L-Glutamine is an α-amino acid that is used in the biosynthesis of proteins. It is essential for cell proliferation and acts as a respiratory fuel and that enhances the function of stimulated immune cells. (1) L-Glutamine might increase plasma arginine and glutamate concentrations, amino acids both potentially capable of eliciting growth hormone secretion. In addition, the conversion of glutamine to glutamate provides a stimulus for directly activating somatotrophic growth hormone release. A study showed that glutamine contributes 64% to the synthesis of arginine from citrulline in plasma under postabsorptive conditions. (2) 

Glutamine plays multiple roles in the maintenance of physiological homeostasis of diverse organs and cell types. It is best known for its ability to serve as a source of fuel for the cells such as enterocytes, renal epithelial cells, hepatocytes, neurons, immune cells, β-cells of the pancreas. (3) It helps maintain the barrier between the intestines and the rest of your body and supports the growth of intestinal cells. From experimental work, it is known that glutamine is an important nutrient for rapidly dividing cells such as cells from the immune system and the gut. (4) A lack of glutamine may result in the deterioration of this intestinal barrier. Due to the major role of the intestines in the immune system, glutamine may benefit your overall immune health by supporting the intestinal cells. 

L-Glutamine also serves as a secondary fuel source for cellular energy and helps create other important compounds, including glucose and purines (the building blocks of DNA). (5) Glutamine metabolism plays multiple roles in nitrogen balance, regulation of glucose metabolism and acid-base homeostasis. It maintains the acid-base balance of body fluids. (6) Athletes can benefit from L-Glutamine supplementation to decrease muscle soreness for faster recovery time. (7) L-Glutamine controls glucose (energy) uptake by the muscles after exercise, which can help restore the energy storage for your next workout. (8) Muscles need glycogen to recover from a tough physical activity. Research suggests that glutamine promotes glycogen synthesis which speeds up the recovery process and reduces muscle soreness associated with weight lifting and strength training. 

Furthermore, when combined with creatine, L-Glutamine supplementation has been shown to increase lean body mass and power with endurance exercise. (9) In one study, subjects underwent a series of rigorous physical endurance tests and after each workout, they took either glutamine or a placebo. In this study, those who supplemented with glutamine maintained their energy levels from the beginning of the trial to the end, while those who took a placebo noticed a significant decrease in their energy over the course of the study. (10) 

A number of studies have suggested that L-Glutamine may be useful in alleviating some of the dermatological side effects of cancer therapy. (11) Of the 50 people recruited for a study, those provided 10 milligrams of L-Glutamine three times daily had less overt dermatitis than those provided a placebo. (12) Also, a 2007 study in Colorectal Disease found that L-Glutamine decreased postoperative complications and reduced the duration of hospital stays in people who had undergone colorectal cancer surgery. (13) 

References: 

1. Newsholme P. Why is L-glutamine metabolism important to cells of the immune system in health, postinjury, surgery or infection? J Nutr. 2001 Sep; 131 (9 Suppl): 2515-22; discussion 2523-2524. 

2. Ligthart-Melis GC, van de Poll MC, Boelens PG, Dejong CH, Deutz NE, van Leeuwen PA. Glutamine is an important precursor for de novo synthesis of arginine in humans. Am J Clin Nutr. 2008 May; 87 (5):1282-9. 

3. Young VR, Ajami AM. Glutamine: the emperor or his clothes? J Nutr. 2001 Sep; 131 (9 Suppl): 2449S-59S; discussion 2486S-7S. 

4. van der Hulst RR, von Meyenfeldt MF, Soeters PB. Glutamine: an essential amino acid for the gut. Nutrition. 1996 Nov-Dec; 12 (11-12 Suppl): S78-81. 

5. N. Barnabé, M. Butler. The effect of glucose and glutamine on the intracellular nucleotide pool and oxygen uptake rate of a murine hybridoma Cytotechnology. 2000 Oct; 34 (1-2): 47–57. 

6. Gstraunthaler G, Holcomb T, Feifel E, Liu W, Spitaler N, Curthoys NP. Differential expression and acid-base regulation of glutaminase mRNAs in gluconeogenic LLC-PK(1)-FBPase(+) cells. Am J Physiol Renal Physiol. 2000 Feb; 278 (2): F227-37. 

7. Legault Z, Bagnall N, Kimmerly DS. The Influence of Oral L-Glutamine Supplementation on Muscle Strength Recovery and Soreness Following Unilateral Knee Extension Eccentric Exercise. Int J Sport Nutr Exerc Metab. 2015 Oct; 25 (5): 417-26. 

8. Melis GC, ter Wengel N, Boelens PG, van Leeuwen PA. Glutamine: recent developments in research on the clinical significance of glutamine. Curr Opin Clin Nutr Metab Care. 2004 Jan; 7 (1): 59-70. 

9. Shao A, Hathcock JN. Risk assessment for the amino acids taurine, L-glutamine and L-arginine. Regul Toxicol Pharmacol. 2008 Apr; 50 (3): 376-99.

10. Mehdi Hakimi, Maryam Ali Mohamadi, Zoleikha Ghaderi. The effects of glutamine supplementation on performance and hormonal responses in non-athlete male students during eight week resistance training Journal of Human Sport and Exercise. 2012 Jan; 7 (4): 770-782. 

11. Gaurav K, Goel RK, Shukla M, Pandey M. Glutamine: A novel approach to chemotherapy-induced toxicity. Indian J Med Paediatr Oncol. 2012 Jan; 33 (1):13-20. 

12. Lopez-Vaquero D, Gutierrez-Bayard L, Rodriguez-Ruiz JA, Saldaña-Valderas M, Infante-Cossio P. Double-blind randomized study of oral glutamine on the management of radio/chemotherapy-induced mucositis and dermatitis in head and neck cancer. Mol Clin Oncol. 2017 Jun; 6 (6): 931-936. 

13. Oguz M, Kerem M, Bedirli A, Mentes BB, Sakrak O, Salman B, Bostanci H. L-alanin-L-glutamine supplementation improves the outcome after colorectal surgery for cancer. Colorectal Dis. 2007 Jul; 9 (6): 515-20.