Abstract
Background
While there are numerous benefits to tea consumption, its long-term impact on patients with chronic kidney disease (CKD) remains unclear.
Method
Our analysis included 17,575 individuals with CKD from an initial 45,019 participants in the National Health and Nutrition Examination Survey (NHANES) (1999–2018). Individuals with extreme dietary habits, pregnancy, or non-CKD conditions were excluded. Key cohort demographics revealed a mean age of 62.3 years, with 52.1% female participants, and 57.3% identified as non-Hispanic White. A total of 5,835 deaths were recorded during follow-up, including 1,823 cardiovascular-related deaths. Cox and restricted cubic spline regression was used to examine the linear or nonlinear association of tea consumption with mortality. The substitution analysis explored the effects of replacing a specific type of tea with another type of tea. Subgroup analysis stratified by sex, age, body mass index (BMI), diabetes, cancer, cardiovascular disease (CVD), and urinary albumin. Sensitivity analysis was performed to ensure the reliability of our findings.
Results
After adjusting for age, sex, race, education level, marital, annual household income, energy intake, total water intake, protein intake, carbohydrate intake, dietary fiber, sugar beverages, milk whole, total monounsaturated fatty acids, total polyunsaturated fatty acids, total saturated fatty acids, smoking, metabolic equivalent of task for physical activity level (MET-PA), BMI, diabetes, hypertension, urinary albumin, estimated glomerular filtration rate (eGFR), CVD, cancer, serum sodium, serum potassium, and serum phosphorus, setting the individuals without tea consumption record as reference. Consuming up to 4 cups of tea per day was significantly associated with lower all-cause mortality compared with that never drinking tea, among CKD patients at 1–2 stages [Hazard Ratio (HR) = 0.89; 95% Confidence Interval (CI) = 0.79, 0.99; p = 0.04], while the association between tea consumption and CVD mortality didn’t reach statistical significance. Dose-response effect was observed, showing that consuming up to three to five cups of tea per day was associated with mitigated risks of all-cause mortality, particularly in early CKD stages (non-linear p > 0.05). A 1 cup per day higher intake of oxidized tea was associated with a 10% lower risk of all-cause mortality in CKD stage 1–2 [HR = 0.90; 95%CI = 0.82, 0.99; p = 0.03]. Replacing 1 cup of green tea with 1 cup of oxidized tea per day was associated with an 8% and 11% lower risk of all-cause mortality [HR = 0.92; 95%CI = 0.86, 0.98; p = 0.01] and CVD mortality [HR = 0.89; 95%CI = 0.80, 1.00; p < 0.05], respectively, in individuals with CKD stages 1–2.
Conclusion
Tea consumption showed protective effects on all-cause mortality in CKD population, with potential benefits observed in terms of both the cups quantity and types of tea consumed. These findings appeared to be more prominent among early stages CKD population.
Keywords: Tea intake, chronic kidney disease, all-cause mortality, cardiovascular mortality
GRAPHICAL ABSTRACT
Introduction
Tea is one of the most prevalently consumed beverages worldwide, with an estimated global intake of over two billion cups per day [1]. It is derived from the leaves of the Camellia Sinensis plant, which can produce different varieties of tea, such as black, green, oolong, and white, depending on the processing methods [2,3]. Tea has different flavors that can be enjoyed hot or cold, plain, or flavored, with or without milk and sugar [4–6]. It has been extensively researched for its health-promoting properties, which are mainly attributed to its bioactive compounds, such as caffeine, theanine, flavonoids, and catechins [7,8]. These compounds can modulate various physiological and biochemical processes, such as energy metabolism, oxidative stress, inflammation, and vascular function [2, 9–17]. Consequently, tea consumption has been associated with lower risks of several chronic diseases, including cancer, diabetes, neurodegenerative disorders, and cardiovascular diseases (CVD) [18–23].
Chronic kidney disease (CKD) has affected people worldwide and has become an increasingly serious global health issue [24–26]. With the decline of renal function and the disruption of nutritional balance, CKD patients are required to adjust their dietary patterns, and the consumption of tea is of great concern [27–33]. According to existing research, on one hand, tea contains antioxidants and anti-inflammatory compounds that can protect the kidneys from damage and lower blood pressure [1, 34,35]. It is also associated with lower risk of CKD and albuminuria and maintain the estimated glomerular filtration rate (eGFR) [35]. On the other hand, patients with advanced CKD need to limit fluid intake to prevent fluid overload [36], and some types of tea are high in oxalates, substances that can contribute to kidney stone formation [37–39]. This complexity makes tea consumption a topic of careful consideration for those with CKD.
Overall, the link between tea consumption and long-term prognosis in the CKD population was currently underexplored in the existing literature. Therefore, we conducted a comprehensive secondary analysis of data obtained from the National Health and Nutrition Examination Survey (NHANES). The primary objective of this analysis was to examine the relationship between tea consumption, encompassing its diverse types, and the mortality attributable to all-cause/CVD among patients with CKD at all stages.
Method
Population and study setting
The data for this study were sourced from the NHANES, which is a program designed to evaluate the health and nutritional status of both adults and children in the United States. Our analysis utilized NHANES data collected from 1999 to 2018. We established a cohort consisting of individuals who, at their initial follow-up, met the following inclusion criteria: (1) were aged 18 years or older, (2) had completed at least one tea consumption questionnaire, (3) met one of the following criteria: an eGFR below 90 mL/min/1.73 m2 as calculated using the CKD-EPI equations, or urinary albumin levels exceeding 200 mg/L, and (4) at least one follow-up record of all-cause death and CVD death. For the purpose of our study, participants were excluded if they had incomplete follow-up data, were pregnant, or exhibited extreme dietary habits. ‘Extreme dietary habits’ were defined as consuming more than 10 cups of tea per day (greater than the mean plus five times the standard deviation). This exclusion criterion aims to eliminate potential biases associated with unusually high intake levels that could skew the overall dietary assessment. The first set of data recorded after a participant is enrolled in NHANES is designated as the baseline.
