Chitosan is a fiber made from the chitin in shellfish shells. Inside your digestive tract, it grabs dietary fat and keeps some of it from getting absorbed. That's why you'll find it in many weight management blends.
Take chitosan right before or with a fatty meal so it can mix with your food. People also use it to help keep cholesterol numbers where they already are, although the proof on that front varies.
Watch out for a few things. Chitosan can latch onto fat-soluble vitamins (A, D, E, and K) and some medications, which may block their uptake. Anyone with a shellfish allergy should steer clear.