L-Theanine Suppresses the Increase in Blood Glucose Levels in Mice During Glucose Tolerance Tests by Promoting Glucose Excretion in the Urine.

L-theanine (N-ethyl-γ-glutamine), a major amino acid in tea leaves, has been reported to exert anti-obesity and antidiabetic effects; however, its mechanisms remain unclear. This study aimed to examine the impact of L-theanine on blood glucose levels during an oral glucose tolerance test (OGTT) in mice, along with its effects on urinary glucose and amino acid excretion. Male Std-ddY mice (5-8 weeks old) received oral L-theanine at 100 or 1000 mg/kg 15 min before OGTT. Blood glucose levels were measured using a blood glucose meter 15 min prior to OGTT and at 0, 15, 30, 60, and 120 min post-OGTT. Urinary glucose concentrations were measured using the electrode method after oral administration of L-theanine for 15 min until the start of OGTT and every 30 min for 180 min after the start of OGTT. Concentrations of urinary amino acids, including L-theanine, were quantified using the PTC-HPLC method. Mice administered 1000 mg/kg L-theanine showed significantly lower blood glucose levels at 15, 30, and 60 min postglucose loading compared to controls. Urinary glucose concentration was also significantly higher. Additionally, the concentration of glutamic acid, glutamine, leucine, serine, methionine, glycine, valine, and lysine increased significantly. These results suggest that L-theanine reduces postprandial blood glucose by promoting urinary glucose excretion. It may interfere with renal glucose reabsorption via competition at amino acid transporters. L-theanine could therefore be a useful compound for managing blood glucose levels.