A detailed study was performed to compare the in vivo ileal digestibility and modulatory effects in fecal microbiota of novel
galacto-oligosaccharides (GOS) derived from lactulose [GOS-Lu; degree of polymerization (DP) 2,14.0andcommercialGOSderivedfromlactose(GOS−La;DP3, 35.1% trisaccharides) in growing rats (5 wk old). Rats were
fed either a control diet or diets containing 1% (wt:wt) of GOS-Lu or GOS-La for 14 d. Quantitative analysis of
carbohydrates from dietary and ileal samples demonstrated that the trisaccharide fraction of GOS-Lu was significantly
more resistant to gut digestion than that from GOS-La, as indicated by their ileal digestibility rates of 12.5 6 2.6% and 52.9
6 2.7%, respectively, whereas the disaccharide fraction of GOS-Lu was fully resistant to the extreme environment of the
upper digestive tract. The low ileal digestibility of GOS-Lu was due to the great resistance of galactosyl-fructoses to
mammalian digestive enzymes, highlighting the key role played by the monomer type and linkage involved in the
oligosaccharide chain. The partial digestion of GOS-La trisaccharides showed that glycosidic linkages (1/6) and (1/2)
between galactose and glucose monomers were significantly more resistant to in vivo gastrointestinal digestion than the
linkage (1/4) between galactose units. The absence of GOS-La and GOS-Lu digestion-resistant oligosaccharides in fecal
samples indicated that they were readily fermented within the large intestine, enabling both types of GOS to have a
potential prebiotic function. Indeed, compared with controls, the GOS-Lu group had significantly more bifidobacteria in
fecal samples after 14 d of treatment. The number of Eubacterium rectale also was greater in the GOS-Lu and GOS-La
groups than in controls. These novel data support a direct relationship between patterns of resistance to digestion and
prebiotic properties of GOS. J. Nutr. 142: 1232–1239, 2012.Peer reviewe
