In the spring of 2018, I conducted a literature review of glutamine used to reduce exercise-induced intestinal permeability for my sports nutrition class. It's quite technical, but may be of interest to anyone who exercises, especially in the heat, either with existing intestinal permeability issues or other health conditions for which increased intestinal permeability is a concern. In my opinion, those who should most be concerned about exercise-induced intestinal permeability are those who exercise frequently in warm environments for long periods of time (e.g. marathon training in the summer, professional athletes, etc.) with existing health concerns.
For those interested in reading scientific literature, read on, at your own discretion. This is much more technical than my regular blog posts, but I thought it's still worth sharing if even one person finds it helpful!
Objective: The objective of this review is to summarize the current literature on the effects of glutamine on exercise-induced intestinal permeability, including the available evidence and clinical recommendations.
Methods: A literature review was conducted through the following online databases: Google Scholar, PubMed, Cochrane Database of Systematic Reviews, MEDLINE Complete, SPORT Discus, and CINAHL. Full-text articles published between 2001 and 2018 were included. Search terms included: “glutamine”, “permeability OR intestinal permeability OR gastrointestinal permeability OR leaky gut”, “exercise OR exercise-induced”. A hand-search of the bibliographies of relevant studies and reviews was also undertaken.
Discussion: This review discusses the possible causes of exercise-induced intestinal permeability and related mechanisms for which glutamine may have an effect on reducing intestinal permeability. Available human trials on the effect of glutamine on exercise-induced permeability are described.
Conclusion: This review determined that glutamine supplementation 2 hours prior to exercise appears to attenuate exercise-induced intestinal permeability in laboratory settings, however further research is warranted to elucidate the clinical significance of these findings.
Key words: glutamine; L-glutamine; intestinal permeability; exercise; leaky gut
The gastrointestinal tract is lined by a single layer of epithelial cells joined together by junctions on their lateral membrane (Achamrah, Déchelotte, & Coëffier, 2017). This lining of the gastrointestinal tract, or intestinal epithelium, has two roles: absorption of nutrients from food and an immune role protecting the rest of the body from potentially harmful substances (e.g., pathogens, microbes, antigens, or other various food components) (Achamrah et al., 2017). The intestinal epithelium has an important barrier function regulating which molecules can pass through from the gastrointestinal lumen to the rest of the body. The permeability of the intestinal epithelium to various substances is controlled via two routes: paracellular and transcellular (Gareau, Silva, & Perdue, 2008). Paracellular permeability refers to the passage of molecules between epithelial cells and is dependent on the integrity of the tight junctions (e.g., occludin and claudin family) (Achamrah et al., 2017). Small molecules, ions, and water pass through in this paracellular pathway in healthy intestines. Transcellular permeability refers to the passage of molecules across the apical membrane and into the epithelial cell, usually via specific transporters. Most nutrients and larger molecules, such as from digested food, are transported into the body in this way. Proper barrier maintenance is important to prevent the transport of pathogens or other potentially harmful molecules into the body and disruption can be a trigger for certain diseases. A disruption of proper barrier function is referred to as intestinal permeability (IP), also commonly called “leaky gut” (Gareau et al., 2008).
IP can be triggered by a range of factors including diet, stress (physical and psychological) (Gareau et al., 2008), and other environmental factors including temperature (Achamrah et al., 2017). Studies have also shown that strenuous exercise for even short amounts of time in both trained athletes and untrained adults induces gastrointestinal changes including IP (Clark & Mach, 2016; Costa, Snipe, Kitic, & Gibson, 2017). Strenuous exercise itself is a physical stressor on the body, which can be exacerbated by the psychological stress of event performance. The body responds to exercise stress in two main pathways: neuroendocrine and circulatory (Costa et al., 2017). The onset of exercise activates the sympathetic nervous system and stress hormones altering the enteric nervous system activity (Clark & Mach, 2016). Splanchnic blood flow is also reduced at the onset of exercise as blood is shunted to the lungs, heart, and peripheral skeletal muscle. Splanchnic hypoperfusion causes gastrointestinal ischemia leading to epithelial cell damage and tight junction barrier damage (Costa et al., 2017). Core temperature increases also play a role in intestinal changes during exercise (Soares et al., 2014). These combined neuroendocrine and circulatory changes in the gastrointestinal tract lead to increased IP (Costa et al., 2017). The increase in popularity of strenuous endurance events such as marathons and Ironman competitions has led to concerns about possible negative gastrointestinal health effects (Costa et al., 2017). Gastrointestinal distress could affect physical performance (Clark & Mach, 2016) and increased IP could lead to complications such as heat stroke (Pires et al., 2017; Soares et al., 2014), endotoxemia (Clark & Mach, 2016; Costa et al., 2017), immunosuppression (Clark & Mach, 2016), a systemic inflammatory response (Pires et al., 2017), or inflammatory bowel diseases (Kim & Kim, 2107). Recent research has explored potential treatments for attenuating this exercise-induced IP.