The study followed the revised December 2021 NHANES protocol, which emphasizes the importance of using sampling weights and sample design variables in all analyses. This approach addresses the survey’s clustered design and the differential probabilities of selection, which is crucial to adjust for any potential selection bias. This bias occurs when the characteristics of the sampled population differ from those of the target population, potentially skewing results if unadjusted. Thus, interview weights for 2-year and 4-year periods were applied in our analyzes to compensate for the complex study design, survey nonresponse, and post-stratification adjustments. The National Center for Health Statistics Ethics Review Board (NCHS ERB) approved the use of this data.
Exposure
Dietary information was obtained via a 24-h dietary recall interview using food frequency questionnaire (FFQ) in the NHANES. Initially, in-person interviews were conducted at the NHANES Mobile Examination Center, followed by phone interviews approximately 3–10 days later. Recorded data included the amount (in grams or in cup) of each food and beverage consumed. Tea was categorized into tea (excluding herbal and presweetened teas) or other tea including herbal teas or infusions and presweetened teas. Average tea consumption was calculated from both surveys and reported in cups per day (1 cup to grams = 236.5), with further division into three categories based on daily consumption: never (0 cup/day), > 0–≤ 4 cups/day, > 4 cups/day. The category of without tea consumption record was used as the reference group. We also recorded tea types (based on oxidation and green tea) and additional sugar content.
The covariates
The collected covariates included demographic variables, lifestyle behaviors and health conditions. The demographic variables included age (years), sex (men/women), ethnicity (non-Hispanic white/non-Hispanic black/Mexican American/Latin/other race), marital (living with partner/married, never married/separated/widowed/divorced), education level (college/more than college, and less than college) and annual household income (< $20,000, ≥ $20,000–≤ $75,000, and > $75,000). Lifestyle behaviors were obtained from a self-report questionnaire that covered metabolic equivalent of task for physical activity level (MET-PA), total energy intake (kcal/day), total water intake (g/day), smoking (defined as individuals who have ever smoked at least 100 cigarettes in their lifetime) (yes/no), alcohol intake (g/day), protein intake (g/day), carbohydrate intake (g/day), dietary fiber (g/day), sugar beverages (g/day), milk whole (g/day), total monounsaturated fatty acids (g/day), total polyunsaturated fatty acids (g/day), total saturated fatty acids (g/day) [40]. Health conditions included self-reported hypertension (yes/no), self-reported diabetes (yes/no), body mass index (BMI, calculated as measured weight (kg) divided by the square of height (m2)), urine albumin levels (mg/L), as well as serum levels of sodium, potassium, and phosphate (mmol/L).
Outcome measures
The primary outcome was mortality from various causes. We utilized the National Death Index to determine the cause of death, with a focus on CVD mortality, classified according to International Classification of Diseases, Tenth Revision (ICD-10) codes (I00–I09, I11, I13, I20–I51, I60–I69). Our analysis encompassed 5835 deaths, including 1823 from cardiovascular events.
Descriptive statistics
Baseline characteristics were presented for three groups based on tea consumption levels (never, > 0–≤ 4 cup/day, > 4 cups/day). Continuous variables were represented as means (with standard deviations), while categorical variables were expressed as numerical values (with corresponding percentages). The multiple imputation method was utilized to impute the missing data, thereby reducing bias and enhancing the efficiency of our analyses.
Cox and restricted cubic spline regression
Cox proportional hazards models were used to assess the association between tea consumption and mortality. The hazard ratio (HR) reflects varying associations within the CKD population depending on the analysis context: it indicates the risk trend with each additional cup consumed or represents the risk for different consumption levels (e.g., ≤4 cups/day or >4 cups/day) compared to non-tea drinkers. Model 1 was adjusted for demographic variables (age, sex, ethnicity, marital, education level, and annual household income); based on Model 1, Model 2 was additionally adjusted for dietary information and lifestyle behaviors (MET-PA, total energy intake, total water intake, smoking, alcohol intake, protein intake, carbohydrate intake, dietary fiber, sugar beverages, milk whole, total monounsaturated fatty acids, total polyunsaturated fatty acids, and total saturated fatty acids); base on Model 2, Model 3 was additionally adjusted for health conditions (BMI, diabetes, hypertension, urine albumin, eGFR, serum sodium, serum potassium, serum phosphorus, CVD, and cancer).
Restricted cubic spline (RCS) regression was utilized to evaluate the dose-response relationship between tea consumption and outcomes among CKD patients, and the knots for the spline were determined using piecewise linear regression. Three knots were placed at the 0.25 quantile, 0.50 quantile, 0.75 quantile of tea consumption, respectively. The nonlinearity of the association was tested using Frank Harrell’s methods.
Tea substitution model
We evaluated the influence of substituting 1 cup of a specified type of tea with another type of tea (e.g., replacing green tea with oxidized tea or replacing sweetened tea with unsweetened tea) on all-cause and CVD mortality, by using partition model that kept total tea consumption constant. The parameter for the substitution effect (substituting another type of tea for the indicated one) and its corresponding 95% confidence interval (CI) were calculated using Cox regression.
Subgroup and sensitivity analyses
We conducted a subgroup analysis based on various factors, including sex, age, BMI, diabetes, cancer, CVD, urinary albumin, water intake and CKD stages. Furthermore, to assess the robustness of our findings, we conducted a sensitivity analysis employing the following criteria: (1) exclusion of participants whose cause of death was related to injuries, such as car accidents, or fire accidents; (2) exclusion of participants with a follow-up time of < 2 years; (3) exclusion of participants whose cause of death was malignant neoplasms; (4) exclusion of participants with mild CKD (Stages 1–2), and (5) using nonspecified substitution model to adjusted for total energy intake.
