Clinical assessment of blood glucose responses to 300-mg dose of L-theanine, an amino acid unique to green tea, in a fixed-sequence, two-period trial

L-theanine (N-ethyl-γ-glutamine), an amino acid found in tea leaves and known for contributing to their umami taste, is widely marketed as a supplement for promoting relaxation. Recently, potential effects on blood glucose regulation have been reported. This study evaluated the effect of oral L-theanine intake on blood glucose levels using an oral glucose tolerance test (OGTT) in healthy adults. Healthy adult participants ingested 300 mg of L-theanine 15 min before the OGTT. Blood glucose levels were measured at 0 (baseline), 30, 60, 90 and 120 min after the start of the OGTT. A fixed-sequence, two-period trial was conducted: at Visit 1 (pre-intervention), the OGTT was performed without L-theanine, whereas at Visit 2 (intervention), the participants ingested 300 mg of L-theanine before undergoing the OGTT. In addition to blood samples, urine samples were collected whenever the participants felt the need to void and analyzed for glucose, sodium ions, and amino acids. Thirty-nine healthy adults participated in the study. Although blood glucose levels tended to be lower at Visit 2 than at Visit 1, there was no statistically significant difference in the primary endpoint, defined as the difference between the peak glucose level and baseline. Thirty minutes after the start of the OGTT, corresponding to the time when serum L-theanine reached its peak, urinary glucose excretion tended to be higher and urinary sodium ions tended to be lower at Visit 2, although the difference was not statistically significant. Urinary excretion of glutamic acid, aspartic acid, and several other amino acids significantly increased during Visit 2. L-theanine may exert a glucose-lowering effect through previously reported mechanisms; however, the effect observed in this study was extremely small and unlikely to raise clinical concerns. Further research is required to clarify the potential effects of long-term L-theanine consumption.