Lactulose is made from lactose, a natural component in milk.
It has several characteristics which make it an attractive and effective energy source for gut bacteria such as lactobacilli and bifidobacteria: 

• It is indigestible by human enzymes in the gut
• It is non-absorbable in the small intestine
• It passes unchanged into the colon

Already in 1957 it was demonstrated that lactulose stimulates the growth of colonic bacteria in adults and infants.
Due to these established beneficial effects, lactulose is used as a food and feed ingredient to support the colonic flora.