Statistical software
The significance level of two-sided p < 0.05 was deemed statistically significant. All analyses were conducted using R version 3.6.1. Multiple imputation method was performed using ‘mice’ package. Cox regression analyses were performed using the ‘survival’ package, cubic spline analysis employed the ‘rcs’ package, and the subgroup analysis utilized the ‘Publish’ package.
Result
Characteristics of the study population
In the process of participant screening, we extracted a total of 45,019 complete profiles from the NHANES database, spanning from 1999 to 2018. 17,692 individuals were defined as CKD patients. Further exclusions were made where 121 individuals were excluded due to an extreme diet or pregnancy. This resulted in a total of 17,575 CKD patients who were analyzed in our study (Supplemental Figure 1).
Among the individuals included, we observed varied tea consumption patterns: 12,958 participants never consumed tea, 3,412 individuals consumed tea at a frequency of 0–4 cups daily, and 1,205 participants consumed more than 4 cups of tea one day. Tea consumers exhibited distinct demographic and lifestyle characteristics: they were more likely to be white, have a higher income and education level, and a greater likelihood of living with a partner or being married. Additionally, their dietary habits were notable for a higher intake of dietary fiber. They also showed a lower intake of other beverages overall, which included water, alcohol, and milk. Tea consumers had lower levels of urinary albumin (Table 1).
Table 1.
The baseline characteristics of total tea consumption among participants with chronic kidney disease.
| Baseline characteristic | Total participants (n = 17,575) | Tea consumption level |
||
|---|---|---|---|---|
| Never (n = 12,958) | >0-≤4 cup/d (n = 3,412) | >4 cup/d (n = 1,205) | ||
| Total Tea, mean (SD), cup/d | 0.87 (1.89) | 0.00 (0.00) | 2.17 (0.91) | 6.58 (2.07) |
| Type of tea, mean (SD), cup/d | ||||
| Sweetened | 0.33 (1.24) | – | 0.74 (1.17) | 2.69 (3.35) |
| Unsweetened | 0.37 (1.29) | – | 0.91 (1.19) | 2.85 (3.43) |
| Green | 0.04 (0.42) | – | 0.10 (0.49) | 0.28 (1.36) |
| Oxidized | 0.21 (1.00) | – | 0.54 (1.05) | 1.57 (2.96) |
| Age, mean (SD), yr | 62.29 (15.24) | 61.86 (15.50) | 64.77 (14.35) | 59.95 (14.09) |
| Sex, N (%) | ||||
| Female | 9,151 (52.1%) | 6,923 (53.4%) | 1,572 (46.1%) | 656 (54.4%) |
| Male | 8,424 (47.9%) | 6,035 (46.6%) | 1,840 (53.9%) | 549 (45.6%) |
| Race, N (%) | ||||
| Non-Hispanic White | 10,063 (57.3%) | 7,212 (55.7%) | 2,008 (58.9%) | 843 (70.0%) |
| Non-Hispanic Black | 3,246 (18.5%) | 2,506 (19.3%) | 604 (17.7%) | 136 (11.3%) |
| Mexican American | 1,908 (10.9%) | 1,551 (12.0%) | 276 (8.1%) | 81 (6.7%) |
| Latin | 1,200 (6.8%) | 959 (7.4%) | 192 (5.6%) | 49 (4.1%) |
| Others | 1,158 (6.6%) | 730 (5.6%) | 332 (9.7%) | 96 (8.0%) |
| Marital, N (%) | ||||
| Living with partner/Married | 10,476 (59.6%) | 7,614 (58.8%) | 2,072 (60.7%) | 790 (65.6%) |
| Never married/Separated/Widowed/Divorced | 7,044 (40.1%) | 5,299 (40.9%) | 1,333 (39.1%) | 412 (34.2%) |
| No data | 55 (0.3%) | 45 (0.3%) | 7 (0.2%) | 3 (0.2%) |
| Household income, N (%) | ||||
| Under 20,000$ | 4,527 (25.8%) | 3,434(26.5%) | 834 (24.4%) | 259 (21.5%) |
| 20,000$–75,000$ | 8,709 (49.6%) | 6,454 (49.8%) | 1,640 (48.1%) | 615 (51.0%) |
| Above 75,000$ | 3,642 (20.7%) | 2,552 (19.7%) | 791 (23.2%) | 299 (24.8%) |
| No data | 697 (4.0%) | 518 (4.0%) | 147 (4.3%) | 32 (2.7%) |
| Educational attainment, N (%) | ||||
| College or above | 8,817 (50.2%) | 6,257 (48.3%) | 1,871 (54.8%) | 689 (57.2%) |
| Less than College | 8,729 (49.7%) | 6,678 (51.5%) | 1,537 (45.0%) | 514 (42.7%) |
| No data | 29 (0.2%) | 23 (0.2%) | 4 (0.1%) | 2 (0.2%) |
