The purpose of Alli is to prevent your body from absorbing some dietary fat. Alli's makers say it blocks about 25 percent of consumed fat.
According to the official product Web site, myalli.com, the active ingredient (orlistat) in Alli attaches to natural enzymes found in your body's digestive system. This action prevents about one-fourth of the fat you consume from being broken down, leaving it undigested.
The undigested fat is not absorbed by your body (and therefore does not turn into calories); it is eliminated when you have a bowel movement.
An article at myalli.com describes this experience as follows: "...you may recognize it in the toilet as something that looks like the oil on top of a pizza."
If you experience unpleasant side effects such as loose or more frequent stools that are hard to control or gas with oily discharge, it is an indication that you are consuming too much fat.
Please note that this content is provided for informational purposes only and does not imply endorsement of this product. Please consult your doctor before taking this -- or any other -- weight loss supplement.