Why do sugar-free sweets upset my stomach?

Almost always the sugar alcohols. Sorbitol, maltitol, xylitol and their relatives are poorly absorbed in the small intestine, so they draw water into the gut and are fermented by bacteria further along. US regulation prescribes the exact warning sentence for foods whose foreseeable consumption could deliver 50 grams of sorbitol a day, which is why that line appears on sugar-free sweets and not on ordinary ones.

Sugar-free sweets and stomach upset: the polyol warning

Most warnings on food packaging are read as legal noise. This one is a description of what will happen.

The mechanism, in one paragraph

Sugar alcohols are sweet and they are not sugar, which is the commercial point. The reason they are not sugar is that the body handles them badly: they are absorbed slowly and incompletely in the small intestine.

What is not absorbed stays in the gut, pulls water in behind it, and then meets the bacteria of the large intestine, which ferment it. Water plus fermentation is the whole story, and the symptoms follow from it directly.

Sorbitol is the most common one, and maltitol, xylitol, isomalt, mannitol and erythritol are the rest of the family. Erythritol is the outlier, being absorbed better than the others and therefore causing less trouble at the same dose.

Why the warning exists

The US regulation does not just allow a warning. It writes it out:

The label and labeling of food whose reasonably foreseeable consumption may result in a daily ingestion of 50 grams of sorbitol shall bear the statement: “Excess consumption may have a laxative effect.”

A named quantity and a fixed sentence. Very few ingredients get that treatment, and Aspartame carries the other well-known example for an entirely different reason. Both are worth reading precisely because they are so rare.

How much is actually in there

The same regulation puts numbers on the levels good manufacturing practice produces: up to 99 percent in hard candy and cough drops, 98 percent in soft candy, 75 percent in chewing gum, 30 percent in baked goods, and 12 percent everywhere else.

So a sugar-free sweet is mostly sorbitol by weight, and fifty grams is a smaller pile than it sounds. That is the arithmetic behind the warning, and it is unusual: for almost every other ingredient a label gives you order and never amount, which is the limit the guide to reading a label sets out.

Children

Sugar-free sweets aimed at children concentrate this, because a small person eating an adult quantity crosses the threshold sooner. The label sentence is accurate and the variable is how much, which is the shape of most real questions on the page for parents.