| Energy intake, mean (SD), kcal/d | 1,943.92 (899.14) | 1,940.14 (903.44) | 1,876.38 (857.65) | 2,175.81 (930.58) |
| Total water intake, mean (SD), g/d | 997.67 (1062.29) | 1,039.87 (1,085.02) | 878.38 (927.32) | 881.69 (1,136.95) |
| Alcohol, mean (SD), g/d | 7.98 (24.08) | 8.98 (26.12) | 5.18 (16.88) | 5.16 (16.44) |
| Protein intake, mean (SD), g/d | 75.41 (39.40) | 75.16 (39.36) | 73.06 (38.24) | 84.67 (41.74) |
| Carbohydrate intake, mean (SD), g/d | 234.91 (114.44) | 232.60 (114.74) | 231.58 (106.88) | 269.20 (125.97) |
| Dietary fiber, mean (SD), g/d | 15.91 (9.79) | 15.59 (9.70) | 16.55 (9.85) | 17.49 (10.33) |
| Sugar beverages, mean (SD), g/d | 335.10 (525.94) | 350.32 (544.15) | 249.41 (394.20) | 414.01 (617.50) |
| Milk whole, mean (SD), g/d | 31.41 (125.62) | 33.69 (133.26) | 24.54 (98.80) | 26.24 (106.46) |
| Total monounsaturated fatty acids, mean (SD), g/d | 27.09 (16.52) | 27.03 (16.43) | 26.12 (16.44) | 30.50 (17.31) |
| Total polyunsaturated fatty acids, mean (SD), g/d | 16.90 (11.47) | 16.71 (11.33) | 16.76 (11.64) | 19.32 (12.17) |
| Total saturated fatty acids, mean (SD), g/d | 24.16 (15.27) | 24.25 (15.32) | 22.92 (14.81) | 26.64 (15.58) |
| Serum sodium, mean (SD), mmol/L | 139.47 (2.56) | 139.46 (2.57) | 139.52 (2.59) | 139.47 (2.45) |
| Serum potassium, mean (SD), mmol/L | 4.07 (0.39) | 4.07 (0.39) | 4.06 (0.39) | 4.05 (0.37) |
| Serum phosphorus, mean (SD), mmol/L | 1.20 (0.19) | 1.20 (0.19) | 1.21 (0.18) | 1.22 (0.19) |
| eGFR, mean (SD), mL/min/1.73 m2 | 71.39 (17.19) | 71.54 (17.35) | 70.33 (17.14) | 72.78 (15.36) |
| Urinary Alb, mean (SD), mg/L | 101.15 (581.21) | 109.12 (622.82) | 83.22 (458.32) | 66.21 (397.85) |
| Physical Activity, mean (SD), MET minutes/wk | 2,901.79 (5,084.86) | 2,950.29 (5,185.73) | 2,547.96 (4,453.10) | 3,382.20 (5,586.63) |
| BMI, mean (SD), kg/m2 | 29.19 (6.48) | 29.30 (6.52) | 28.60 (5.95) | 29.72 (7.30) |
| DM, N (%) | ||||
| Yes | 3,682 (21.0%) | 2,766 (21.3%) | 683 (20.0%) | 233 (19.3%) |
| No | 13,893 (79.0%) | 10,192 (78.7%) | 2729 (80.0%) | 972 (80.7%) |
| HTN, N (%) | ||||
| Yes | 9,066 (51.6%) | 6,721 (51.9%) | 1,785 (52.3%) | 560 (46.5%) |
| No | 8,509 (48.4%) | 6,237 (48.1%) | 1,627 (47.7%) | 645 (53.5%) |
| Smoking, N (%) | ||||
| Yes | 8,695 (49.5%) | 6,597 (50.9%) | 1,479 (43.3%) | 619 (51.4%) |
| No | 8,880 (50.5%) | 6,361 (49.1%) | 1,933(56.7%) | 586 (48.6%) |
| CVD, N (%) | ||||
| Yes | 3,668 (20.9%) | 2,729 (21.1%) | 709 (20.8%) | 230 (19.1%) |
| No | 13,907 (79.1%) | 10,229 (78.9%) | 2703 (79.2%) | 975 (80.9%) |
| Cancer, N (%) | ||||
| Yes | 2,810 (16.0%) | 2,032 (15.7%) | 575 (16.9%) | 203 (16.8%) |
| No | 14,765 (84.0%) | 10,926 (84.3%) | 2,837 (83.1%) | 1,002 (83.2%) |
All variables are expressed as number and percentage. The individuals without tea consumption record have been set as reference.
Abbreviations: BMI: body mass index; eGFR: estimated glomerular filtration rate; Urinary Alb: Urinary micro-albumin; MET: metabolic equivalent of task; HTN: hypertension; DM: diabetes; CVD: cardiovascular disease; d: day.
Association of tea consumption with all-cause/CVD mortality in CKD population
An analysis of all-cause mortality and CVD mortality in populations with stages 1–2 and 3–5 CKD was associated with tea consumption (Table 2). After accounting for potential confounders, a pattern emerged indicating that consuming up to 4 cups of tea per day was significantly associated with lower all-cause mortality in patients with stage 1–2 CKD [HR = 0.89; 95%CI = 0.79, 0.99; p = 0.04]. However, the association between tea consumption and mortality from CVD did not reach statistical significance (p > 0.05).
Table 2.
The association between tea consumption and all-cause/CVD mortality in patients with CKD stages 1–2 and 3–5.
