Lactulose is a non-digestible carbohydrate that passes the small intestine and reaches the colon unchanged. There 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.