This cross-sectional study was based on the Tianjin Chronic Low-grade Systemic Inflammation and Health (TCLSIHealth) Cohort Study, which is a large prospective dynamic cohort study that focuses on the relationships between chronic low-grade systemic inflammation and the health status of a population living in Tianjin, China . Participants were recruited during their annual health examinations at the Tianjin Medical University General Hospital-Health Management Center and community management centres in Tianjin. This dynamic cohort study was launched in 2007. Furthermore, a detailed lifestyle questionnaire had been administered to about 70–80% of randomly selected subjects from this population since May 2013. The questionnaire covered questions about family income, marital status, employment status, educational level, physical activity, sleep habits, dietary habits, overall computer/mobile device usage time, television time, history of prior infections, and use of medicines as well as physical performance tests.
A total of 32,165 participants without acute inflammatory disease completed a comprehensive health examination and the study questionnaire. We excluded participants who changed their lifestyles (including dietary intake, drinking, smoking, physical activity, and sleeping habits) (assessed via a question in the questionnaire) in the last five years (n = 7364), or those had a history of cardiovascular disease (n = 1057), cancer (n = 173), NAFLD (n = 3344), alcoholic fatty liver disease (n = 775), and other live diseases (including chronic hepatitis B or C, autoimmune liver disease, and liver surgery, n = 102). The final study population comprised 19,350 individuals.
Assessment of tea consumption
Dietary intake was assessed using a validated food frequency questionnaire (FFQ) that included 100 food items with specified serving sizes. The FFQ included seven frequency categories ranging from ‘almost never eat’ to ‘twice or more per day’ for foods and eight frequency categories ranging from ‘almost never drink’ to ‘four or more times per day’ for beverages. The reproducibility and validity of the questionnaire have been assessed in a random sample of 150 participants living in Tianjin by comparing the data from the questionnaire with the data from two dietary questionnaires collected approximately three months apart and four-day weighed dietary records (WDRs). Spearman’s rank correlation coefficient for energy intake between two food frequency questionnaires administered three months apart was 0.68. The correlation coefficients for food items (fruits, vegetables, fish, meat, and beverages) between two food frequency questionnaires administered three months apart ranged from 0.62 to 0.79. Spearman’s rank correlation coefficient for energy intake as assessed via the WDRs and FFQ was 0.49. Correlation coefficients for nutrients (vitamin C, vitamin E, polyunsaturated fats, saturated fats, carbohydrates and calcium) as assessed via the WDRs and FFQ ranged from 0.35 to 0.54.
The mean daily intake of nutrients was calculated using an ad hoc computer program developed to analyse the questionnaire. The Chinese food composition tables  were used as the nutrient database. Factor analysis was applied to generate the major dietary patterns and food loading on all 100 food items and beverages (gram). Factors were named descriptively according to the food items that showed high loading (absolute value > 0.3) with respect to each dietary pattern as follows: sweet foods pattern, vegetable pattern and animal foods pattern (Additional file 1: Table S1). The scores of dietary patterns were used for further analyses as confounding factors.
The participants indicated the frequency of consumption of green tea, oolong tea, black tea, and jasmine tea over the previous one month in terms of the specified serving size by selecting one of the following eight frequency categories: almost never, < 1 cup/week, 1 cup/week, 2–3 cups/week, 4–6 cups/week, 1 cup/day, 2–3 cups/day, and ≥ 4 cups/day (200 ml per cup). In the study region, a cup of tea has a typical volume of 200 ml. In the present study, the frequency of intake of green tea, oolong tea, and black tea were categorised as: almost never, < 1 cup/week, 1–6 cups/week and ≥ 1 cup/day.
Liver ultrasonography and definitions of NAFLD
Liver ultrasonography was performed by trained sonographers using a TOSHIBA SSA-660A ultrasound machine (Toshiba, Tokyo, Japan) with a 2–5–MHz curved array probe. According to the revised definition and treatment guidelines for NAFLD published by the Chinese Association for the Study of Liver Disease in 2010 , we defined ‘heavy drinking’ as an intake of > 140 g of alcohol per week among men and > 70 g per week among women. Total alcohol intake in the preceding week was assessed using the FFQ. Participants were diagnosed as having NAFLD if they had evidence on abdominal ultrasonography (brightness of liver and a diffusely echogenic change in the liver parenchyma) and no history of heavy drinking.
