Turin A, et al. - Researchers investigated the link between vitamin D deficiency and incident atrial fibrillation (AF). They focused on characterizing this link in the setting of renin-angiotensin-aldosterone system (RAAS) inhibition. Retrospective identification of patients with 25-hydroxyvitamin D (25[OH]D) laboratory testing was followed by propensity score matching to select comparable groups of those who initiated angiotensin converting enzyme inhibitors (ACEI)/angiotensin receptor blockers (ARB) (AI) vs those who did not (NAI). They found no link between 25(OH)D deficiency and incident AF. 25(OH)D deficiency resulted in a reduced benefit of ACE/ARB use, indicating a possible role of 25(OH)D as a cofactor in the mitigation of AF by RAAS inhibition.