REN and atrial fibrillation: Vitamin D deficiency can activate the renin–angiotensin system and inflammation, leading to a poor prognosis in heart failure, and vitamin D supplementation decreases plasma renin concentration.23 While some epidemiological studies have demonstrated the higher prevalence of AF after CABG, some other studies have shown no relationship between vitamin D levels and postoperative AF.24-27 Accordingly, given the controversy surrounding this issue, we sought to evaluate the effects of vitamin D levels on the occurrence of AF after CABG.