Kidney stones are one of the most common urologic conditions that can progress to kidney disease. A global perspective study showed that the prevalence of kidney stones ranges from 0.1% to 14.8% worldwide^[1]. In China, the prevalence is 10.34% in males and 6.62% in females, with the general morbidity increasing from 5.95% to 10.63% from the year 1991 through 2016^[2]. In addition, the recurrence rate of kidney stones is very high. Therefore, kidney stones have been recognized as an important public health issue.

Diet habits, especially dietary intake, were showed to play important roles in the pathogenesis of kidney stones. Tea is the second widely consumed beverage all over the world. Some specific nutritional components such as oxalate (the main component of tea) have been reported to increase the formation of kidney stones^[3]. Some studies revealed that increased tea consumption is associated with a reduced risk of kidney stones^[4-5], while others demonstrated that tea consumption is a risk factor for kidney stones^[6-7]. This inconsistency may be due to the frequency of tea consumption. Therefore, we conducted a large-scale, cross-sectional study to investigate the relationship between tea consumption and kidney stones in northern China.

From July 2013 to August 2014, a convenience sample including 9, 078 participants was recruited from Chaofeidian district, Tangshan city, in northern China. Inclusion and exclusion criteria for this study were described previously^[8]. In brief, all adults aged 18 to 65 years old were included, while those suffering any cancer or cerebrovascular conditions (including self-reported transient ischemic attack, myocardial infarction, and stroke) were excluded. Four participants with incomplete data on the questionnaire survey and 267 participants who did not take the ultrasonography examination were excluded. The final analysis included 8, 807 (97.01%) participants. This study was approved by the ethics committee of the Staff Hospital of Jidong Oil-field of Chinese National Petroleum (2013-1). All participants provided written informed consent.

Ultrasonography was performed using a 3.5-MHz transducer (ACUSON X300, Siemens, Germany) by well-trained examiners. Kidney stones on ultrasonic images have strong waves, bright echogenic structures, and particles ≥ 4 mm were defined as kidney stones. As described previously^[8], a standard questionnaire was used to collect information on biographical information (age, sex, education level, and income), diet habit (a high-fiber diet was defined as eating vegetables ≥ 1 time/day, high-protein diet was defined as eating meat ≥ 1 time/day), and lifestyle such as current smoking status (Yes/No), current alcohol drinking status (Yes/No), and tea consumption. Tea consumption was categorized into never drinking, occasional drinking (defined as ≤ 3 drinks/week), and often drinking (> 4 drinks/week). Types of tea consumed were categorized as black tea, green tea, scented tea, and others.

Blood samples were collected after a 12-h fast. Laboratory tests included fasting plasma glucose, triglycerides (TG), total cholesterol (TC), high-density lipoprotein (HDL), low-density lipoprotein (LDL), and uric acid (UA). Blood samples were analyzed at the central laboratory of Staff Hospital of Jidong Oil-field of Chinese National Petroleum as described in detail previously^[8]. Body mass index (BMI) was calculated based on weight (accurate to 0.1 kg) and height (accurate to 0.1 cm) measurements of participants, as body weight (kg)/ square of height (m²). Hypertension was defined as having a history of hypertension, using anti-hypertensive medication, or a systolic blood pressure (SBP) ≥ 140 mmHg, or a diastolic blood pressure (DBP) ≥ 90 mmHg during the investigation. Diabetes mellitus (DM) was defined as a self-reported history of diabetes, taking oral hypoglycemic agents, or fasting glucose ≥ 126 mg/dL. Dyslipidemia was defined as TC > 6.2 mmol/L, or TG > 2.3 mmol/L, or LDL > 2.6 mmol/L, or HDL > 3.4 mmol/L. Metabolic syndrome (MS) was defined as having central adiposity (elevated waist circumference ≥ 90 cm for Chinese males and ≥ 80 cm for Chinese females) plus any two of following four factors: triglycerides (TG) ≥ 1.7 mmol/L or specific treatment for this lipid abnormality; high-density lipoprotein (HDL) < 1.03 mmol/L in males or < 1.29 mmol/L in females or specific treatment for this lipid abnormality; SBP ≥ 130 mmHg or DBP ≥ 85 mmHg or treatment of previously diagnosed hypertension; and fasting plasma glucose (FPG) ≥ 100 mg/L or previously diagnosed type 2 diabetes.