L-Glutamine (glutamine) is the most abundant amino acid in the human body, and can be consumed via the diet or synthesized in the body (Windmueller & Spaeth, 1978). Since the body can manufacture glutamine it is considered a non-essential amino acid, but under catabolic states such as illness or stress the levels of glutamine in the body can be depleted and requirements for glutamine increase, making it a conditionally essential amino acid. Glutamine is a precursor to the major antioxidant glutathione and the primary fuel source for rapidly dividing intestinal enterocytes and immune cells, stimulating cell growth in the small intestine (Soares et al., 2014). More recently glutamine has been found to play other important roles as an intermediate in many metabolic pathways, in stimulating protein synthesis (e.g., tight junction proteins, heat shock proteins) (Achamrah et al., 2017), attenuating cell apoptosis (Jiang, Chen, Liu, Yao, & Yin, 2017), preventing bacterial translocation (Soares et al., 2014), and inhibiting inflammatory pathways and oxidative stress, which all may play a role in increased IP (Achamrah et al., 2017). As a result of promising animal models, supplemental glutamine has been studied extensively and found to reduce IP in patients with acute illness (e.g. sepsis, burns, surgery) (Shu, Yu, Kang & Zhao, 2016), cancer (Zuhl et al., 2014), HIV (Achamrah et al., 2017) and those receiving parenteral nutrition (Gleeson, 2008). Findings are mixed in studies of glutamine supplementation in patients with intestinal diseases, but some clinical research has shown reduced IP in Crohn’s disease (Kim & Kim, 2017). Due to its numerous critical roles in the proper functioning of the intestinal epithelium and promising research findings in select populations, glutamine has become a common dietary supplement to repair intestinal damage and restore barrier function over-the-counter and in clinical settings (Clark & Mach, 2016).
Oral glutamine supplementation may also attenuate exercise-induced IP. It is well established that even short bouts of strenuous exercise induce permeability in trained and untrained individuals and glutamine supplementation can decrease IP in certain populations of patients under physical catabolic stress. The purpose of this paper is to review the current literature on the effects of glutamine on exercise-induced IP, summarizing the available evidence and clinical recommendations.
A literature review was conducted through the following online databases: Google Scholar, PubMed, Cochrane Database of Systematic Reviews, MEDLINE Complete, SPORT Discus, and CINAHL. Full-text articles published between 2001 and 2018 were included. Search terms included: “glutamine”, “permeability OR intestinal permeability OR gastrointestinal permeability OR leaky gut”, “exercise OR exercise-induced”. A hand-search of the bibliographies of relevant studies and reviews was also undertaken. There were no exclusion criteria.
Methods used to measure intestinal permeability
IP can be measured by urinary lactulose-to-rhamnose ratio, lipopolysaccharide (LPS) or intestinal fatty acid binding protein (I-FABP) in blood, or radioisotopes (e.g. DTPA). A healthy intestinal epithelium acts as a barrier preventing the passage of large molecules. As the permeability of the intestinal epithelium increases, larger molecules permeate across the barrier. Each method measures the passage of a molecule across the intestinal epithelium which should not pass through under healthy conditions. To measure the effectiveness of glutamine on IP most human exercise studies use the urinary lactulose-to-rhamnose or lactulose-to-mannitol ratio. All studies of glutamine and exercise in humans have used the urinary lactulose-to-rhamnose ratio (Lambert et al., 2001; Pugh et al., 2017; Zuhl et al., 2014; Zuhl et al., 2015). Lactulose is a large disaccharide while rhamnose and mannitol are smaller monosaccharides. Since they are not metabolized by the body, the ratio of lactulose to the smaller monosaccharide measured in the urine correlates to greater IP. Some studies measure IP by the level of LPS and I-FABP translocation from the lumen to the blood. LPS is present in the gastrointestinal lumen on the surface of Gram-negative bacteria but not usually present in blood and I-FABP is found in the intestinal villi. While this method requires a blood draw, the results are more accurate at exact points in time than urinary measurements, which are collected across 4-5 hours after exercise. Radioisotope (e.g., DTPA) measurements are commonly used in animal studies and some human studies, but not used in any exercise studies (Pires et al., 2017).
