From the American Herbal Products Association (AHPA):

“Although this study was done in rats, it demonstrates an indirect pathway for antioxidant compounds to exert an antioxidant effect. Instead of a direct antioxidant effect from antioxidant compounds in coffee that could correlate with an ORAC assay, the antioxidant effect produced here was at least in part due to an increase in liver enzyme activities. It is notable that this effect was seen at physiological levels corresponding to a human dose of about four medium cups of coffee and that it lasted for four hours.”

Increase of the Activity of Phase II Antioxidant Enzymes in Rats after a Single Dose of Coffee (J. Agric. Food Chem., 2011, 59 (20), pp 10887–10892)

Abstract

This study evaluated the acute effect of the administration of coffee brew in the activity of phase II antioxidant enzymes in the hepatic tissue of rats. A single dose of this beverage increased the activity of the enzymes SOD, CAT, and GPx; the maximum increase occurred 1 h after administration (19.1, 22.1, and 25.1%, respectively). These changes were statistically significant (p < 0.05), the response was shown to be dose-dependent (p < 0.05), and the return to basal levels took >4 h from the intervention, suggesting a long-term effect. The total antioxidant capacity of the hepatic tissue also exhibited a peak 1 h after the intervention (6.5%), but the increase was not statistically significant and the response was not dose-dependent due to the low exposure to coffee. These results indicate that coffee increases the activities of antioxidant enzymes, improving protection against oxidative stress.