Gastritis is an inflammation, irritation, or erosion of the lining of the stomach (and gastritis can worsen to the point of causing an ulcer).

High amounts of pressure on the abdomen can cause the muscle at the end of the food pipe (the lower esophageal sphincter) to grow slack or weak, allowing the acid to backwash up into the esophagus. This is why acid reflux is common in people who are overweight, obese, or pregnant. This is also why weight loss helps reduce symptoms because it decreases pressure on the food pipe.

For more information, here’s a reference sheet.