When you're young, anything goes down your mouth. Your metabolism burns calories quickly, and eating or drinking only bothers you going in or out.
As you age, though, things appear to slow down and the body begins to provide more input than it formerly did.
The fermentable oligosaccharides, disaccharides, monosaccharides, and polyols (FODMAPs) found in onions and other foods may be responsible for onion-related indigestion.
FODMAPs are essentially certain sugars & other carbohydrates that are more difficult for stomach acid to break down, resulting in fermentation in the stomach & prolonged digestion.
The University of Michigan Health reports that as people age, their stomachs produce less acid, making it more difficult to digest meals.
As FODMAPs ferment in the stomach, they can lead to gas buildup and bloating as the stomach works to digest the food.
In addition to virtually every form of onion, Healthline recognizes wheat and wheat products, garlic, and artichokes as FODMAP-rich foods.
FODMAPs can also contribute to acid reflux. Gases in the stomach can trigger belching and other reactions, causing stomach acid to backflow into the esophagus.
Cooking meals high in FODMAPs has been shown to reduce some of the symptoms, as prepared food is easier for your stomach to digest.