Possible causes of exercise-induced intestinal permeability
The current body of evidence clearly shows that exercise induces IP. As exercise intensity, exercise duration, and temperature increases permeability increases proportionally (Pires et al., 2017). A 2017 systematic review of seven studies on the impact of exercise on gastrointestinal permeability confirmed these findings, additionally finding permeability higher in running than cycling (Costa et al., 2017). Costa also found permeability to be greater in running than cycling studies, however only two cycling studies were included and neither measured body temperature, an important factor contributing to IP.
The mechanisms leading to increased IP during exercise are complex but are thought to primarily stem from the changes seen in the body due to exercise stress (neuroendocrine and circulatory) and hyperthermia. It is worth noting other factors can also contribute to IP including dehydration or fluid restriction (Costa et al., 2017), NSAID use, glutamine status, dietary habits, and concurrent carbohydrate intake during exercise (Lambert et al., 2001).
Glutamine and exercise-induced hyperthermia
The body produces heat during exercise, increasing core temperature. The environmental conditions (temperature and humidity) contribute to this effect and exercising in high heat and/or humidity can further increase the individual’s core temperature above ideal conditions. According to a systematic review (n=16) by Pires et al. (2017), exercise-induced hyperthermia is an independent risk factor for IP, with a strong correlation between core temperature levels and the lactulose-to-rhamnose ratio (r = 0.793; r2 = 0.629; p < 0.001). Interestingly, IP was observed in all experimental trials (10/10 100%) when core temperature was >39.0°C after exercise (Pires et al., 2017). Although this study does not prove a causal relationship between heat and IP, it is known that heat is cytotoxic and has been hypothesized that heat may affect tight junction proteins and further direct blood away from the gastrointestinal tract, leading to an inflammatory cascade (Zuhl 2014).
Glutamine has been shown to activate the heat shock response through heat shock factor-1 (HSF-1) and heat shock protein 70 (HSP70), which stabilize tight junction proteins and inactivate the NFkB proinflammatory pathway (Zuhl et al., 2014; Zuhl et al., 2015). Supplementation of 0.5 g/kg of glutamine attenuated hyperthermia in mice heated to a temperature of 39°C (39.86°C, SEM = 0.11 v. 40.33°C, SEM = 0.15 at minute 70; p < 0.05) and prevented an increase in IP whereas permeability was significantly higher in mice not given glutamine (p < 0.01) (Soares et al., 2014). Attenuated hyperthermia with glutamine supplementation has not been found in studies of humans exercising in 22.4°C (48% humidity) & 30°C (12-20% and 40-45% humidity), however (Lambert, 2001; Pugh, 2017; Zuhl, 2014). Of note, only participants in the Zuhl et al. (2014) trial exceeded a core temperature of 39°C. Regardless of the lack of core temperature reduction, supplementation of glutamine prior to exercise has shown to significantly attenuate IP in three double-blind placebo-controlled crossover studies (Pugh et al., 2017; Zuhl et al., 2014; Zuhl et al., 2015). Due to these findings, it is possible that exercise may cause hyperthermia under certain environmental conditions, inducing IP, and that glutamine supplementation is able to protect the intestinal epithelium from heat stress and attenuate IP but does not, in fact, reduce core temperature in humans. The proposed mechanism of action according to Zuhl et al. is the enhancement of heat shock proteins that protect the intestinal enterocytes from exercise-induced intestinal temperature damage.
