Serum Ethylamine Levels as an Indicator of <scp>l</scp>-Theanine Consumption and the Risk of Type 2 Diabetes in a General Japanese Population: The Hisayama Study

OBJECTIVE This study investigated the association between serum ethylamine levels as an indicator of l-theanine consumption and the development of type 2 diabetes in a Japanese community. RESEARCH DESIGN AND METHODS A total of 2,253 community-dwelling Japanese individuals aged 40–79 years without diabetes were monitored for 7 years. Serum ethylamine levels were divided into quartiles: ≤0.86, 0.87–2.10, 2.11–5.28, and ≥5.29 ng/mL. Kinetic analysis of serum ethylamine concentrations was performed after ingestion of l-theanine–rich green tea products containing 8 mg of l-theanine by 12 healthy volunteers. RESULTS During follow-up, 282 subjects developed type 2 diabetes. The age- and sex-adjusted cumulative incidence of type 2 diabetes decreased significantly with elevating levels of serum ethylamine (P for trend = 0.04). This association remained unchanged after adjusting for potential confounding factors. The multivariable-adjusted hazard ratio (HR) for type 2 diabetes was significantly lower in the fourth quartile of serum ethylamine than in the first quartile (HR 0.69, 95% CI 0.49–0.98). This trend of decrease in diabetic risk across serum ethylamine levels was more prominent in middle-aged subjects and in subjects with prediabetes, obesity, or insulin resistance. Kinetic analysis estimated that the minimum concentration at the steady state was &gt;5.90 ng/mL in the case of twice-daily ingestion with an interval of 12 h. CONCLUSIONS Higher serum ethylamine was significantly associated with lower risk of the development of type 2 diabetes in a general Japanese population. The measurement of serum ethylamine concentration would be a useful biomarker for the objective estimation of l-theanine consumption.