How does it work? The active ingredient in alli attaches to some of the natural enzymes in the digestive system, preventing them from breaking down about a quarter of the fat you eat. Undigested fat cannot be absorbed and passes through the body naturally. The excess fat is not harmful. alli only works on enzymes that break down fat, not on other enzymes. So carbohydrates and proteins are not affected. You still absorb those nutrients. But, since fat grams have more than twice as many calories as carbs and proteins, alli capsules work on the worst offenders - the calorie-dense fat grams. Generic alli is used together with a reduced-calorie diet and weight maintenance to treat obesity in people with certain risk factors (such as diabetes, high blood pressure, and high cholesterol or triglycerides).