Chitosan has a sticky structure which is said to act a bit like velcro trapping small amounts of fatty substances in your intestines, however will also catch the carbs before it can be absorbed.
So instead of being absorbed within your body, it ends up as waste material and your body gets rid of it in the normal way - if you know what I mean.
Research Title: Supplemental products used for weight loss.
Year Published: 2004
Research Summary: A review was undertaken of several well known weight loss supplements on the market. One of the supplements reviewed was chitosan. The review found that there was little evidence to support its use as a weight loss supplement, but conceded that there was too little research done in this area.
Research Title: Herbal preparations for obesity: are they useful?
Year Published: 2003
Research Summary: Double blind studies into chitosan has revealed that it is largely ineffective, despite its ability in the lab to bind and set aside fat.
Research Title: The effect of the dietary supplement, Chitosan, on body weight: a randomised controlled trial in 250 overweight and obese adults.
Year Published: 2004
Research Summary: The authors conducted a double blind, placebo controlled trial over 24 weeks. The trial used 250 obese participants mostly women with a mean age of 48 years. The group that received 3 g per day of chitosan did lose more weight than the placebo group, however, the amount lost was so small, that it did not result in clinically significant fat loss.
Research Title: Chitosan supplementation and fecal fat excretion in men.
Year Published: 2003
Research Summary: Fifteen subjects consumed as much food within five meals per day as they wanted, over 12 days. They received no supplementation for the first 4 days (control period) and then received 4.5 grams per day of chitosan - administered 30 minutes before the five meals. Because chitosan and the nutrients it trapps ends up as stool, the researchers collected fecal samples of the participants and analysed the fat content of the feces. They found that the fat content of the feces only increased by 1.1 g per day which does not even equal the amount of calories you'd consume in a teaspoon of butter. Therefore the researchers concluded that chitosan does not trap fat as commonly thought.
Research Title: Effect of chitosan in complex management of obesity.
Year Published: 2002
Research Summary: Fifty obese women participated in a randomized, placebo-controlled, double-blind study, while also receiving regular counselling from a doctor, psychologist and dietition as well as a low calorie diet. The women were divided into a treatment group and placebo. The treatment group received 750 mg pure chitosan per tablet, two tablets three times daily before each main meal. The group that received treatment lost significantly more weight than the placebo group, (15.9 kg, as opposed to 10.9 kg of the placebo group). The researchers concluded that chitosan can be a valuable and safe addition to a long term weight loss strategy.
As you can see, for but one, the majority of studies I found indicated that there is little evidence to support the use of this supplement as a fat blocker.
The overall consensus is that, while having promising results in the laboratory working with rats, the same fat blocking results do not occur reliably with human trials.
The exception is where the supplement was administered over a long period of time (six months) where weight loss benefits were observed and chitosan was integrated into more conventional weight loss strategies.
Based on Zahorska-Markiewicz's work listed above, the supplement may be a useful addition therefore to healthy eating and a sensible exercise regime. However, may not be of use when used as the only weight loss technique.
While the supplement is undergoing investigation for other beneficial uses, most research is indicating that it has little effect as a weight loss supplement.
