Lactulose is a non-absorbable sugar used in the treatment of constipation and hepatic encephalopathy. It is a disaccharide (double-sugar) formed from one molecule each of the simple sugars (monosaccharides) fructose and galactose. Lactulose is not normally present in raw milk but is a product of heat-processed: the greater the heat, the greater amount of this substance. Lactulose is not absorbed in the small intestine nor broken down by human enzymes, thus stays in the digestive bolus through most of its course, causing retention of water through osmosis leading to softer, easier to pass stool. It has a secondary laxative effect in the colon, where it is fermented by the gut flora, producing metabolites which have osmotic powers and peristalsis-stimulating effects (such as acetate), but also methane associated with flatulence. Lactulose is metabolized in the colon by bacterial flora to short chain fatty acids including lactic acid and acetic acid. These partially dissociate, acidifying the colonic contents (increasing the H+ concentration in the gut).[14] This favors the formation of the nonabsorbable NH+4 from NH3, trapping NH3 in the colon and effectively reducing plasma NH3 concentrations. Lactulose is used in the treatment of chronic constipation in patients of all ages as a long-term treatment. Lactulose is used for chronic idiopathic constipation, i.e. chronic constipation occurring without any identifiable cause. Lactulose may be used to counter the constipating effects of opioids, and in the symptomatic treatment of hemorrhoids as a stool softener.

Epilactose (4-O-beta-galactopyranosyl-D-mannnose) is a rare disaccharide in cow milk that can be synthesized from lactose by the cellobiose 2-epimerase of Ruminococcus albus. Epilactose is a promising prebiotic. Dietary supplementation with epilactose (4.5%) increased cecal wall weight and cecal contents and decreased the pH of the cecal contents in Wistar-ST rats. The number of total anaerobes tended to be greater in rats fed epilactose than in those fed the control diet. Epilactose inhibited the conversion of primary bile acids to secondary bile acids. Epilactose also increased the calcium absorption in everted small intestinal sacs. In addition, the levels of plasma total cholesterol and nonhigh-density lipoprotein cholesterol were lower in epilactose-fed rats. These results indicate that epilactose promotes calcium absorption in the small intestine and possibly lowers the risk of arteriosclerosis.

Lactose is the most important carbohydrate in the milk of most species. Its biosynthesis takes place in the mammary gland. The molecular structures of α- and β -lactose differ in the orientation of a hydrogen- and a hydroxyl group on carbon atom no.1 in the glucose moiety. Both forms change into one another continuously. At room temperature, the equilibrium results in a ratio of about 40% α-lactose and 60% β-lactose. The fact that two forms of lactose exist which differ in molecular structure has profound effects on various properties of lactose such as crystallization behavior, crystal morphology, solid-state properties, and solubility. The intestine does not actively absorb lactose unless it is split into its two-monosaccharide components, i.e. glucose and galactose. This hydrolysis of lactose is affected by the enzyme lactase, which is produced by the epithelium cells in the brush-border of the small intestine. Thus, the capacity of mammals to digest lactose is dependent on the lactase activity in the intestine. The maximum activity of the enzyme occurs shortly after birth and declines during the weaning period, after which it remains at a relatively constant level. Genetically determined factors governing residual lactase activity also exist. Individuals having low lactase activity are called lactose malabsorbers. Lactose intolerance is a condition in which people have symptoms due to the decreased ability to digest lactose. The principal symptom of lactose intolerance is an adverse reaction to products containing lactose (primarily milk), including abdominal bloating and cramps, flatulence, diarrhea, nausea, borborygmi, and vomiting (particularly in adolescents). These appear one-half to two hours after consumption.

Fructose, or fruit sugar, is a simple ketonic monosaccharide found in many plants, where it is often bonded to glucose to form the disaccharide sucrose. In nature, fructose is produced as D-enantiomer. D-Fructose is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream. L-fructose is an enantiomer of D-fructose. L-fructose does not occur naturally and can be produced synthetically from the racemic mixture of D- and L-fructose. L-fructose was investigated as a non-caloric sweetener; however it was detected that digestibility of L-fructose is 10% lower than D-fructose, and L-fructose did not found significant commercial application.

Galactose, a monosaccharide sugar, is a key source of energy and is an important compound for early human development. Galactose is present in dairy products, the pectin of some fruits, vegetables, and some herbs. Children get most of their dietary galactose from milk. D-galactose is freely available in health food stores and is promoted for stimulating the immune system and improving gut motility in healthy individuals. Galactose as a part of food supplement participates in phase II of clinical trials for congenital disorders of glycosylation (CDG) patients. CDG is a group of more than 130 inborn errors of metabolism affecting N-linked, O-linked protein and lipid-linked glycosylation. In addition, monosaccharide is used in clinical trials phase I in patients with focal segmental glomerulosclerosis (FSGS), where the galactose lowers the level of a circulating factors that increase glomerular permeability to albumin in patients with resistant FSGS.