Chitosan is a polysaccharide polymer derived from a homopolymer of modified glucose Chitin derived from the exoskeleton of shellfish such as shrimp, lobster, crabs, fungi, arthropods and other marine invertebrates.

Specifically, Chitosan is an undigested fiber similar to what you find in cellulose of vegetables and outer shell of whole grains. The difference is that Chitosan fiber, unlike other fibers, carries a positive ionic charge. This unique property gives Chitosan ability to attract fats - Since lipids, fats, and bile acids all possess negative charges, there is a chemical bond between the two and thus they attract naturally to one another. So, it has been theorized that consuming Chitosan, or the Chitin fiber during a meal will minimize the amount of calories which can be absorbed by the digestive tract. The fat is attracted to Chitosan and excreted as a byproduct.

It makes sense that the described bond of Chitosan fiber with fat is better than any other fiber However, what's good in theory is not always good in real life as there is a great chance of mal-absorption. As a result, you may not be able to get the vital vitamins and minerals, particularly the fat-soluble A, D, E, and K.