The shells of crabs, lobsters, and other crustaceans contain a nondigestible fiber called chitosan. Extracted and taken in supplement form with meals, chitosan reportedly encourages weight loss by binding to fat molecules in the digestive tract, preventing the body from absorbing the fat.

Whether chitosan is safe and effective as a diet aid remains controversial, however. Some small studies have found that taking the supplement may result in weight loss. In one trial, people who used chitosan for a month in conjunction with a calorie-controlled diet lost, on average, 8% of their body weight. This translates into 16 pounds for a 200-pound person. How much this was due to the chitosan and how much to the calorie restriction is unknown.

Other studies, however, have not reported such positive findings. A carefully designed 1999 trial in England involving 34 overweight individuals, for instance, found that chitosan supplements were no better than a placebo in reducing body weight. 

Some advertisements encourage taking chitosan so that you can "indulge" in a fatty meal. This is definitely not the case. While chitosan does work to bind to some of the fats in your digestive tract, it certainly doesn't cover them all. The bottom line is that anyone wishing to try chitosan for weight loss will have to watch their total calorie intake, reduce the fat in their diet, and exercise.

Researchers are still examining whether chitosan may lower cholesterol (another effect of its fat-binding qualities) or even have a role to play in controlling ulcers, high blood pressure, and arthritis.