Original Article: Clinical Investigation Green tea intake and risk of incident kidney stones: Prospective cohort studies in middle-aged and elderly Chinese individuals Xiang Shu, 1 Hui Cai, 1 Yong-Bing Xiang, 2 Honglan Li, 2 Loren Lipworth, 1,3 Nicole L Miller, 4 Wei Zheng, 1 Xiao-Ou Shu 1 and Ryan S Hsi 4 1 Division of Epidemiology, Department of Medicine, Vanderbilt Epidemiology Center, Vanderbilt-Ingram Cancer Center, Vanderbilt University Medical Center, Nashville, Tennessee, USA, 2 State Key Laboratory of Oncogene and Related Genes, Department of Epidemiology, Shanghai Cancer Institute, Renji Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai, China, 3 Vanderbilt O’Brien Center for Kidney Disease, and 4 Department of Urologic Surgery, Vanderbilt University Medical Center, Nashville, Tennessee, USA Abbreviations & Acronyms BMI = body mass index CI = confidence interval FFQ = food frequency questionnaire HR = hazard ratio SMHS = Shanghai Men’s Health Study SWHS = Shanghai Women’s Health Study WHR = waist–hip ratio Correspondence: Xiang Shu Ph.D., Division of Epidemiology, Vanderbilt University Medical Center, 2525 West End Avenue, Suite 600, Nashville, TN 37203, USA. Email: xiang.shu@vumc.org Received 26 June 2018; accepted 15 October 2018. Online publication 8 November 2018 Objectives: To investigate the association between green tea intake and incident stones in two large prospective cohorts. Methods: We examined self-reported incident kidney stone risk in the Shanghai Men’s Health Study (n = 58 054; baseline age 40–74 years) and the Shanghai Women’s Health Study (n = 69 166; baseline age 40–70 years). Information on the stone history and tea intake was collected by in-person surveys. Multivariable Cox proportional hazards models were adjusted for baseline demographic variables, medical history and dietary intakes including non-tea oxalate from a validated food frequency questionnaire. Results: During 319 211 and 696 950 person-years of follow up, respectively, 1202 men and 1451 women reported incident stones. Approximately two-thirds of men and one-quarter of women were tea drinkers at baseline, of whom green tea was the primary type consumed (95% in men, 88% in women). Tea drinkers (men: hazard ratio 0.78, 95% confidence interval 0.69–0.88; women: hazard ratio 0.8, 95% confidence interval 0.77–0.98) and specifically green tea drinkers (men: hazard ratio 0.78, 95% confidence interval 0.69–0.88; women: hazard ratio 0.84, 95% confidence interval 0.74– 0.95) had lower incident risk than never/former drinkers. Compared with never/former drinkers, a stronger dose–response trend was observed for the amount of dried tea leaf consumed/month by men (hazard ratio highest category 0.67, 95% confidence interval 0.56– 0.80, P trend < 0.001) than by women (hazard ratio highest category 0.87, 95% confidence interval 0.70–1.08, P trend = 0.041). Conclusions: Green tea intake is associated with a lower risk of incident kidney stones, and the benefit is observed more strongly among men. Key words: adult, diet, kidney calculi, prospective studies, risk factors. Introduction Kidney stone disease is highly prevalent and recurrent. 1–4 Among the known risk factors for calcium oxalate stone recurrence is hyperoxaluria. 5,6 A number of oxalate-rich foods have been described that contain varying amounts of oxalate and corresponding bioavailability, including tea. 7 Whether tea consumption increases risk for kidney stones is controversial. On the one hand, fluid intake has been shown to be protective against stone recurrence. 8,9 How- ever, tea leaves are a source of dietary oxalate that might contribute towards hyperox- aluria. 7,10–12 As a result, tea is frequently found on the do-not-consume lists given to stone formers who often become frustrated with the lack of permissible beverage options. Black tea has the highest oxalate content and is consumed more often in Western coun- tries. 10 It undergoes a full oxidation stage before steaming and drying. When black tea is con- sumed with milk, the soluble oxalate content is reduced through binding with dietary calcium. Green tea is produced without oxidation and drying fresh leaves directly after harvest. Green tea has significantly less oxalate content than in black tea, and is preferred in many Asian and North African countries. 10 © 2018 The Japanese Urological Association 241 International Journal of Urology (2019) 26, 241--246 doi: 10.1111/iju.13849