(Cue Minion voice) Whhhaaaaaaaatt??? I know, who knew?
There are two main reasons why sugar-free gum bloats you. First off, you naturally swallow a lot of extra air while chewing gum. Secondly, and arguably the main reason, is the presence of sugar alcohols in sugar-free gum. Sugar alcohols are a form of carbohydrate that are not digested well.
Once in your intestinal tract, the sugar alcohol meets bacteria that ferments it, releasing gas, leading to bloating, cramps, pain, and/or diarrhea. So while sugar alcohols contain less calories, they actually cause you to look bigger by bloating you.
Watch out for sugar alcohols by looking for ingredients ending in –ol. Some common names in labels are xylitol, mannitol, sorbitol, maltitol, and erythritol.