Effect of Monosodium Glutamate on Microbiome-Derived Imidazole Propionate Production in Healthy Volunteers

The goal of this clinical trial is to learn whether dietary monosodium glutamate (MSG), a common food flavor enhancer, increases the blood level of a gut-microbiome-derived compound called imidazole propionate (ImP) in healthy adult volunteers. ImP is a compound made by gut bacteria from the amino acid histidine, and higher blood levels have been linked to problems with blood-sugar control. The main questions this trial aims to answer are: Does taking MSG together with L-histidine increase blood ImP levels compared with taking a salt (sodium chloride) placebo together with L-histidine? How do blood ImP levels change over time in response to MSG and L-histidine taken together? Researchers will compare a 7-day period of L-histidine plus MSG against a 7-day period of L-histidine plus sodium chloride (a salt placebo with matching sodium content) to see whether MSG changes blood ImP levels. Each participant completes both periods in random order and serves as their own comparison. Participants will: Take L-histidine capsules twice daily (morning and evening with meals) during two separate 7-day periods. Dissolve either MSG powder or salt powder in water and drink it twice daily during each period, with the two periods separated by a 7-day break (washout). Give a fasting blood sample at the start of the study and on day 7 of each period. Optionally provide a stool sample at the end of each period. Complete a daily treatment diary and a follow-up phone call after the final sample.