A reliable tool for measurement of caffeine consumption was developed and used to demonstrate that caffeine intake above a threshold was associated with less severe fibrosis on liver biopsy. The protective association of caffeine was most pronounced in patients with HCV infection, however the number of patients with other liver diseases was relatively small (n=56; 32%). In the HCV cohort, the protective association of caffeine on liver fibrosis remained significant whether evaluated as a continuous variable, categorized as coffee-cup equivalents or dichotomized above or below the 75th percentile for the study population. After controlling for other factors known to affect fibrosis (age, sex, race, BMI and alcohol consumption), the apparent protective effect of caffeine persisted. In keeping with the reduced fibrosis on liver biopsy, patients with greater caffeine consumption also had lower AST, alkaline phosphatase, direct bilirubin and increased serum albumin levels. Together these data suggest that increased caffeine consumption is associated with less advanced liver fibrosis.
Categorization of caffeine intake by coffee-cup equivalents or quartile suggested the protective effect of caffeine may not be linear and there appears to be a threshold effect. The proportion with advanced fibrosis and the mean liver test values were similar between patients consuming 0-1 and 1-2 coffee-cup equivalents of caffeine per day, but patients reporting greater than 2 coffee-cup equivalents of daily caffeine had a lower rate of advanced fibrosis and a trend toward lower aminotransferase levels and improved hepatic synthetic function (bilirubin, prothrombin time). Notably when compared to patients in the lowest quartile of caffeine consumption, those in the 2nd and 3rd quartile showed a trend toward more advanced fibrosis (). Whether this truly implies that at low levels of caffeine intake, there is a harmful effect to increasing caffeine consumption is hard to discern. The numbers of patients in each group were relatively small and after controlling for other factors, the apparent associations were not significant. This finding did however strengthen the suggestion that the potential beneficial effects of caffeine were not linear and that consumption above a threshold (approximately 2 coffee-cup equivalents per day) was necessary to have an effect on hepatic fibrosis. Clarification of whether there is a hepatoprotective threshold and whether the benefits plateau with further consumption will be important for understanding the biology and potentially for therapeutic recommendations.
Most previous studies of caffeine's health effects have focused largely on coffee consumption rather than total caffeine intake. The instrument developed for this study allowed for a relatively detailed breakdown of sources of dietary caffeine intake. However, for the purposes of analysis, it was necessary to assume that all caffeine sources of a given type contained equal amounts of caffeine irrespective of brand, the process of production and other factors. The use of visual aids likely improved the reliability of estimates. Responses were consistent on repeat testing suggesting that the instrument can provide reproducible results and that caffeine consumption stays relatively constant over time, at least for the study period.
To tease apart whether the beneficial effects seen were related to caffeine or coffee intake, each component was evaluated individually. Consistent with previous reports, no beneficial effect was seen with green or black tea, caffeinated soda or any other sources of caffeine5
. However, a significant protective effect could have been missed due to small numbers, as 71% of total caffeine consumed came from coffee. Alternatively, if the beneficial effect of caffeine on fibrosis requires consumption above a threshold of daily caffeine, any benefit of non-coffee related caffeine may have been inapparent because the absolute amount of caffeine consumed from sources other than coffee was relatively low (75th
percentile: 61 mg from non-coffee sources vs 270 mg from coffee). The observation that the association with less advanced liver fibrosis was seen only with caffeinated coffee implies either that the benefit is derived from caffeine (all caffeine or only that in coffee) or possibly to a substance removed by the decaffeination process. Different decaffeinating procedures were not evaluated.
Race was an important effect modifier of the caffeine-fibrosis relationship. Caucasian patients consumed the most caffeine and the protective association with advanced fibrosis was most apparent in this group. It is difficult to draw strong conclusions about the results in the non-Caucasian patients due to the relatively small numbers. The observation that non-Caucasian patients in the highest quartile of caffeine consumption for this group did not have a lower odds of advanced fibrosis may simply be due to the fact that even the highest quartile in this group consumed much less caffeine than the apparent protective threshold.
Previous studies have shown that increased coffee consumption is associated with lower liver enzymes, reduced rates of liver cancer and possibly even reduced hepatic decompensation and liver-related mortality2, 4-11
. The assumption has been that reduced fibrosis was due to a reduction in disease activity as reflected by serum aminotransferase levels. However, because most studies have relied on population surveys, liver histology was not evaluated, and the possible effects of coffee/caffeine on liver fibrosis had to be indirectly assessed. The distinction between anti-fibrogenic effects and protection against decompensation is important in understanding the underlying beneficial mechanism. With complete liver biopsy data on all 177 patients, across the spectrum of liver fibrosis, the data from this study suggest that the beneficial effect of caffeine is mediated through reduced rate of progression of fibrosis. However the lack of association between caffeine intake and hepatic inflammation, suggests that rather than reducing fibrosis by minimizing ongoing inflammation, the protective effect of caffeine may be mediated through a direct anti-fibrogenic mechanism
Recent in vitro
data suggest possible mechanisms by which coffee and/or caffeine may affect liver disease and specifically hepatic fibrogenesis. Studies in mice and rats as well as human hepatoma cell lines, have shown that coffee and some of its major components (caffeine, cafestol and kahweol) alter expression and activity of enzymes involved in xenobiotic metabolism25-28
. Inhibition of Phase I enzymes and up-regulation of Phase II enzymes such as glutathione-S-transferase have been reported, both of which would favor reduced accumulation of toxic metabolites within hepatocytes27
. Pre-treatment with cafestol and kahweol protected mice from carbon tetrachloride (CCL4
) hepatotoxicity by inhibiting cytochrome CYP 2E1, the enzyme responsible for CCL4
. With respect to caffeine specifically, Gressner and colleagues recently reported that caffeine inhibits expression of connective tissue growth factor (CTGF) by interfering with transforming growth factor beta (TGFβ) signaling through the SMAD pathway30
. Caffeine was also found to up-regulate peroxisome proliferator-activated receptor gamma (PPARγ) levels, which further reduce CTGF expression. Although these results from primary cell culture clearly need in vivo
confirmation, inhibition of the TGFβ pathway is an attractive explanation for anti-fibrogenic effects attributed to caffeine.
It is important to consider potential confounding factors when interpreting the data from this study. The study was cross-sectional in nature, and caffeine consumption was estimated at the time of liver biopsy despite the fact that any protective effect would likely occur over many years. Patients consuming the greatest amount of caffeine had less fibrosis on biopsy. Although it is tempting to conclude that caffeine has a protective effect on fibrogenesis, other explanations are also possible. Patients with more advanced liver fibrosis may have reduced their caffeine intake because of a presumption that caffeine may not be good for their health. Caffeine is metabolized by the liver and therefore it is also possible that as hepatic function deteriorated, patients may have required less caffeine to achieve the same physiological effects, leading them to reduce their intake over time. By asking whether caffeine consumption patterns had changed in the past 6 months or 5 years, an attempt was made to discern if patients with more advanced fibrosis were decreasing their caffeine intake. Most patients did not report a change in caffeine consumption patterns over time, but this is clearly an imperfect measure of this trend. Importantly however, of patients reporting a change in intake over the past 5 years, there were similar numbers with and without advanced fibrosis, suggesting that worsening liver disease was not the impetus to alter consumption of caffeine. Other factors that may affect caffeine consumption such as socioeconomic status, education level and recreational drug use, were also not considered in this analysis.