Lactose. Computer-generated molecular model of lactose. Lactose is composed of carbon (green), hydrogen (grey-white) and oxygen (red). Lactose is a saccharide (sugar) that occurs naturally in both human and cow's milk. Saccharides consist of rings of four or five carbon atoms and one oxygen atom. Attached around the ring are hydroxyl (OH) groups. Disaccharides comprise two different monosaccharide units joined together, in this case glucose and galactose. A deficiency of the enzyme lactase, which breaks down lactose, can lead to lactose intolerance.