Given that the prevalence of kidney stones is 10.63% in mainland China^[2] the odds ratio for suffering kidney stones in those who drank teas compared with those who never drank tea is 1.50. Also, since 40% of participants drank tea, at the significance level of 0.05 and a power of 80%, the sample size was estimated to be 3, 118 (for those who never drank teas) and 4, 677 (for those who drank teas) by using PASS 11 software (NCSS, LLC. Kaysville, Utah, USA. www.ncss.com). Categorical variables were described using percentages (%) and compared with Chi-square tests whereas continuous variables were described using mean ± SD (standard deviation) and compared with an ANOVA or a t-test. A series of multinomial logistic regression models were used to calculate the odds ratios (ORs) and 95% confidence intervals (95% CIs). Adjustments were made for other confounding variables such as sex, age, income, smoking, education, drinking, hypertension, DM, dyslipidemia, BMI, MS, and UA. Statistical analyses were performed using SAS software (Version 9.4, SAS Institute, Cary, NC, USA). All tests were 2-sided, and significance levels were 0.05.

A total of 8, 807 individuals participated in this study, including 128 patients with kidney stones (Table 1). In total, 57.33% (5, 049/8, 807) participants did not drink tea, and 42.67% (3, 758/8, 807) participants had habits of drinking tea, among whom 30.35% drank green tea, 3.11% drank black tea, 5.81% drank scented tea, and 3.40% drank others. The prevalence of kidney stones was 1.07%, 1.73%, and 2.25% in participants with tea consumption frequency of never, occasionally, and often groups, respectively. The age of patients (kidney stones) and controls were 46.55 ± 11.96 years and 42.19 ± 13.14 years, respectively. Kidney stones were present predominantly in males (P < 0.001). Education level, smoking, and tea consumption were significantly associated with kidney stones, while income, drinking, high-fiber diet, and high-protein diet were not statistically significant (P > 0.05). In addition, clinical parameters (BMI, MS, hypertension, dyslipidemia, and UA) were also significantly associated with kidney stones (P < 0.05). In contrast, DM was not associated with kidney stones (P < 0.05).

Table 1

Table 1 Demographic and Clinical Characteristics of Participants Variables Total Kidney Stones P Value No Yes n, % 8, 807 8, 679 (98.55) 128 (1.45) Age (Years) 42.25 ± 13.13 42.19 ± 13.14 46.55 ± 11.96 < 0.001 Sex < 0.001   Male (n, %) 4, 612 (52.37) 4, 518 (52.06) 94 (73.44)   Female (n, %) 4, 195 (47.63) 4, 161 (47.94) 34 (25.56) Income, (￥/month) 0.642    < 3, 000 3, 338 (38.53) 3, 286 (38.49) 52 (40.95)   3, 001-5, 000 4, 675 (53.96) 4, 607 (53.97) 68 (53.54)   ≥ 5, 001 651 (7.51) 644 (7.54) 7 (5.51) Education level (n, %) 0.010   Illiteracy/Primary 326 (3.7) 322 (3.71) 4 (3.13)   Middle School 3, 147 (35.73) 3, 085 (35.55) 62 (48.43)   College/University 5, 334 (60.57) 5, 272 (60.74) 62 (48.43) Smoking (n, %) 0.001   No 6, 521 (74.04) 6, 443 (74.24) 78 (60.94)   Yes 2, 286 (25.96) 2, 236 (25.76) 50 (39.06) Drinking (n, %) 0.072   No 5, 884 (66.81) 5, 808 (66.92) 76 (59.37)   Yes 2, 923 (33.19) 2, 871 (33.08) 52 (40.63) High-fiber diet (n, %) 0.247   No 728 (8.27) 721 (8.31) 7 (5.47)   Yes 8, 079 (91.73) 7, 958 (91.69) 121 (94.53) High-protein diet (n, %) 0.936   No 2, 998 (34.04) 2, 954 (34.04) 44 (34.38)   Yes 5, 829 (65.96) 5, 725 (65.96) 84 (65.62) Tea consumption (n, %) 0.001   Never 5, 049 (57.33) 4, 995 (57.55) 54 (42.19)   Occasionally 2, 025 (22.99) 1, 990 (22.93) 35 (27.34)   Often 1, 733 (19.68) 1, 694 (19.52) 39 (30.47) Tea category (n, %) 0.004   Never 5, 049 (57.33) 4, 995 (57.55) 54 (42.19)   Black tea 274 (3.11) 266 (3.06) 8 (6.25)   Green tea 2, 673 (30.35) 2, 624 (30.23) 49 (38.28)   Scented tea 512 (5.81) 503 (5.80) 9 (7.03)   Others 290 (3.40) 291 (3.35) 8 (6.25) Hypertension (n, %) 0.021   No 6, 054 (68.74) 5, 978 (68.88) 76 (59.37)   Yes 2, 753 (31.26) 2, 701 (31.12) 52 (40.63) Diabetes (n, %) 0.207   No 8, 220 (93.33) 8, 097 (93.29) 123 (96.09)   Yes 587 (6.67) 582 (6.71) 5 (3.91) MS (n, %) 0.019   No 5, 876 (66.72) 5, 803 (66.86) 73 (57.03)   Yes 2, 931 (33.28) 2, 876 (33.14) 55 (42.97) Dyslipidemia (n, %) 0.001   No 3, 909 (44.39) 3, 869 (44.59) 39 (30.47)   Yes 4, 898 (55.61) 4, 809 (55.41) 89 (69.53) BMI (kg/m2) 24.54 ± 3.72 24.52 ± 3.73 25.37 ± 3.29 0.012 UA (mmol/L) 298.02 ± 89.33 297.7 ± 89.33 321.3 ± 86.49 0.003 Note.Data are shown as mean ± SD or n (%). BMI, body mass index; UA, uric acid; MS, metabolic syndrome.