| CKD stages | Cause of death | Tea consumption | Model 1 |
Model 2 |
Model 3 |
|||
|---|---|---|---|---|---|---|---|---|
| HR (95%CI) | p | HR (95%CI) | p | HR (95%CI) | p | |||
| CKD 1–2 stages | All-cause mortality | ≤ 4 cup/d | 0.86 (0.78, 0.96) |
0.01 | 0.89 (0.80, 0.99) |
0.04 | 0.89 (0.79, 0.99) |
0.04 |
| > 4 cup/d | 0.86 (0.71, 1.03) |
0.10 | 0.88 (0.73, 1.07) |
0.20 | 0.87 (0.72, 1.05) |
0.15 | ||
| CVD mortality | ≤ 4 cup/d | 0.86 (0.71, 1.04) |
0.12 | 0.87 (0.72, 1.05) |
0.15 | 0.87 (0.72, 1.05) |
0.15 | |
| > 4 cup/d | 0.90 (0.65, 1.24) |
0.51 | 0.91 (0.66, 1.26) |
0.56 | 0.89 (0.64, 1.22) |
0.46 | ||
| CKD 3–5 stages | All-cause mortality | ≤ 4 cup/d | 0.87 (0.76, 0.98) |
0.02 | 0.91 (0.80, 1.03) |
0.12 | 0.91 (0.80, 1.04) |
0.16 |
| > 4 cup/d | 0.86 (0.66, 1.11) |
0.25 | 0.90 (0.69, 1.17) |
0.42 | 0.98 (0.76, 1.26) |
0.86 | ||
| CVD mortality | ≤ 4 cup/d | 0.96 (0.78, 1.18) |
0.71 | 0.99 (0.80, 1.23) |
0.93 | 1.01 (0.81, 1.25) |
0.96 | |
| > 4 cup/d | 0.86 (0.55, 1.35) |
0.51 | 0.91 (0.58, 1.42) |
0.67 | 1.01 (0.65, 1.59) |
0.96 | ||
Cox proportional hazard regression analysis was performed. The individuals without tea consumption have been set as reference.
The hazard ratio (HR) indicates the risk for different consumption levels (e.g., ≤ 4 cups/day or > 4 cups/day) compared to non-tea drinkers.
The model 1 was adjusted for demographic variables (age, sex, race, education level, marital, and annual household income).
The model 2 was adjusted for covariates in Model 1 + dietary information and lifestyle (energy intake, total water intake, protein intake, carbohydrate intake, dietary fiber, sugar beverages, milk whole, total monounsaturated fatty acids, total polyunsaturated fatty acids, total saturated fatty acids, smoking, and MET-PA).
The model 3 was adjusted for covariates in Model 2 + health status (BMI, diabetes, hypertension, urinary albumin, eGFR, cardiovascular disease, cancer, serum sodium, serum potassium, and serum phosphorus).
Abbreviations: CKD: chronic kidney disease; CVD: cardiovascular disease; d: day.
Next, we assessed the impact of green tea, oxidized tea, sweetened tea, unsweetened tea (Table 3). After adjusting for confounding factors, our study revealed a significant association between an increase of one cup of oxidized tea per day and a lower all-cause mortality rate in the CKD stage 1–2 population [HR = 0.90; 95%CI = 0.82, 0.99; p = 0.03]. In the CKD stage 3–5 population, an increase of one cup of unsweetened tea per day was linked to a lower in all-cause mortality [HR = 0.95; 95%CI = 0.91, 1.00; p = 0.03]. Remarkably, our study also found that an additional cup of green tea per day was associated with higher CVD mortality among the CKD stage 3–5 population [HR = 1.39; 95%CI = 1.09, 1.79; p = 0.01]. Furthermore, while the consumption of oxidized tea and unsweetened tea showed positive trends toward reducing all-cause mortality across various CKD stages, these associations did not reach statistical significance.
Table 3.
The type of tea consumption and all-cause/CVD mortality in CKD 1-2 and 3-5 stages patients.
| Cause of death | CKD stages | Types of tea | Model 1 |
Model 2 |
Model 3 |
|||
|---|---|---|---|---|---|---|---|---|
| HR (95%CI) | p | HR (95%CI) | p | HR (95%CI) | p | |||
| All-cause mortality | CKD 1–2 stages | Oxidized tea | 0.91 (0.83, 1.00) |
0.04 | 0.92 (0.84, 1.00) |
0.05 | 0.90 (0.82, 0.99) |
0.03 |
| Green tea | 0.96 (0.84, 1.10) |
0.56 | 0.98 (0.85, 1.13) |
0.80 | 1.01 (0.87, 1.16) |
0.92 | ||
| Unsweetened tea | 0.98 (0.94, 1.01) |
0.18 | 0.99 (0.96, 1.02) |
0.51 | 0.99 (0.96, 1.02) |
0.52 | ||
| Sweetened tea | 0.99 (0.95, 1.03) |
0.53 | 0.98 (0.94, 1.02) |
0.35 | 0.98 (0.94, 1.02) |
0.24 | ||
| CKD 3–5 stages | Oxidized tea | 0.96 (0.88, 1.05) |
0.38 | 0.97 (0.90, 1.06) |
0.55 | 0.99 (0.91, 1.08) |
0.84 | |
| Green tea | 1.01 (0.74, 1.38) |
0.93 | 1.04 (0.75, 1.43) |
0.81 | 1.06 (0.77, 1.46) |
0.72 | ||
| Unsweetened tea | 0.95 (0.91, 1.00) |
0.03 | 0.96 (0.92, 1.01) |
0.08 | 0.97 (0.93, 1.01) |
0.16 | ||
| Sweetened tea | 1.00 (0.95, 1.05) |
0.99 | 1.00 (0.96, 1.06) |
0.86 | 1.02 (0.97, 1.07) |
0.40 | ||
| CVD mortality | CKD 1–2 stages | Oxidized tea | 0.88 (0.74, 1.05) |
0.15 | 0.89 (0.75, 1.05) |
0.16 | 0.87 (0.73, 1.03) |
0.11 |
| Green tea | 0.98 (0.78, 1.23) |
0.87 | 1.00 (0.81, 1.25) |
0.97 | 1.05 (0.85, 1.30) |
0.64 | ||
| Unsweetened tea | 0.97 (0.92, 1.03) |
0.34 | 0.98 (0.93, 1.04) |
0.54 | 0.98 (0.93, 1.04) |
0.60 | ||
| Sweetened tea | 1.01 (0.94, 1.08) |
0.86 | 1.00 (0.92, 1.07) |
0.90 | 0.98 (0.92, 1.06) |
0.69 | ||
| CKD 3–5 stages | Oxidized tea | 0.96 (0.84, 1.09) |
0.53 | 0.97 (0.85, 1.10) |
0.63 | 0.99 (0.87, 1.13) |
0.93 | |
| Green tea | 1.31 (1.02, 1.68) |
0.04 | 1.34 (1.04, 1.73) |
0.03 | 1.39 (1.09, 1.79) |
0.01 | ||
| Unsweetened tea | 0.95 (0.89, 1.02) |
0.18 | 0.96 (0.90, 1.03) |
0.30 | 0.97 (0.91, 1.05) |
0.46 | ||
| Sweetened tea | 1.02 (0.95, 1.11) |
0.57 | 1.03 (0.95, 1.11) |
0.52 | 1.05 (0.97, 1.13) |
0.23 | ||
The HR indicates the risk associated with all-cause mortality and CVD mortality for each additional cup of a specific type of tea consumed.
