L-glutamine is a nonessential and naturally occurring amino acid. It is the most abundant free amino acid in the human body and is found in high levels in plasma and muscle. The proposed benefits of glutamine supplementation include immune system support, anti-catabolic effects, increased glycogen synthesis, and improved fluid/electrolyte uptake. This can lead to improvements in performance and recovery from acute exercise. It is hypothesized that supplementing glutamine may mitigate performance loss by enhancing muscle recovery. (Gleeson 2008, Piattoly 2013)
Prolonged exercise is related to a decrease in plasma and muscle glutamine concentrations, which may lead to impaired immune function. Supplementing with L-glutamine is able to increase liver and muscle glutamine concentrations that may lessen the oxidative stress caused by long exhaustive exercise. L-glutamine promotes fluid uptake (increase in sodium ions and release of potassium ions), which improves cellular hydration and volume contributing to injury resistance and enhanced recovery. Glutamine supplements can also promote glycogen synthesis during the first few hours of post-exercise recovery. (Gleeson 2008, Cruzat 2014)
There is some evidence of improved aerobic performance with glutamine supplementation, but effects on anaerobic performance are not well documented. Some evidence shows an increase in endurance performance and improved power, but only in those who are moderate to well-trained. This is demonstrated with short-term glutamine supplementation; however, no improvements in recovery from intense endurance exercise are evident. (Piattoly 2013)
Similarly, there is no scientific evidence for a beneficial effect of oral glutamine supplementation on reduced muscle soreness or muscle repair following exercise-induced damage (Gleeson 2008). Glutamine supplementation does not appear to affect aerobic performance, but effects on strength and anaerobic performance are not definitive due to limited research (Ramezani 2018).
In general, the current evidence is not strong enough to recommend glutamine supplementation in athletes to improve performance. Glutamine appears to have no substantial effect on an athlete's immune system or aerobic performance and data is controversial for anaerobic and strength measures (Ramezani 2018). For athletes, ensuring adequate protein intake is sufficient to restore decreased glutamine levels, therefore supplementing glutamine is not essential (Gleeson 2008).
Cruzat VF, Krause M, Newsholme P (2014). Amino acid supplementation and impact on immune function in the context of exercise. Journal of the International Society of Sports Nutrition, 11(1), 61-61.
Gleeson M (2008). Dosing and efficacy of glutamine supplementation in human exercise and sport training. The Journal of Nutrition, 138(10), 2045S-2049S.
Piattoly T, Parish TR, Welsch MA (2013). l-glutamine supplementation: Effects on endurance, power and recovery. Current Topics in Nutraceuticals Research, 11(1/2), 55.
Ramezani Ahmadi A, Rayyani E, Bahreini M, Mansoori A (2018). The effect of glutamine supplementation on athletic performance, body composition, and immune function: A systematic review and a meta-analysis of clinical trials. Clinical Nutrition. Retrieved March 31, 2019, from https://www.clinicalnutritionjournal.com/article/S0261-5614(18)30173-0/fulltext