What Is Acid Reflux Disease? At the entrance to your stomach is a valve, which is a ring of muscle called the lower esophageal sphincter (LES). Normally, the LES closes as soon as food passes through it. If the LES doesn't close all the way or if it opens too often, acid produced by your stomach can move up into your esophagus. This can cause symptoms such as a burning chest discomfort called heartburn. If acid reflux symptoms happen more than twice a week, you may have acid reflux disease, also known as gastroesophageal reflux disease (GERD). What Causes Acid Reflux Disease? One common cause of acid reflux disease is a stomach abnormality called a hiatal hernia. This occurs when the upper part of the stomach and LES move above the diaphragm, a muscle that separates your stomach from your chest. Normally, the diaphragm helps keep acid in our stomach. But if you have a hiatal hernia, acid can move up into your esophagus and cause symptoms of acid reflux disease. ...