Galacto-oligosaccharides (GOS) belong, together with fructo-oligosaccharides (FOS) and inulin, to the group of non-digestible carbohydrates (NDO). NDO may be regarded as soluble dietary fibres as regards their compliance with a generally accepted definition of dietary fibre including both biochemical and nutritional/physiological criteria. GOS, along with other soluble fibres, is a fermentable substrate for anaerobic bacteria in the large intestines. The main fermentation products are the short chain fatty acids acetate, propionate and butyrate. Short chain fatty acids reduce the colonic pH, which results in precipitation of bile acids and inhibition of the formation of secondary bile acids. High concentrations of secondary bile acids have been associated with an increased colon cancer risk.