The model 1 was adjusted for demographic variables (age, sex, race, education level, marital, and annual household income).
The model 2 was adjusted for covariates in Model 1 + dietary information and lifestyle (energy intake, total water intake, protein intake, carbohydrate intake, dietary fiber, sugar beverages, milk whole, total monounsaturated fatty acids, total polyunsaturated fatty acids, total saturated fatty acids, smoking, and MET-PA).
The model 3 was adjusted for covariates in Model 2 + health status (BMI, diabetes, hypertension, urinary albumin, eGFR, cardiovascular disease, cancer, serum sodium, serum potassium, and serum phosphorus).
Abbreviations: CKD: chronic kidney disease; CVD: cardiovascular disease.
Furthermore, in our stratified analyses across different stages of CKD (Figure 1), we observed that drinking up to 4 cups of tea per day seemed to demonstrate a trend toward a lower risk of all-cause mortality, especially in the early stages (CKD stages 1–2, non-linear p > 0.05). Nonetheless, there was no significant association between tea consumption and CVD mortality.
Figure 1.
Restricted cubic spline models for the relationship between an increased cups of tea intake and all-cause/CVD mortality among CKD population. (A) The all-cause mortality of all CKD stages patients. (B) The CVD mortality of all CKD stages patients. (C) The all-cause mortality of CKD stages 1–2 patients. (D) The CVD mortality of CKD stages 1–2 patients. (E) The all-cause mortality of CKD stages 3–5 patients. (F) The CVD mortality of CKD stages 3–5 patients. The analysis was conducted by adjusting for demographic variables (age, sex, race, education level, marital, and annual household income) + dietary information and lifestyle (energy intake, total water intake, protein intake, carbohydrate intake, dietary fiber, sugar beverages, milk whole, total monounsaturated fatty acids, total polyunsaturated fatty acids, total saturated fatty acids, smoking, and MET-PA) + health status (BMI, diabetes, hypertension, urinary albumin, eGFR, cardiovascular disease, cancer, serum sodium, serum potassium, and serum phosphorus). The individuals without tea consumption record have been set as reference. The value of the X-axis represents the cups of tea intake. The hazard ratio (HR) demonstrates the overall association between tea consumption and mortality in the CKD population. It shows the risk trend for tea drinkers with increasing tea consumption. Abbreviations: CKD: chronic kidney disease; CVD: cardiovascular disease.
Substitution analysis of different tea types
In our substitution analysis, the results indicated that replacing one cup of green tea with one cup of oxidized tea was associated with a lower in both all-cause mortality [HR = 0.92; 95%CI = 0.86, 0.98; p = 0.01] and CVD mortality [HR = 0.89; 95%CI = 0.80, 1.00; p < 0.05] in individuals with CKD stages 1–2 (Table 4). Additionally, substituting one cup of green tea with one cup of oxidized tea per day, or one cup of sweetened tea with one cup of unsweetened tea, appeared to lower all-cause mortality rates across various CKD stages, although these associations did not reach statistical significance.
Table 4.
The substitution analysis of tea consumption and all-cause/CVD mortality in CKD 1–2 and 3–5 stages patients.
| Cause of death | CKD stages | Type of tea and instead | Model 1 |
Model 2 |
Model 3 |
|||
|---|---|---|---|---|---|---|---|---|
| HR (95%CI) | p | HR (95%CI) | p | HR (95%CI) | p | |||
| All-cause mortality | CKD 1–2 stages | Oxidized tea replaces green tea | 0.94 (0.88, 1.00) |
0.04 | 0.94 (0.88, 1.00) |
<0.05 | 0.92 (0.86, 0.98) |
0.01 |
| Unsweetened tea replaces Sweetened tea | 0.99 (0.97, 1.01) |
0.50 | 1.00 (0.98, 1.02) |
0.90 | 1.00 (0.98, 1.03) |
0.77 | ||
| CKD 3–5 stages | Oxidized tea replaces green tea | 0.96 (0.89, 1.05) |
0.39 | 0.97 (0.90, 1.06) |
0.53 | 0.99 (0.91, 1.07) |
0.76 | |
| Unsweetened tea replaces Sweetened tea | 0.98 (0.95, 1.01) |
0.12 | 0.98 (0.95, 1.01) |
0.17 | 0.98 (0.95, 1.01) |
0.12 | ||
| CVD mortality | CKD 1–2 stages | Oxidized tea replaces green tea | 0.92 (0.83, 1.02) |
0.10 | 0.92 (0.82, 1.02) |
0.11 | 0.89 (0.80, 1.00) |
<0.05 |
| Unsweetened tea replaces Sweetened tea | 0.98 (0.94, 1.02) |
0.41 | 0.99 (0.95, 1.03) |
0.72 | 1.00 (0.96, 1.04) |
0.93 | ||
| CKD 3–5 stages | Oxidized tea replaces green tea | 0.91 (0.77, 1.08) |
0.30 | 0.92 (0.77, 1.10) |
0.34 | 0.93 (0.77, 1.13) |
0.47 | |
| Unsweetened tea replaces Sweetened tea | 0.97 (0.92, 1.02) |
0.18 | 0.97 (0.92, 1.02) |
0.25 | 0.97 (0.92, 1.02) |
0.19 | ||
Substitution analysis was performed. The HR evaluates the impact of replacing one cup of a specified type of tea with another on all-cause and CVD mortality, using substitution model that maintain constant total tea consumption.
