discovered that the subjects were supplemented with only branch chain amino acids, and no glutamine at all, so it was excluded from the final analysis.


Statistical Analysis

The major objective of this study was to quantify the effect of glutamine supplementation on plasma glutamine concentration levels, and in turn determine whether or not it had any effect on the immune system.

A method of data standardization is through calculating an effect size (ES) for each study. The pre and post values for both placebo and experimental groups for plasma glutamine levels, total leucocytes, and total lymphocytes were input into an Excel spreadsheet (See Appendix) for ES transformation, where ES for each outcome measure was calculated in accordance with the method outlined by Glass (1981) (as cited in Thomas & Nelson, 2001). An ES is defined as a unit less measure of the efficacy of glutamine centred at zero if the effect of glutamine supplementation is no different from that of the placebo. A scale for ES has been suggested by Cohen (1988), with 0.8 reflecting a large effect, 0.5 a moderate effect, and 0.2 a small effect.

The resulting pre- and post-study ES achieved for each study were then corrected (See Appendix) for positive and negative effects (Hedges, 1981, as cited in Thomas & Nelson, 2001).

The pooled ES for each outcome measure were then analyzed to determine whether or not there were any statistical differences on plasma glutamine concentration, total lymphocytes, and total leucocytes, with glutamine supplementation, Statistical analysis was performed using a T-test two sample assuming equal variances for testing differences. Results were considered significant is P < 0.05 was obtained.

A Correlation between plasma glutamine concentration of the experimental group post-trial and the dose of glutamine given was made to determine whether or not dosage had any effect on plasma glutamine concentration levels. Variables were considered to be associated if r was close to either 1 or -1.

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RESULTS


Included Studies. Only 9 of the 15 studies met the inclusion criteria for analysis. Coding Scores of the included studies are presented in Table 2.


Plasma Glutamine Concentration
9 studies met the inclusion criteria for plasma glutamine concentration, with the average quality score being 58.50%. The studies were largely published in applied physiology and exercise journals between 1995 and 2006. The studies were all acute in their treatment. The average glutamine dose and the number of doses given per day was 5.6 grams (range 2-12 grams) and 1to 12 respectively. Expressing the data as an ES indicated a net decrease in plasma glutamine concentration in both the glutamine supplemented and placebo groups of -0.51 + 1.86 and -0.86 + 1.71 respectively (Fig 1 and Fig 2). There was no significance (P < 0.699) in plasma glutamine concentration in those supplemented with glutamine compared to those who received the placebo.

Overall, glutamine supplementation resulted in a decrease of plasma glutamine concentration of 5.66% (SD + 12.11), which was not different from the decrease of 5.12% (SD + 17.21) in the placebo group (Fig 3, P < 0.802).

The correlation conducted between the ES of plasma glutamine concentration and the glutamine dose given resulted in r = 0.018, which demonstrated that there was no correlation.


Total Leucocytes
3 studies qualified for analysis for total leucocytes, with the average quality score being 56.54%. All studies involved supplementation of glutamine of between 0.9 to 5 g/day, and resulted in a net increase in total leucocytes in both the placebo and treatment groups. Expressing the data as an ES indicated a large ES in total leucocytes of 2.48 + 1.00 and 2.84 + 1.50 in the placebo and treatment groups respectively (Fig 4).

Despite all the studies having a large ES (See Fig 5), the increase in total leucocytes in both groups was not significant (P < 0.796).


Total Lymphocytes
4 studies qualified for analysis for total lymphocytes, with the average quality score being 55.06%. All studies involved supplementation of glutamine of between 0.9 to 5 g/day, and resulted in a net increase in total lymphocytes (Fig 6 and Fig 7). However, the increase in total lymphocytes was not significant (P < 0.741), even though it produced a moderate ES of 0.365 in the treatment group.

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DISCUSSION


Plasma Glutamine Concentration

The findings of this meta-analysis on glutamine supplementation not having a significant effect on plasma glutamine concentration is consistent with previous papers that conclude that glutamine supplementation is ineffective at increasing plasma glutamine concentration (Castell & Newsholme, 1997). However, glutamine supplementation in said paper only increased from baseline after two hours of having taken it (Castell & Newsholme, 1997). The paper is also older than 10 years, which makes it dated.

