The current meta-analysis included data from 22,963 patients with kidney stones from four 6 cohort studies[9–14], 5 case-control studies[15–18]. Compared with controls, tea drinking had a significant preventive effect on the occurrence of kidney stones (OR = 0.86; 95% CI, 0.81 − 0.91). This is the first comprehensive analysis of the protective effect of tea consumption on kidney stones. Subgroup analysis showed that the risk of kidney stones decreased with increasing tea consumption, there were no significant differences between men and women, howere, Tea consumption has been shown to reduce kidney stones more in Chinese than in Europeans/Americans
The sensitivity analysis in this meta-analysis confirmed the validity of our conclusions about the comparison between tea drinking and never tea drinking. The strength of our study lies in the complete adherence to a systematic review approach, including a two-stage screening process for published titles and abstracts by independent researchers, an assessment of the quality of each study, and an assessment of the risk of bias. All of the studies were well-designed and included people recruited from the general population, suggesting that the effects of the intervention were unlikely to depend on specific environmental factors.
Changes in dietary habits are key to preventing stone formation and recurrence[24], and coffee and tea intake have previously been reported to be inversely associated with kidney stones[25, 26]. Coffee and tea are sources of caffeine, which has been reported to moderately increase diuresis, while promoting excretion of magnesium, potassium, calcium, and sodium[27]. Recent evidence suggests that caffeine-induced natriuresis may be caused by Shirley DG[28]. Based on an experimental model, Ming and Lautt[29] proposed that caffeine, as an inhibitor of adenosine receptors, may have diuretic and natriuretic effects by interfering with hepatic A1 adenosine receptor-mediated liver-kidney reflex. Caffeine also reduces Na/KATPase and Na/H exchange activities and increases ANP and NO activities in the kidney[30]. Jeong et al.[31] evaluated the cytotoxicity of epigallocatechin gallate (EGCG), one of the main components of green tea catechins, against oxalate in an animal model and found that EGCG inhibited oxalic acid-induced free radical production. In addition, tea can reduce the excretion of oxalic acid in the urine, and green tea has an inhibitory effect on the formation of urinary stones, which is largely due to the antioxidant effect of EGCG. Taken together, these changes increase diuresis and have a potentially protective effect against stone formation. Tea is one of the most common drinks in China, next only to water. Even though the oxalic acid content in tea poses a risk for stone formation, based on a large number of clinical studies, we found that the vast majority of people favor the idea of a protective effect against urolithiasis[32]. There are several factors for this. First, tea consumption is essentially a supplement to daily fluid intake, and this is only the beneficial part of explaining the effects of tea on stone formation. In addition, tea contains caffeine, which has diuretic and natriuretic effects on the kidneys; however, the specific mechanism by which tea exerts a protective effect against kidney stones is still unclear.
Several potential limitations need to be considered, as follows: (1) lacking accurate information regarding exposure to tea intake. Despite a feasibility of crude classifications, typo was inevitable; (2) tea is divided into different species and different substances, which may affect the results; (3) due to the different measurement methods used in the study on the amount of tea consumed, it is difficult to unify the information on the dose-response relationship in this study; and (4) we selected study subjects adjusted for potential confounders, however, it is difficult to demonstrate the presence of other risk factors that may contribute to kidney stones. For example, there are people in the Chinese population who drink both tea and coffee, and studies have shown that drinking coffee can reduce the incidence of kidney stones[33]. In conclusion, this meta-analysis showed that tea consumption was more effective in reducing the risk of kidney stones in the population and may play a key role in preventing stone formation and recurrence.