The mechanism by which coffee protects the liver has not been fully clarified, and it is hypothesized that it may be related to lowering liver enzymes (alanine aminotransferase, glutamate aminotransferase, glutamic oxalate aminotransferase), and slowing down the progression of liver disease. At the same time, coffee can also reduce insulin resistance, improve glucose metabolism to reduce the incidence of diabetes, and indirectly reduce the incidence of liver disease. It has been reported that the main active ingredients of coffee for liver protection are likely to be caffeine, diterpenes and chlorogenic acid. So is caffeine one of the main substances? Decaffeinated coffee in the end there is no lowering effect of liver enzymes? In response to these questions, Prof. Xiao and others from the National Cancer Institute conducted a cross-sectional study, the results of which were published online in the journal Hepatology. The study collected data from the 1999-2010 National Health and Nutrition Examination Survey (NHANES), which selected 62,539 subjects. Exclusion criteria included: failure to complete a personal dietary review survey; failure to complete a liver enzyme test; and <20 years of age. The final enrollment was 27,793. Coffee used in the study was categorized into three types: regular coffee (caffeinated), decaffeinated coffee, and unspecified coffee. The frequency of consumption was assessed by a 24-hour dietary review. Subjects' serologic levels of ALT, AST, ALP and γ-GT were also measured. Finally, weighted multivariate logistic regression (abnormally elevated levels of liver enzymes) and linear regression (continuous liver enzyme levels) were used for analysis. The study found that coffee consumption among the subjects was more skewed toward non-black, older, and smokers. Total coffee consumption was negatively associated with abnormal levels of four liver enzymes, as well as with continuous levels of three liver enzymes: AST, ALP, and γ-GT. The ratio of abnormal levels of ALT, AST, ALP and γ-GT in subjects who consumed more than 3 cups of coffee per day (including 3 cups of coffee) was 0.75, 0.82, 0.73 and 0.69, respectively.The same negative correlation was found between decaffeinated coffee and abnormal levels of ALT, AST and γ-GT. In conclusion, regular coffee consumption, with or without caffeine, reduces liver enzyme levels. A plausible explanation could be that coffee contains some other liver-friendly component, which needs to be confirmed by further experiments in the future. Another study conducted by Bravi F, a professor of epidemiology at the University of Milan-Bicocca in Italy, and his team was published online in the May 2013 issue of Clin Gastroenterol Hepatol. The study found that coffee intake was associated with a reduced risk of liver cancer. The study used PubMed (American Medical Index) to search for original English-language research articles, including case-control or cohort studies, published from 1966 through September 2012 on the association between coffee intake and the development of hepatocellular carcinoma or primary hepatocellular carcinoma, and meta-analyzed these articles. The researchers calculated the combined relative risk (RR) for any coffee intake, low intake, and high intake compared to no coffee intake. Three cups of coffee per day was considered a low intake in nine of the studies, and one cup of coffee per day was considered a low intake in five of the studies. The combined RR for the 16 studies of 3153 liver cancer cases was 0.60 for any coffee intake compared with the no coffee intake group, 0.56 for the 8 case-control studies, and 0.64 for the 8 cohort studies, and 0.72 for the low intake group, 0.72 for the high intake group, and 0.64 for the high intake group, as compared with the no coffee group. The relative risk in the low intake group was 0.72 and the relative risk in the high intake group was 0.44 compared with the no coffee intake group, and the relative risk in the study group with an increase of 1 cup of coffee per day was 0.80. Regardless of the sex of the subjects, their history of alcohol consumption, and their history of hepatitis or liver disease, the negative correlation between coffee intake and the risk of primary hepatocellular carcinoma development was consistent. The negative correlation between coffee intake and primary liver cancer risk was consistent regardless of the sex of the subjects, and was consistent with the risk of primary liver cancer in subjects with a history of hepatitis or liver disease. However, coffee has been shown to affect liver enzymes and the development of cirrhosis, so it may have a protective effect against liver cancer. In summary, coffee can reduce the incidence of liver cancer and the mortality rate of chronic liver disease, and there is no variability among ethnic groups and whether it contains caffeine or not, but the exact mechanism of its prevention of liver cancer and chronic liver disease and its active ingredients have not yet been fully clarified, and further research is needed. The above studies show that coffee can prevent the occurrence of liver cancer and chronic liver disease.