The negative mean ES in plasma glutamine concentration could be due to a variety of factors. The literature has a lack of consensus on the effects of exercise on plasma glutamine concentration, with studies showing both increases and decreases in plasma glutamine levels after both continuous and/or intermittent, high-intensity, and prolonged light-moderate intensity exercise (Walsh, et al., 1998). However, the majority of the research shows a decrease in plasma glutamine concentration following exhaustive, prolonged exercise (Walsh, et al., 1998). Few of the studies in the meta-analysis took into account the physiological response to exercise on plasma glutamine concentration, with 7 of the studies (Bowtell, et. al., 1999; Castell, et. al., 1997; Castell, & Newsholme, 1997; Krieger, et al., 2004; Krzywkowski, et. al., 2001b; Rohde, et al., 1998; Varnier, et al., 1995) included for analysis taking the post-exercise measurement of plasma glutamine concentration immediately following the trial. Of the 4 studies (Bowtell, et al.,1999; Krzywkowski, et. al., 2001a; Varnier, et al., 1995; Wilkinson, et al., 2006) where plasma glutamine concentration increased following supplementation, Krzywkowski, et. al. (2001a) and Wilkinson, et al. (2006) took their post-exercise measurement at 2.3 hours and 1.5 hours following the end of the exercise bout/protocol, instead of immediately post-exercise. Thus reasons for the increase in plasma glutamine concentration in the latter 2 studies may be because plasma glutamine concentration had had time to return to physiological levels.

Of the 2 studies that took measurements immediately post-exercise, Varnier, et al (1995) used untrained subjects, and Bowtell, et al (1999) failed to mention the training status of their subjects.

Glutamine dosage is very important on the effects of plasma glutamine concentration. According to Castell (2003) 5 grams of glutamine increased plasma glutamine concentration by 2.5-fold in resting subjects, before returning to baseline levels. The average dose of glutamine given was 5.6 grams per day; and the subjects were not resting, but undertaking exercise of a minimum of a variety of duration and intensity, from 10 minutes duration at an intermittent, high-intensity (Krieger, et al., 2004), up to 2 hours of intensity set at 79% VO2max (Krzywkowski, et. al., 2001b), and sometimes a mix of prolonged exercise with intermittent high-intensity efforts included in the protocol (Bowtell, et al., 1999). It has been observed that in healthy adults undergoing heavy exercise protocols and training, high doses of glutamine of even 45 grams per day, produces no significant effects (Candow, et al., 2001). The range of glutamine given was 2 to 12 grams per day, has been shown to increase plasma glutamine concentration (Kreider, Miriel & Bertun, 1993, as cited in Kreider, 1999), albeit not significantly.

Timing of the glutamine supplementation may have had an impact on plasma glutamine concentration in each study where this outcome measure was increased. Varnier, et al (1995) infused their subjects with a saline solution of 3.75 ml/min (that included in total 30mg/kg/bw glutamine) over the 2 hours post-exercise. Bowtell, et al (1999) gave their subjects one dose of 8 grams of glutamine in a glucose polymer solution within 20-minutes post-exercise. Krzywkowski, et al. (2001a) gave their subjects a total of 3.5 grams of glutamine, split into 5 doses during (at 60 minutes and 105 minutes; exercise protocol lasted 120 minutes) and up to 2 hours post-exercise. Wilkinson, et al. (2006) gave their subjects a total of 0.3g/kg/bw of glutamine spread over 12 doses given in 165 ml of a glutamine, amino acid and carbohydrate beverage every 15 minutes for 3 hours post exercise.

