Levels of oxidative stress in the circulation have been shown to reduce in response to xanthine oxidase inhibition in subjects with heart failure [69, 70], diabetes [79], metabolic syndrome [85], obstructive sleep apnoea [86], coronary artery disease [88, 90], chronic obstructive pulmonary disease [96, 97], and liver disease [99]. This evidence concerns the gene XDH and diabetes mellitus.