The disclosure relates to a method for providing a composition comprising sialic acid containing oligosaccharides, comprising the steps of: a) providing a source of non-digestible galactooligosaccharides (GOS) containing at least two terminally bonded beta-linked galactose residues； b) providing a sialic acid donor having (alpha2-3)-sialylated O-glycans； c) contacting said GOS with said sialic acid donor in the presence of an enzyme having trans-sialidase activity in an enzyme reaction mixture； and d) obtaining from said enzyme reaction mixture a fraction comprising at least 5 percent by weight of disialylated galactooligosaccharides (di-Sia-GOS) based on the dry matter.
