What is Chitosan? Chitosan is a non-digestible amino polysaccharide fiber derived from ground or powdered shellfish like crabs and prawns. It is not absorbed into the body, but rather passes through the gastrointestinal tract. What are the effects of Chitosan? It can help with weight loss by reducing the absorption of fats. Chitosan works by binding with dietary fats in the gastrointestinal tract and preventing the digestion and absorption of fats, and ultimately the storage of fats.