Onions also contain a natural fiber called inulin, which is basically a VIP pass for your gut bacteria. Inulin acts as a prebiotic—feeding the good bugs that keep your digestive system running smoothly. But here’s the catch: too much inulin too fast, and you get a rapid, bubbly evacuation.
There are also compounds in onions called thiosulfinates. These are the chemicals that make you cry when chopping, but inside your gut, they can relax the muscles of your intestinal wall. That muscle relaxation can speed up transit time, meaning what goes in must come out sooner.
Purple Onions And Their Effect On Stool And Urine Color | MedShun
The FODMAP Factor
If you’ve ever heard of the low-FODMAP diet, onions are the supervillains of that list. FODMAPs are short-chain carbs that ferment quickly, and onions are loaded with them. For someone with irritable bowel syndrome (IBS), a single onion ring can feel like a personal attack.
One study found that about 76% of people with IBS report onions as a trigger food. That’s not just a gut feeling—it’s a statistical stampede. So if you’re prone to dramatic bathroom scenes, you might want to swap onions for their milder cousins, like chives or scallions.