Table 1 Demographic and Clinical Characteristics of Participants

A previous study reported an inverse relationship between tea consumption and kidney stones^[5]. A systematic review and a dose-response meta-analysis was conducted to assess the association between fluid intake and kidney stones risk, and the results showed that subjects who consumed 1 to 2 cups of tea per day had a non-significant relative risk (RR) of kidney stones of 1.06 (range, 0.94-1.20), compared with those who never drank tea^[4]. When modeled as a continuous variable, each 110 mL/day increase in tea intake was associated with an RR of kidney stones of 0.96 (95% CI: 0.93-0.99)^[4]. In contrast, our study indicated that tea consumption is associated with an increased risk of kidney stones (Table 2). In a crude model, when compared with the 'never' group, the OR (95% CI) of the 'occasionally' and 'often' groups are 1.63 (range, 1.06-2.50) and 2.13 (range, 1.41-3.28) for the risk of kidney stones, respectively (P for trend < 0.001). When age, sex, education level, income, smoking, drinking, hypertension, DM, dyslipidemia, BMI, and UA were further controlled, tea consumption remained to be associated significantly with kidney stones. Similar to our findings, a case-control study reported that tea consumption was positively associated with kidney stones^[7]. In another 1:1 matched case-control study, drinking tea regularly was reported to have an OR of 1.46 (range, 1.03-2.07) for kidney stones, when compared with those who never drank tea^[6].

Table 2

Table 2 ORs and 95% CIs for Occurrence of Kidney Stones, According to Tea Consumption Frequency Model Never Occasionally vs. Never Often vs. Never P for Trend Crude model Ref 1.63 (1.06-2.50) 2.13 (1.41-3.23) < 0.001 Model 1 Ref 1.55 (1.00-2.39) 1.65 (1.08-2.52) 0.015 Model 2 Ref 1.59 (1.03-2.47) 1.65 (1.07-2.55) 0.016 Model 3 Ref 1.57 (1.00-2.46) 1.65 (1.06-2.57) 0.019 Note.Model 1: adjusted for sex and age; Model 2: adjusted for sex, age, income, education level, smoking, and drinking; Model 3: adjusted for sex, age, income, education level, smoking, drinking, hypertension, DM, dyslipidemia, UA, and BMI.

Table 2 ORs and 95% CIs for Occurrence of Kidney Stones, According to Tea Consumption Frequency

This inconsistency may be related to excess oxalate consumption. One study showed that the mean oxalate content in the infusions from 3 g of black teas and green teas were 115.68 mg/100 cm³ and 87.64 mg/100 cm³, respectively^[9]. Tea was the main source of oxalate in diets of patients with kidney stones. About 80%-85% of oxalate in diets of Chinese individuals was derived from tea and coffee^[3], so increasing the frequency of tea consumption would also increase oxalate consumption. Furthermore, one survey found that 17.67% of Chinese people like strong tea, and 52.3% of patients have more than 4 cups of tea every day^[10]. Heavy tea-drinking habits could lead to excess oxalate consumption. As a result, increasing the frequency of tea consumption will increase the excretion of urinary oxalic acid, which may lead to kidney stone formation.

There are several potential limitations in this study. First, the inherent design of a cross-sectional study makes it unable to evaluate the causality between tea consumption and kidney stones. Second, the convenience sampling method used to recruit participants might restrict generalization of the findings. Therefore, other independent replications of a case-control study or a longitudinal study are urged to confirm our findings. Third, only the combined frequency of tea (green tea, black tea, scented tea, and others) were collected, which made it impossible to analyze the association between tea category and kidney stones. Fourth, the status of tea consumption was based on self-reporting; therefore recall bias may be present. In addition, tea consumption was only categorized into never, occasionally (≤ 3 drinks/week), and often (> 4 drinks/week). Finally, the concentration of the tea consumed and the volume of total fluid intake were not included in the questionnaire.

To the best of our knowledge, this was the first large, community-based, cross-sectional study conducted to investigate the association between the frequency of tea consumption and kidney stones in Northern China. Our findings indicated that tea consumption might be one of the risk factors for kidney stones in the Northern Han Chinese. Reducing tea consumption may be helpful to prevent kidney stones.

We appreciate all the participants in this study. We thank all the members of survey teams from the Jidong community. We also thank the Sorfa Institute of Life Science and the Beijing Recdata Technology Co. Ltd., for their important contributions.