Improvement in endothelial function has been observed following xanthine oxidase inhibition in patients with heart failure [65, 69, 70], coronary artery disease [75, 90], diabetes [78], hypercholesterolaemia [81], smokers [83, 95], high overall cardiovascular risk [84], metabolic syndrome [85], obstructive sleep apnoea [86], and stroke [87]. This evidence concerns the gene XDH and metabolic syndrome.