Lactulose is a non-digestible carbohydrate that is not absorbed in the small intestine. It passes unchanged into the colon, where it is food for gut bacteria. This makes it an example of so-called soluble fibre.
As such it is used as an ingredient for food, food supplements or animal feed.
Moreover, it is applied as a pharmaceutical ingredient to make the stool bulkier, wetter and softer, which helps to relieve constipation.