Several studies have revealed the benefits of moderate caffeine intake to healthy adults by reducing the risk of cardiovascular disease, obesity, diabetes and cancer(20). However, the dietary patterns of healthy people do not apply to CKD patients(21). Currently, there is limited research on the impact of caffeine intake on the prognosis of CKD patients. A previous study showed that caffeine intake in CKD patients was linearly and negatively correlated with all-cause mortality, and had no significant association with cardiovascular mortality and cancer mortality(12). Nevertheless, nonlinear association between caffeine intake and mortality among CKD patients has not been explored and the optimal caffeine intake for CKD patients remains unknown. Thus, our study aimed to select updated NHANES datasets to analyze the linear and nonlinear relationship between caffeine intake and mortality in CKD patients and explore the optimal caffeine intake for CKD patients.
In our study, the results of linear analysis suggested that caffeine intake was not significantly associated with all-cause mortality, cardiovascular mortality, cancer mortality, cerebrovascular mortality, nephropathy mortality and influenza and pneumonia mortality among CKD patients. Our results are not consistent with the previous study mentioned above. The reason may be that updated NHANES datasets were selected in our research so that our included population tended to consume more caffeine. This may account for the failure of fitting the relationship between caffeine intake and prognosis among CKD patients with linear model.
In comparison to the simple and intuitive interpretation ability provided by linear regression, nonlinear regression is more suitable for fitting complicated data patterns. Therefore, RCS analysis was performed to further explore the nonlinear association between caffeine intake and mortality among CKD patients. The results showed a U-shaped relationship (inflection point = 277mg) between caffeine intake and all-cause mortality. A tall of Americano typically contains about 150mg caffeine. For healthy adults, the recommended daily caffeine intake is less than 400mg(22). Thus, CKD patients may be suitable to consume less caffeine than healthy adults. It is recommended that CKD patients intake moderate amount of caffeine daily (about 1.85 talls of Americano) to reduce the risk of all-cause death. In addition, the results showed a J-shaped relationship between caffeine intake and cardiovascular (inflection point = 252mg, about 1.68 talls of Americano) and cancer mortality (inflection point = 79mg, about 0.53 talls of Americano).
The association between coffee consumption and the risk of CKD is controversial. Three relevant meta-analyses were retrieved, two of which supported the role of coffee consumption in reducing the risk of developing CKD(16, 23), while the other did not find a statistical association(24). The molecular mechanism by which caffeine affects the kidney is unclear. The protective effect of caffeine on the kidney may be attributed to the antioxidant ability of caffeine. Oxidant-antioxidant system imbalance always occurs in CKD patients, which can result in kidney damage and progression of CKD by inducing inflammatory responses and renal interstitial fibrosis(25). Reactive oxygen species production can be reduced and oxidative stress can be inhibited by caffeine and its methylxanthine metabolites through antagonizing adenosine receptor(26). However, adenosine also acts as an endogenous anti-inflammatory agent in humans. The antagonistic effects of caffeine on adenosine receptors may also exacerbate glomerular and interstitial inflammation(27). It has been demonstrated in animal studies that caffeine does not cause pathological effects on the kidneys with normal function and structure, but aggravates adverse effects on the kidneys with preexisting diseases(28–31). Thus, combined with the results in our research, we speculated that caffeine may play a dual role on the kidneys. Moderate caffeine intake may protect the kidney through antioxidants, but excessive caffeine intake may cause kidney damage through pro-inflammatory effects.
The effects of caffeine intake on the occurrence of cardiovascular events have been extensively studied. The results of a large prospective cohort study based on the general population showed that moderate caffeine intake from coffee (121-180mg/ day) had the highest protective effect against coronary heart disease(32). However, our research did not find the protective effect of caffeine against cardiovascular mortality in CKD patients and suggested that excessive caffeine intake may increase cardiovascular mortality. It has been reported in a recent study that low density lipoprotein (LDL) levels in the blood can be reduced by caffeine through inducing the expression of LDL receptors on liver cells(33). The reduced LDL levels may account for the protection effect of caffeine on coronary heart disease. However, excessive caffeine intake can lead to a sharp increase in blood pressure, which may explain the increased cardiovascular risk caused by excessive caffeine consumption(34).
Caffeine has the potential to prevent and control various types of cancer(35–37). It may play a role in inhibiting proliferation, promoting apoptosis, preventing metastasis, enhancing immune response(36, 38–40). However, it was suggested in our study that excessive caffeine consumption may increase the cancer mortality in CKD patients. There is few research on the cancer risk of caffeine, which deserves further research.
Our research has several advantages. First, CKD patients, a special population, were selected as our research objects. It is the first time to investigate the nonlinear relationship between caffeine intake and mortality and explore the optimal daily caffeine intake in CKD patients. Second, our study was based on the NHANES database, which adopts the method of sampling weights to make the data representative. All of our analyses took appropriate weights into account. Anyway, there are some limitations in our study. Although some potential covariates were adjusted in this study, the influence of other possible confounding factors could not be completely excluded.