Why lactose is non fermentable?

Lactose is a disaccharide composed of glucose and galactose, and it is considered non-fermentable by organisms that lack the enzyme lactase, which is necessary to break it down into its monosaccharide components. In individuals who are lactose intolerant, the absence of lactase prevents the fermentation of lactose in the gut, leading to its accumulation and causing digestive discomfort. Certain microorganisms can ferment lactose if they possess the appropriate enzymes, but in many cases, lactose remains unfermented in the human digestive system.