Whether or not glutamine was taken in its pure form, and whether it was ingested as a stand alone or with other factors, such as carbohydrates and protein would also have had an impact on the plasma glutamine concentration measured (Castell, 2003). All of the studies that had an increase in plasma glutamine concentration ingested the glutamine with carbohydrates of some form: Varnier, et al. (1995) gave subjects a constant infusion of 50 mg/kg/bw/hr of glucose for the 2 hours post-exercise along with the glutamine infusion; Bowtell, et al. (1999) gave subjects their dose of glutamine in a 18.5% glucose polymer, as well as a primed constant of 8.5 mg/kg/bw/hr of glucose for 2 hours post-exercise; Krzywkowski, et. al. (2001b) gave subjects their glutamine in 0.5 litres of maltodextrin of 10% carbohydrate; Wilkinson, et al. (2006) gave subjects their glutamine with a total of 2 litres of a solution that contained in total 9.25 grams of essential amino acids and 1g/kg//bw/hr of carbohydrate. In all of the studies that did not see an increase in plasma glutamine concentration post-exercise, the glutamine dose was given with 330 ml of either water (Castell, et al., 1997; Castell & Newsholme, 1997; Krzywkowski, et. al., 2001a) or carbohydrate-free lemonade (Krieger, et al., 2004; Rohde, et al., 1998).

The training status of the subjects may also have impacted on the effect of the glutamine supplementation. Of the 9 studies analyzed, 2 did not state the training status of their subjects (Bowtell, et al, 1999; Rohde, et al., 1998), 1 used untrained subjects (Varnier, et al., 1995), 2 used trained subjects, but did not state how trained the subjects were, or what mode they were trained in (Krieger, et al., 2004; Krzywkowski, et. al., 2001a), 1 used moderately trained subjects (Wilkinson, et al., 2006), and 3 used endurance trained subjects (Castell, et al., 1997; Castell & Newsholme, 1997; Krzywkowski, et. al. 2001b); and of those 9 studies 4 used elite athletes (Castell, et al., 1997; Castell & Newsholme, 1997; Krzywkowski, et. al. 2001a), while the others used volunteers (Bowtell, et. al., 1999; Krieger, et al., 2004; Krzywkowski, et. al., 2001b; Rohde, et al., 1998; Varnier, et al., 1995; Wilkinson, et al., 2006).

The diet of the subjects is another variable that could have influenced the resultant plasma glutamine concentration. In only 2 studies (Krzywkowski, et. al., 2001a; Krzywkowski, et. al., 2001b) were the subjects’ diets recorded and then standardized among the subjects. In the rest of the studies (Bowtell, et. al., 1999; Castell, et. al., 1997; Castell, & Newsholme, 1997; Krieger, et al., 2004; Rohde, et al., 1998; Varnier, et al., 1995; Wilkinson, et al., 2006) no dietary restrictions were placed on the subjects, although for all the exercise-test protocols, subjects were tested in a fasted state.


Total Leucocytes

In all studies where total leucocytes were measured there was a large positive effect size in the post-exercise increase of leucocytes. Despite this, the findings of this meta-analysis on glutamine supplementation indicate that glutamine supplementation does not have a significant effect on increasing total leucocytes following exercise.

There are many variables that could have influenced the results reached in the studies included for analysis, including the intensity, duration, and mode of the exercise protocol used in each study.


Total Lymphocytes

Despite a moderate effect size of 0.365, glutamine supplementation had no significant effect on the total lymphocytes in the body post-exercise. This finding is in agreement with previous research by Krzywkowski, et al. (2001b) that shows that glutamine supplementation has no affect on and does not influence the exercise-induced change seen in total lymphocytes and lymphocyte function following exercise.

In the literature there are variable responses in trained populations, compared to sedentary and/or untrained subjects, with total lymphocytes having been reduced, elevated, and/or unchanged in the post-exercise recovery period (Nielsen & Pedersen, 1997). This could explain the different responses in the total lymphocytes post-exercise following glutamine supplementation in the studies included for analysis, as all the studies, with the exception of Rohde, et al. (1998) used elite athletes as subjects.

Previous research by Rohde, et al (1998) suggests that there is a dose-dependent increase in lymphocytes in a healthy individual at rest. Therefore, looking at research by Candow, et al. (2001), perhaps the glutamine doses given in the studies included for analysis were not high enough to elicit a significant increase in total lymphocyte numbers.

