Lactulose is a disaccharide derivative of lactose used to treat constipation and portal systemic encephalopathy. Lactulose is indicated for use as a laxative in the treatment of chronic constipation in adults and geriatric patients. Additionally, lactulose is also employed as an adjunct to protein restriction and supportive therapy for the prevention and treatment of portal-systemic encephalopathy (PSE), including both the hepatic pre-coma and coma variations. In particular, lactulose solution has been effective at managing PSE resulting from surgical portacaval shunts or from chronic hepatic diseases like cirrhosis. Lactulose formulations are most commonly administered via the oral route or the rectal route. Consequently, because the substance experiences minimal absorption by the gut it typically remains localized in the gastrointestinal tract environment and ultimately demonstrates almost all of its pharmacologic effects within the gut. In particular, as lactulose elicits its laxative effects in enhancing stool amounts and softening stool, such biochemical and physiologic activities can cause increased bowel sounds (borborygmi), a feeling of bloatedness, belching, frequent flatus, and diarrhea. The human small intestinal mucosa does not have the enzymes to split lactulose, and hence lactulose reaches the large bowel unchanged. Lactulose is metabolized in the colon by colonic bacteria to monosaccharides, and then to volatile fatty acids, hydrogen, and methane. Lactulose reduces intestinal ammonia production and absorption in three ways. First, the colonic metabolism of sugars causes a laxative effect via an increase in intraluminal gas formation and osmolality which leads to a reduction in transit time and intraluminal pH. This laxative effect is also beneficial for constipation. Next, lactulose promotes increased uptake of ammonia by colonic bacteria which utilize the trapped colonic ammonia as a nitrogen source for protein synthesis. The reduction of intestinal pH facilitates this process, which favors the conversion of ammonia (NH3) produced by the gut bacteria, to ammonium (NH4+), an ionized form of the molecule, unable to cross biological membranes. Finally, lactulose also causes a reduction in intestinal production of ammonia. The acidic pH destroys urease-producing bacteria involved in the production of ammonia. The unabsorbed disaccharide also inhibits intestinal glutaminase activity, which blocks the intestinal uptake of glutamine, and its metabolism to ammonia. As one of the principal features of PSE involves the accumulation of nitrogenous waste products like ammonia in the systemic circulation, a state in which the colonic contents become more acidic than blood allows ammonia in the circulation to diffuse into the colon.

Lactulose Pathway References