Lactose sugar molecule structure, illustration. Lactose (bottom) is a disaccharide sugar found in milk. It is made of two monosaccharide units (shown across top), one of galactose (left) and one of glucose (right), forming a beta 1-4 glycosidic linkage (shown at lower centre). Lactose is an important sugar in the diet, but some people are lactose intolerant, and cannot digest the sugar, causing cramps and weight loss. Here, the molecules are shown as ball-and-stick models, with the atoms as colour-coded spheres: carbon (blue), hydrogen (green) and oxygen (pink).