Lactulose (galacto-fructose) is a semi-syntetic disaccharide derived from lactose by chemical or enzymatical isomerization. Lactulose is a prebiotic substance, mostly used for the pharmaceutical treatment of specific medical conditions (i.e., to alleviate constipation and to prevent/treat hepatic encephalopathy) but also in food applications. Although not present in nature, it occurs in heat-treated milk products as a result of catalyst-free isomerization of lactose. Galacto-fructose has a long history of use in humans as a prebiotic (since the 1950s) and has been recognized as the first prebiotic with a bifidogenic properties. From a regulatory perspective, lactulose is considered as a standard nutritional food ingredient (not a novel food). Its prebiotic and intestinal effects have been demonstrated and the related health claims approved (Quierzy, 2009).
The chemical structure of galacto-fructose is key for its prebiotic effect. The galacto- frutcose passes through the stomach and ileum without any degradation and is able to reach the colon intact about one and a half hours after its ingestion. In the colon, there are mainly saccharolytic and proteolytic bacteria; the former are able to degrade the carbohydrates by fermentation and the latter break down the proteins. Galacto-fructose is degraded by saccharolytic bacteria, like bifidobacteria, lactobacilli and some species of streptococci. Several studies have shown that after supplementation of galacto-fructose, the bifidobacteria population increases, confirming its bifidogenic effect (Terada et al., 1992; Tomoda et al., 1991). Since increased populations of bifidobacteria and other probiotic species can contrast the growth of undesirable bacteria like Bacteroides and Clostridia, for example, the synergistic effect of galacto-fructose and probiotics is well recognized. The prebiotic effect of galacto-fructose can address several factors, such as acidification of gut contents, ammonia depletion, increased osmotic pressure, and increased peristalsis.
Galacto-fructose improves the gut transit on healthy people, exerting an osmotic action in the small intestine: It draws water into the intestinal lumen, increasing the mass of the stool and consequently stimulating the intestinal musculature and bowel movements. In the colon, the biomass, stool volume, and osmotic pressure are increased and pH decreased, leading to an accelerated transit time. The dosage of galacto-fructose as a prebiotic substance, associated with gut transit, is 10 g per day, as recognized by EFSA (EFSA Journal, 2010, p. 1806).
Several scientific evidences (e.g., in post-menopausal women) have shown a better absorption of calcium and magnesium after supplementation of galacto-fructose (Van den Heuval et al., 1999).
Arriving undigested in the colon, galacto-fructose is an ideal promotor of positive and safe bacteria in the bowel microbiota, and for this reason may have positive protective effects on the human gut microbiota. In fact, galacto-fructose allows a differential growth in colonic bacteria that reflects well the changes in enzymatic activities. A reduced activity of some enzymes at the origin of potentially toxic compounds like putative co-carcinogens has been observed (Ballonguee et al., 1997; Quierzy, 2009). Galacto-fructose may inhibit bacterial ammonia production by acidifying the content of the colon and reducing the activity of the proteolytic bacteria. Galacto-fructose, by reducing colonic transit time, also helps to reduce the time available for ammonia production and speeds up ammonia elimination. A possible DNA-protective effect of galacto-fructose has been proposed (Rowland et al., 1996), but this issue is still under investigation.