High oolong tea consumption predicts future risk of diabetes among Japanese male workers: a prospective cohort study

AIMS: Acute administration of oolong tea decreases blood glucose levels. We investigated the association between long-term oolong tea intake and subsequent risk of developing diabetes among men of working age. METHODS: Data were analysed from a cohort of participants in the High-risk and Population Strategy for Occupational Health Promotion Study (HIPOP-OHP), conducted in Japan from 1999 to 2004. Oolong tea intake at baseline and subsequent risk of diabetes was evaluated using a Cox proportional hazards model. RESULTS: Of 4975 male workers, a total of 201 cases of diabetes were reported over a median of 3.4 years of follow-up. Mean age and BMI of all participants at baseline were 38.3 years and 22.9 kg/m(2) , respectively. Compared with those not consuming oolong tea, multivariable adjusted hazard ratios for developing diabetes were 1.00 (95% CI 0.67-1.49) for those who drank one cup of oolong tea per day and 1.64 (95% CI 1.11-2.40) for those drinking two or more cups per day. Fasting blood glucose increment per year was 0.11 mmol/l (95% CI 0.09-0.12 mmol/l), 0.12 mmol/l (95% CI 0.09-0.15 mmol/l) and 0.15 mmol/l (95% CI 0.11-0.18 mmol/l), respectively, for oolong tea consumption of 0, 1 and ≥ 2 cups/day, with a significant linear trend (P < 0.0001). CONCLUSIONS: Long-term consumption of oolong tea may be a predictive factor for new onset diabetes. Further studies are necessary to elucidate the role of oolong tea in the risk of developing diabetes.