GPT and Cholestatic liver disease: Although ALT, AST and GGT are considered markers of liver injury because they are released from disrupted hepatocytes in patients with NAFLD, the levels of GGT, a key enzyme involved in glutathione and cysteine metabolism, are increased in not only patients with NAFLD but also patients with many other conditions, including oxidative stress, cholestatic liver disease and ethanol exposure [32].