ATP12A and gastroesophageal reflux disease: PPI: proton pump inhibitors; MMA: methylmalonic acid; holoTC: holotranscobalamin; Hcy: homocysteine; H2RAs: histamine receptor antagonists; ZES: Zollinger-Ellison syndrome; MCV: mean corpuscular volume; GERD: gastro-esophageal reflux disease; H. pylori: Helicobacter pylori