Chitosan is made from the shells of seafood, such as crab and shrimp. It is used in various products from cosmetics to waste-water treatment. It is also marketed as a "Fat Blocker" which supposedly prohibits fat absoprtion. Manufacturers claim that you can eat all the fatty foods you want, and the fat will not be digested. They claim that the chitosan binds to the fat and carries it out of your system, allowing quick weight loss. Chitosan, as a fat blocker, is sold alone or in other weight loss supplement combinations.
Chitosan was initially found to block fat absorption when tested in rats. Supplement manufacturers jumped on this, and have made millions of dollars from sales. Clinical trials on humans, however, were less promising. The International Journal of Obesity recently conducted a clinical study which determined that chitosan had no effect on fat absorption. Other studies reached the same result.
One popular product was marketed as the "Fat Trapper" and "Exercise in a Bottle", claiming that you could eat all the fat you wanted, not exercise, and still lose weight. The Federal Trade Commission fined the company $10 million due to deceptive advertising charges.
Chitosan is not effective as a fat blocker in humans. If you feel that you want a product that blocks fat absorption, ask your physician for a prescription for a drug such as Zenical. If you really want to buy crab shells, go to Red Lobster.