Glutamine and exercise stress
Exercise is a form of physical stress and the onset of exercise initiates a systemic circulatory response and activates the sympathetic nervous system. The complex response stimulates free radical production, increases proinflammatory cytokines, decreases immunoprotective mast cells, and causes mucosal damage (Lambert et al., 2001). Zuhl et al. (2015) were able to elucidate glutamine as an anti-inflammatory agent in exercising adults leading to an attenuation of IP. In a double-blind placebo-controlled crossover study, 7 physically active adults consumed 0.9 g/kg fat-free mass of glutamine or placebo 2 hours prior to a 60-minute treadmill run at 70% VO2max. IP was measured by the urinary lactulose-to-rhamnose ratio 2 and 4 hours post-exercise. The placebo group showed significantly greater IP than at rest (0.06 ± 0.01 vs. 0.02 ± 0.01; p < 0.05), whereas the glutamine group showed attenuated IP, not statistically different from resting (0.04 ± 0.02 vs. 0.02 ± 0.01; p > 0.05). They also demonstrated glutamine supplementation significantly decreased TNF-alpha, a proinflammatory cytokine, 4 hours post-exercise (p < 0.05), indicating a mechanism by which glutamine may reduce proinflammatory cytokine production.
Acute doses of up to 20-30 g of glutamine are generally considered safe and well tolerated (Walsh, Blannin, Bishop, Robson, Gleeson, 2000). The glutamine dosage used in human studies to attenuate IP has ranged from a single dose of 0.9 g/kg of fat-free mass 2 hours prior to exercise (Zuhl et al., 2015), 0.9 g/kg of fat-free mass per day for seven days with a final dose 2 hours prior to exercise (Zuhl et al., 2014), and either 0.25, 0.5, or 0.9 g/kg of fat-free mass 2 hours prior to exercise (Pugh et al., 2017). While all three trials significantly reduced markers of IP, Zuhl et al. (2015) were able to show that seven days supplementation provided was not superior to a single acute dose of 0.9 g/kg of fat-free mass. Furthermore, Pugh et al. (2017) demonstrated for the first time a dose-dependent relationship, indicating the dosage of glutamine may be more important than the length of supplementation. An acute dose as low as a 0.25 g/kg 2 hours prior to exercise had a moderate effect on attenuating IP (ES = 0.6; ± 0.5) but a larger dose of 0.9 g/kg had a larger effect (ES = 0.9; ± 0.6).
While these studies show glutamine supplementation can attenuate IP, the question of clinical significance remains. Pugh et al. (2017) collected data on global GI symptoms but they were unaffected by glutamine supplementation, however, low GI symptoms were reported in all groups. There is no other existing evidence for glutamine supplementation in the prevention or management of exercise-induced gastrointestinal syndrome (Costa et al., 2017). Acute glutamine supplementation 2 hours prior to exercise at 0.9 g/kg fat-free mass may best be recommended for strenuous (high intensity and/or endurance) exercise in the heat, especially if there is a known deficiency in glutamine or existing IP issue.
This review had many limitations, foremost of which is the limited number of human studies evaluating the effect of glutamine supplementation prior to exercise, of which there is considerable heterogeneity. The number of participants in each study is also limited to between 7 and 17. All studies are conducted in laboratory settings at 22.4 - 30°C in healthy young endurance-trained individuals on a treadmill. For these reasons, the results make extrapolation to real-life exercise settings or other types of exercise difficult. Future studies would benefit from larger sample sizes and evaluating the effect of glutamine during real-life exercise conditions that replicate the additional stress of performance.
Exercise-induced IP occurs in most exercising individuals participating in strenuous exercise, especially in the heat. Supplemental glutamine has been used in various conditions to repair gastrointestinal mucosal damage and reduce IP. Glutamine has only been studied in the use of exercise-induced IP in four experimental human trials, which show promising results. Glutamine supplementation of 0.25 - 0.9 g/kg fat-free mass 2 hours prior to strenuous exercise may attenuate IP. However, due to the small number of studies focusing exclusively on glutamine (n=3) conducted in laboratory settings for no longer than 60 minutes, further research in real-life conditions is warranted. Additional research should also be conducted to elucidate the clinical significance of such findings.
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