Glutamine is a neutral amino acid that is used by rapidly dividing cells such as erythrocytes, lymphocytes, and fibroblasts. It is also the substrate of glutathione synthesis. In normal metabolic rates, glutamine is an amino acid synthesized endogenously, but in high metabolic conditions such as cancer, it must be taken exogenously. Animal studies strongly demonstrate that glutamine protects both the upper and lower gastrointestinal tract mucosa from the effects of chemotherapy, radiotherapy, or other causes of injury. In this study, we investigated the protective effect of glutamine on radiation-induced diarrhea.