Assessment of other variables
The educational level was assessed by asking the question ‘what is the highest degree you earned?’ with two possible response categories: <college graduate or ≥ college graduate. Employment status was classified as either senior officials and managers or professionals. Information on smoking (‘never,’ ‘former,’ and ‘current smoking’) and drinking (‘never,’ ‘former,’ ‘current drinking everyday’, and ‘current drinking sometimes’) status of the participants was obtained via a questionnaire survey. PA in the most recent week was assessed using the short form of the International Physical Activity Questionnaire . The questionnaire asked whether subjects had performed any activities from the following categories during the preceding week: walking, moderate activity (household activity or child care), and vigorous activity (running, swimming, or other sporting activities). Metabolic equivalents (METs; hours per week) were calculated using corresponding MET coefficients (3.3, 4.0, and 8.0, respectively) according to the following formula: MET coefficient of activity × duration (hours) × frequency (days). Total PA levels were assessed by combining separate scores for different activities. Weight and height were assessed using an automatic height and weight measurement instrument (Omron HNH-219; Omron, Kyoto, Japan) with a standard protocol. Body mass index (BMI) was calculated by dividing the weight in kilograms by the square of the height in meters (kg/m2). Fasting blood samples were obtained via venipuncture of the cubital vein and immediately mixed with EDTA. Waist circumference was measured at the umbilical level with participants standing and breathing normally. Blood pressure was measured twice from the upper left arm using a TM-2655P automatic device (A&D Co., Tokyo, Japan) after five minutes of rest in a seated position. The mean of these two measurements was taken as the blood pressure value. Hypertension was defined as average systolic BP ≥ 140 mmHg or average diastolic BP ≥90 mmHg or use of antihypertension medications . Blood samples for the analysis of fasting blood glucose and lipid levels were collected in siliconised vacuum plastic tubes. Fasting blood sugar level was measured via the glucose oxidase method, triglyceride level was measured via enzymatic methods, and high-density lipoprotein cholesterol level was measured via the chemical precipitation method using reagents obtained from Roche Diagnostics on an automatic biochemistry analyser (Roche Cobas 8000 modular analyser; Roche, Mannheim, Germany). HbA1c separation and quantification were conducted by a high-performance liquid chromatography analyzer (HLC-723 G8; Tosoh, Tokyo, Japan). In addition, an oral glucose tolerance test was performed, and postprandial glucose was determined in blood samples obtained 2 h after oral administration of a standard 75 g glucose solution. Type 2 diabetes was defined as having fasting blood glucose ≥7.0 mmol/, or 2-h postprandial blood glucose ≥11.1 mmol/l, or HbA1c ≥ 6.5% (48 mmol/mol), or a history of diabetes based on the 2014 American Diabetes Association criteria . Hyperlipidemia was defined as TC ≥ 5.20 mmol/L, or TG ≥ 1.70 mmol/L, or self-reported clinically diagnosed hyperlipidemia according to 2016 Chinese guidelines for the management of dyslipidemia in adults .
In the assessment of characteristics of participants according to NAFLD status, continuous variables have been presented as the least square mean (with 95% confidence interval, [CI]) and examined using analysis of variance. Categorical variables have been presented as percentages and examined using chi-square test. Associations between tea drinking and the prevalence of newly diagnosed NAFLD were assessed via conditional logistic regression analysis. Odds ratios (ORs) and 95% CIs were calculated. All P values for linear trend were calculated according to the categories of tea consumption (almost never: 1, < 1 cup/week: 2, 1–6 cups/week: 3, ≥1 cup/day: 4) based on logistic regression. Model 1 was adjusted for age, BMI, and sex. Model 2 was additionally adjusted for energy intake (kJ/d), type 2 diabetes, hypertension, hyperlipidemia, physical activity, educational level, household income, smoking status, drinking status, employment status, family history of cardiovascular disease, cancer, and diabetes, intake of sweet foods pattern, vegetable pattern and animal foods pattern, and consumption of two other kinds of tea. All analyses were performed using the Statistical Analysis System 9.3 edition for Windows (SAS Institute Inc., Cary, NC, USA) and STATA (version 12.1; Stata Corp LP, College Station, TX, USA). All P-values were two-tailed and differences with P < 0.05 were considered statistically significant.