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 Orlistat, the first prescription treatment for obesity that does not act as an appetite suppressant, works by interfering with the action of gastrointestinal (GI) lipase in the GI tract. As a result of this mechanism of action, 30% of ingested dietary fat is not absorbed. Orlistat therefore reduces total energy intake to produce a significant weight loss. Orlistat is a reversible inhibitor of lipases. It exerts its therapeutic activity in the lumen of the stomach and small intestine by forming a covalent bond with the active serine residue site of gastric and pancreatic lipases. The inactivated enzymes are thus unavailable to hydrolyze dietary fat in the form of triglycerides into absorbable free fatty acids and monoglycerides. As undigested triglycerides are not absorbed, the resulting caloric deficit may have a positive effect on weight control. Systemic absorption of the drug is therefore not needed for activity. At the recommended therapeutic dose of 120 mg three times a day, orlistat inhibits dietary fat absorption by approximately 30%. How Orlistat Works Orlistat has a unique molecular structure, which allows it to bind to the active site of GI lipase and block that enzyme’s activity. The enzyme is thus unable to break triglycerides down into their component parts. A significant proportion of dietary fat therefore remains undigested and unabsorbed, passing through the GI tract unchanged. However, 70% of ingested fat is digested in the normal fashion, ensuring sufficient absorption of fat-soluble vitamins. On average, dietary fat accounts for 34% of daily energy intake, 36 although healthy eating guidelines recommend that it should make up no more than 30% of calorie intake. 41 Orlistat has been shown to reduce absorption of dietary fat by an average of 30% at a dose of 120 mg 3 times daily. 20 By blocking the digestion and absorption of some, but not all, dietary fat, calorie intake is significantly reduced while still allowing the absorption of necessary fat-soluble dietary constituents that ensure a healthy, nutritious diet. When taking XENICAL, patients should be advised to take a multivitamin supplement containing fat-soluble vitamins to ensure adequate nutrition. Less absorption of dietary fat effectively reduces energy intake so that a significant weight loss may occur. Summary
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