Other factors that may have influenced the findings of this meta-analysis is the fact that the studies where total lymphocytes decreased (Castell, et al., 1997; Castell & Newsholme, 1997) were conducted on athletes who had just finished competing in a marathon and there was no control over what and when and how they had been training in the week leading up to the event; whereas, the studies where total lymphocytes increased (Rohde, et al., 1998: Krzywkowski, et. al., 2001b) were clinical, and controlled for variables such as subject training, using protocols as strict as the subject’s not being allowed to exercise for 8 days prior each exercise trial (Krzywkowski, et. al., 2001b).


Comparing the studies included in the meta-analysis on their quality scores, Wilkinson, et al. (2006) had the most credibility out of any of the studies with 69.62%, followed by Krieger, et al. (2004) and Krzywkowski, et al. (2001a) even at 68.35%, followed closely by Krzywkowski, et. al. (2001b) with a score of 67.09%.


This meta-analysis shows that glutamine supplementation does not increase plasma glutamine concentration. It has also shown that, although not significant, total leucocytes have increased following glutamine supplementation. It has, however, not shown that lymphocytes have increased, with 50% of the studies that were included in analysis for total lymphocytes increasing in number, and the other 50% resulting in a decrease in lymphocytes. The decrease in plasma glutamine concentration means that the glutamine is being utilized elsewhere. It can be speculated that decreased plasma glutamine concentrations and higher leucocyte numbers post-exercise following glutamine supplementation indicates that the immune system is utilizing some of that glutamine (as well as it being used by other organs and tissues, such as the gut and the liver for gluconeogenesis) (Walsh, et al., 1998). If the glutamine supplemented is being used by the cells of the immune system, then it can be hypothesized that the immune response and function will be enhanced, and thus possibly reduce the incidence of infections (as reported in Castell & Newsholme, 1997), and speed up the recovery process of athletes post-exercise.


Limitations

This study, like others, has limitations. The limitations of a meta-analysis arise from the limitations of the individual studies that it is comprised of. The mean quality score of the individual studies (n = 9) reported here was 58.51%, which is not that high.

Another potential bias could be the nonreporting of studies, which could have an impact.

Another important limitation of this meta-analysis is to generalize the effects of the subjects’ sex, age and training status across the individual studies.

Finally, the small number of subjects in the majority of studies is also a limitation.

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CONCLUSION

In conclusion, the individual studies reported in this meta-analysis show that glutamine supplementation does not significantly increase plasma glutamine concentration. Although the included studies support glutamine supplementation for increasing the number of leucocytes, it does not necessarily increase the number of lymphocytes in the body following exercise.

More research is required to determine the complete effects of glutamine supplementation on the immune response of athletes and the recovery process following intense, prolonged exercise.


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Bowtell, J. L., Gelly, K., Jackman, M. L., Patel, A., Simeoni, M. & Rennie, M. J. (1999). Effect of oral glutamine on whole body carbohydrate storage during recovery from exhaustive exercise. Journal of Applied Physiology, 86(6). (p. 1770-1777).

Burke, L., Cort, M., Cox, G., Crawford, R., Desbrow, B., Farthing, l., Minehan, M., Shaw, N. & Warnes, O. (2006). Supplements and sports foods. In L. Burke & V. Deakin (Eds.) Clinical sports nutrition (3rd ed.). (p. 485-579). NSW, Australia: McGraw-Hill.

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APPENDIX

See the Attached File APPENDIX.doc at the end of post #2 (i.e. Glutamine Supplementation, Immune Response, and Post-Exercise Recovery (Part II))

[Trauma1]

Being a murse, you know i liked that post rosie....NICE JOB!

[Guejsn]

Originally Posted by Trauma1

Being a murse, you know i liked that post rosie....NICE JOB!

Cheers, John. It was actually VERY interesting doing it (once I got into it, LOL). I learnt a lot. Now just sharing that with everyone else

[Trauma1]

Originally Posted by Guejsn

Cheers, John. It was actually VERY interesting doing it (once I got into it, LOL). I learnt a lot. Now just sharing that with everyone else

Immunology is actually a great interest of mine among other things in the medicine. I need to get back to school so i can become "Physicain Assistant" John, instead of "Murse" John haha.

Great read for sure rosie!