The model 1 was adjusted for demographic variables (age, sex, race, education level, marital, and annual household income).
The model 2 was adjusted for covariates in Model 1 + dietary information and lifestyle (energy intake, total water intake, protein intake, carbohydrate intake, dietary fiber, sugar beverages, milk whole, total monounsaturated fatty acids, total polyunsaturated fatty acids, total saturated fatty acids, smoking, and MET-PA).
The model 3 was adjusted for covariates in Model 2 + health status (BMI, diabetes, hypertension, urinary albumin, eGFR, cardiovascular disease, cancer, serum sodium, serum potassium, and serum phosphorus).
Abbreviations: CKD: chronic kidney disease; CVD: cardiovascular disease.
Subgroup analyses and sensitivity analyses
No significant results were apparent in subgroup analysis stratified by participant characteristics, such as sex, age, BMI, diabetes, preexisting CVD, urinary albumin levels, water intake or CKD stages, which did not suggest effect modification, except potentially by cancer status for some outcomes (Supplemental Figures 2–16, P for interaction > 0.05).
In the sensitivity analysis, even after excluding the participants whose causes of death were related accidents or malignant tumors, with a follow-up time of < 2 years or with early CKD (Stages 1–2) and using nonspecified substitution model to adjusted for total energy intake, there remained no alteration our results that the association between tea consumption and all-cause/CVD mortality in specific CKD population (Supplemental Tables 1–2).
We conducted comprehensive diagnostics for the Cox proportional hazards models to ensure the validity of our findings. Model fit was confirmed using the likelihood ratio test, Wald test, and score test, with satisfactory results. Schoenfeld residuals and global chi-square tests were used to evaluate the proportional hazards assumption, with non-significant results (p > 0.05) across all models, indicating no evidence of time-dependent effects. Residual plots also showed random distribution around zero, further supporting the assumption’s validity. Multicollinearity was assessed using variance inflation factors (VIFs), which were all near 5, indicating no significant issues. Deviance residuals identified outliers, but their influence on the results was minimal. Detailed diagnostics are provided in Supplementary Figures 17–19 and Tables 3–8.
Discussion
This study investigated the association between tea consumption and all-cause/CVD mortality in CKD population. Our findings indicated limiting 4 cups or less of tea per day was significantly associated with lower all-cause mortality in patients with CKD stages 1–2 and 3–5, whereas tea consumption outside of this range had no significant effect. Furthermore, consuming an additional cup of oxidized tea per day was associated with a 10% reduction in all-cause mortality among individuals with CKD stages 1–2. In contrast, green tea consumption was linked to an increased risk of CVD mortality in those with CKD stages 3–5. Additionally, substituting one cup of green tea with one cup of oxidized tea daily was correlated with an 8% and 11% lower risk of all-cause and CVD mortality, respectively, in individuals with CKD stages 1–2. These results suggested that tea consumption might have a protective effect on all-cause/CVD mortality in CKD patients, especially in stages 1–2, with both the quantity and quality of tea being relevant.
CKD has emerged as a significant public health concern in the past few decades, as it has been associated with a substantial rise in mortality worldwide [26]. According to global estimates, CKD-related deaths increased by 41.5% from 1990 to 2017, affecting all age groups and reaching approximately 1.2 million people [41]. CKD patients should follow specific dietary guidelines to decrease metabolic byproduct accumulation and mitigate uremic symptom onset and severity [28]. However, there is currently a lack of evidence on how much tea should be consumed per day for CKD patients, although it has been reported to prevent various diseases in previous research [19–23]. These findings suggest that tea consumption, particularly the type and quantity, may offer protective benefits against mortality in CKD patients, notably in the early stages of the disease.
Existing research suggests that individuals with early-stage kidney disease may have distinct dietary needs compared to those with advanced disease. Importantly, our findings show that consuming up to four cups of tea per day is statistically associated with reduced all-cause mortality in CKD patients at stages 1–2, offering practical dietary guidance for these patients. In contrast, studies in the general population have demonstrated that increasing tea consumption by 3–5 cups per day is linked to decreased CVD risk [2, 42,43], with prospective cohort studies reporting a 31% and 39% reduction in cardiovascular risk among moderate and heavy tea drinkers, respectively [20, 44]. Some studies even suggest that consuming more than ten cups of green tea daily is associated with a lower relative risk of death from CVD [20, 45]. Tea consumption has been found to have a potentially protective effect on the cardiovascular system, which may be attributed to the alteration of certain oxidation indexes [46]. A meta-analysis has indicated that long-term daily consumption of 600–1500 mL of green tea may have a preventative effect on CVD [46]. Some evidence suggests that green tea consumption can decrease lipid oxidation and enhance the overall antioxidant capacity [46]. These benefits of green tea may have a more profound impact on CKD patients who are exposed to high oxidative stress (OS) [47]. However, for the CKD population, the beneficial effects of tea polyphenols, theaflavins, and thearubigins must be balanced with the need to manage fluid intake carefully due to compromised kidney function [12, 36, 48]. Regrettably, our study did not find a significant association between tea consumption and CVD mortality among CKD patients.
