The active ingredient in Orlistat is a lipase inhibitor. Lipase is the enzyme that breaks down dietary fats. The aim of a lipase inhibitor is to prevent dietary fats from absorbing into the body. Orlistat prevents the absorption of approximately 25% of dietary fat that is consumed within the diet. This idea is based on the fact that fat is the most calorie-dense of all the macronutrients, contributing 9 kcal/gram of fat. If only a small percentage of that fat is absorbed, then this will reduce overall energy intake and induce weight loss.