Prebiotics are dietary fibres that are not digestible by humans, but which stimulate the growth of beneficial lactic acid cultures such as bifidobacteria and lactobacilli in the large intestine, which positively influence the intestinal flora.

Prebiotics in breast milk serve as "food" for probiotics (natural lactic acid cultures) and stimulate their growth. Prebiotic dietary fibres such as galacto-oligosaccharides (GOS) and fructo-oligosaccharides (FOS) are also present in some infant formulas. Here there is a difference between plant-based oligosaccharides (FOS, e.g. from chicory, leeks, etc.) and lactose-based oligosaccharides (GOS).

Because breast milk contains only lactose and not fructose, the lactose galacto-oligosaccharides (GOS) are closer to the prebiotic dietary fibres in breast milk than the fructo-oligosaccharides (FOS).