Onions, especially raw onions, are a common trigger for heartburn.

Like other foods on this list, onions may relax the lower esophageal sphincter, which may cause acid reflux and symptoms of heartburn.

In one study, people with heartburn ate a plain hamburger on one day, followed by an identical hamburger with onions on another day. Eating the burger with onions significantly worsened heartburn symptoms, compared to eating the burger with no onions

In addition, onions are a rich source of fermentable fiber, which may cause belching. Belching can aggravate acid reflux symptoms (25).

The fermentable fiber onions contain is made up of FODMAPs, a group of compounds that may trigger digestive issues.

Onions, especially raw onions, may cause acid reflux and heartburn symptoms by relaxing the lower esophageal sphincter. In addition, onions are high in fermentable fiber, which may promote acid reflux symptoms