Chitosan
This fiber-like substance, derived from crustacean shells, is said to block fat absorption in the intestines. Although there is some evidence that chitosan may help in the short term, studies have had mixed results and most are poor quality. Better-quality trials have found no clinically meaningful effect on weight. Plus, there are potential risks. If Chitosan does block some fat, it may also reduce absorption of some fat-soluble vitamins (A,D,E,K), as well as calcium, iron and magnesium. People with shellfish allergies should not take chitosan.