The effects of tea consumption on all-cause and CVD mortality in CKD patients may vary based on their tea-drinking habits, including the addition of sugar, artificial sweeteners, or milk, which are often used to enhance the flavor of tea [12, 49–52]. Notably, previous studies did not distinguish between different types of tea, which led us to examine how various teas affect mortality rates in the CKD population [53]. Our study identified a significant protective effect of consuming unsweetened tea on all-cause mortality among CKD patients [6, 54]. Adding sugar to tea is a common practice that can undermine the health benefits of tea polyphenols, which are known to help inhibit insulin resistance [55–58]. Conversely, sugar itself can promote insulin resistance, suggesting that the potential health benefits of tea might not fully offset the adverse effects of added sugar. Moreover, the established dose-response relationship between sugar intake and mortality indicates that CKD patients should be cautious about adding sugar to tea, carefully considering both the amount and the necessity of such additions [58–62].
The health effects of tea can vary depending on the type, such as green, black, oolong, and white tea. Our findings suggest that the consumption of oxidized tea, such as black tea, provides a protective effect against all-cause mortality in the CKD population, especially among those with early-stage disease. This protective effect may be largely attributed to the primary components of oxidized tea, thearubigins and theaflavins, which constitute over 20% and 2–6% of the water-extractable fraction of oxidized tea, respectively [63–65]. These compounds are produced during the enzymatic fermentation of fresh tea leaves and result in structural variations of catechin molecules [66–69]. Extensive research has highlighted the beneficial effects of black tea polyphenols on renal health. These benefits include the reduction of uremic toxins, decreased production of nitric oxide, preservation of antioxidant status, improved liver function, and better regulation of blood urea nitrogen and filtration [69,70]. Although research has often focused more on green tea, with numerous studies highlighting its benefits for cardiovascular health, the significant protective qualities of black tea, particularly for the CKD population, should not be overlooked [20, 71,72].
Interestingly, our research findings revealed a significant association between increased green tea intake and higher CVD mortality rates among individuals with CKD stages 3–5. Regrettably, there is currently inadequate clinical research to substantiate any deleterious effects of green tea on the cardiovascular system of patients with advanced CKD.
Conclusion
In summary, this study was the first to explore the relationship between tea consumption and all-cause/CVD mortality in CKD population. We suggested that CKD patients limit their daily intake of tea to no more than 4 cups, and choose appropriate varieties and flavors, such as oxidized tea and sugar-free tea. Our study results will provide scientific guidance for tea consumption in CKD patients.
Strengths and limitations
This study possesses several notable strengths. First, it is one of the few studies that examined the association between tea consumption and mortality in CKD patients, using a large and nationally representative sample from NHANES. Second, it delved into the impacts of diverse tea types and quantities consumed, with regards to all-cause mortality and CVD mortality. Third, it performed various analyses to test the robustness and validity of the results, such as dose–response analysis, substitution analysis, subgroup analysis, and sensitivity analysis.
This study also exhibits certain limitations. Firstly, the NHANES dietary assessment is subject to recall bias and lacks detailed information on tea preparation methods, which may affect the accuracy of our findings. Secondly, as an observational study, it cannot establish causal relationships between tea consumption and mortality, despite adjusting for various potential confounding variables. There remains the potential for residual or unmeasured confounders to exist. Thirdly, the study employed a limited set of variables to assess tea consumption and types, which may not encompass the full diversity and intricacy of tea-drinking habits. For instance, information pertaining to brewing time, temperature, indicator of oxidant status, or additives used in tea preparation was not available, despite their potential influence on the bioavailability and health effects of tea polyphenols. Fourthly, the study lacked information on cause-specific mortality among CKD patients, which could have provided further insights into the mechanisms and pathways through which tea consumption impacts mortality, beyond cardiovascular causes. Fifth, the study did not incorporate data on genetic variants associated with tea metabolism or response, which may influence the association between tea consumption and mortality. Sixth, we were unable to evaluate the situation of stage 5 CKD patients because only 103 stage 5 CKD patients (accounting for 0.59% of the total sample size) were included in our cohort. Lastly, we recommend limiting tea consumption to less than four cups per day for CKD patients, as no clear additional benefits were observed beyond this amount, aligning with both our findings and existing guidelines.
Based on the above analysis, we speculate that the health benefits associated with tea consumption are dependent on varying amounts and types. For individuals with CKD in stages 1–2, it is suggested that opting for oxidized tea without added sugar and consuming less than 4 cups per day may yield more favorable outcomes.
Supplementary Material
Funding Statement
This work was supported by a grant from the National Science Foundation of China [No. 82374406], Guangdong Basic and Applied Basic Research Foundation [2023A1515030146], Guangzhou University of Chinese Medicine’s Youth Elite Talents Cultivation “List Unveiling and Leadership” Team Project, the Excellent Doctoral Dissertation Cultivation Project of the First Clinical School of Guangzhou University of Chinese Medicine in 2023 [YB202301], and the Funding Project from Guangdong Provincial Department of Education [2024KTSCX114].
Author contributions
L.J., and C.X.L., contributed to study design, literature search, data analysis, statistical analysis and manuscript writing. L.J., C.X.L., OY.X.L., Z.X.J., L.Y., and S.S.N. were involved in literature search, data analysis and manuscript editing. Y.Z.Q., and W.L.J. contributed to the funding acquisition. L.L. and N.S.H. contributed to the study design, funding acquisition, data analysis and the decision to submit the manuscript for publication. All authors were involved in the interpretation of the results and revision of the manuscript and approved the final version of the manuscript.
Disclosure statement
No potential conflict of interest was reported by the author(s).
Data availability statement
Data described in the manuscript, code book, and analytic code will be made available upon request pending application and approval.
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Supplementary Materials
Data Availability Statement
Data described in the manuscript, code book, and analytic code will be made available upon request pending application and approval.