What is Chitosan?
Chitosan Reviews - Do Chitosan Products Work?
Chitosan comes from the shell of crustaceans. There are many different chitosan supplements or products containing chitosan as an ingredient on the market, and depending on where you look, you'll see various claims made such as using Chitosan to block fat absorption, help with weight loss, reduce cholesterol levels, and it claims to have the ability to "dissolve and grab a
hold of fats and cholesterol in the stomach". You may have
seen some outrageous claims that you can eat whatever you want,
and Chitosan will block out all the fat from the media. Sounds
too good to be true doesn't it?
So does it really work as claimed? Or is this all a hoax trying to scam your money? To answer these questions, we dug deep, looking at many different scientific studies involving Chitosan including research that used human subjects and as well as lab animals. Unfortunately, the overwhelming results were that there doesn't appear to be a whole lot of evidence to support the various claims of Chitosan of being an effective fat loss and weight control supplement.
Chitosan is extracted from the shell of crustaceans
like shrimp and crab. Contrary to what you might think, Chitosan
has been used for many decades. It's not some new, revolutionary
discovery, nor is it a new weigh loss miracle pill. So don't get fooled
by the media hype. In fact, FDA had sent several warning letters back in 2004 to companies who made unsubstantiated claims for this supplement. Indeed, after reviewing and looking through quite a few scientific papers on Chitosan, our conclusion here is that the Chitosan supplement doesn't quite live up to all the various claims made, and furthermore, there are some indications that Chitosan may also block / remove beneficial nutrients from your body.
Most weight loss experts will agree that Chitosan
simply doesn't work. Previous studies done on Chitosan were 'small'
studies and were not published in peer reviewed journals. Many of these studies used lab animals. And with the many studies involving human subjects that we looked at, the results were quite poor. We'll discuss all of that here.
Note: you shouldn't take Chitosan if you have
any type of shellfish allergies.
Does Chitosan Work? What are some Chitosan Benefits?
If you're thinking of trying Chitosan to lose
some weight, read this article carefully, and you might just change your mind afterwards. Chitosan makes grand promises of 'absorbing'
fat and preventing it from getting into your body - it's the ultimate
'fat blocker' - or is it? Far from it. Thorough clinical studies
on humans have show that Chitosan does NOT work! Let me explain...
Initial Chitosan research were done on lab animals,
not humans. These studies did show some effectiveness, and did
show some fat-absorbing properties; however, studies involving human subjects found little evidence to support its weight loss claims. Some studies find little support, while other studies find no support at all for chitosan as a fat loss supplement. We'll very briefly describe several human studies done using chitosan products below.
In a research paper that reviewed 15 different studies involving 1219 subjects, the researchers looked and analyzed these different studies. They found that some of the results indicated some benefits of chitosan for helping with weight loss and cholesterol reduction, however, they commented that many studies had sub-optimal quality, and that while there appears to be some limited evidence to support Chitosan for treatment of obesity, the results they found from high-quality trials indicate that Chitosan effects on body weight was minimal. 
To be fair and present supporting evidence on both sides, we have also found some studies that supports the use of Chitosan for weight control. In one study involving 150 overweight adults, treatment group took six 500mg chitosan capsules a day. The study results found that the treatment group lost on average 2.6lbs more fat compared to the placebo group, and had experienced a better body composition improvement. This provided some support for using chitosan to reduce excess body fat without negatively affecting lean body mass. 
But that's about as far as I could go with supporting human studies (without resorting to animal studies). Another study involving human subjects had this conclusion: "The fat trapped was clinically insignificant. For men, it would take more than 7 months to lose 1 pound of body fat. For women, no fat was trapped. This product, as with other similar products, fails to meet claims."  In another study with 250 participants lasting 24 weeks, the chitosan treated group lost minimally more weight compared to the control group, leading the researchers to conclude that Chitosan supplements did not result in clinically significant loss of body weight when compared to placebo. 
Chitosan Benefits & Chitosan Side Effects
Okay, so now you know why you shouldn't waste
your money on Chitosan as a fat loss supplement, but you should know that Chitosan
does have some benefits, although not weight loss benefits. Studies
have shown Chitosan to have cholesterol lowering abilities. One
study had shown that 3 to 6 grams of Chitosan a day for 2 weeks
lowered LDL(bad) cholesterol by 6% and increase HDL (good) cholesterol
by 10%. Another study stated that Chitosan could decrease total cholesterol levels and improve lipid metabolism.  However, please note that these studies were done using lab animals and not humans. Again when it came to human trials, studies find little evidence to support chitosan for having an effect on cholesterol.  Although this study did find the consumption of Chitosan to be safe with no side effects.
From the many studies we looked at, several had made brief mentions on how the human subjects reacted in terms of adverse side effects, and it will be good to note here that none of the studies had reported any chitosan side effects. However, one concern that we want to point out here is that it appears that although Chitosan may be able to block some fats, it may also block and affect beneficial nutrients as well. One study stated that it may also interact with other molecules in the body, and evaluated how it would affect vitamin B12. Their results found that Chitosan modified the photophysical properties of vitamin B12 and that it could eliminate vitamin B12 - hence, prolonged chitosan supplementation is cautioned. 
As you can see, there's much more evidence against the use of chitosan products as a fat loss and weight control supplement. Most studies found little to no evidence supporting its claims. However, if you're still keen on trying it out, you can find some with the links below:
>> Click here for Fat Blocker Supplements
1. Cochrane Database Syst Rev. 2008 Jul 16;(3):CD003892.
Chitosan for overweight or obesity.
Jull AB, Ni Mhurchu C, Bennett DA, Dunshea-Mooij CA, Rodgers A.
2. J Am Coll Nutr. 2006 Oct;25(5):389-94.
Evaluating efficacy of a chitosan product using a double-blinded, placebo-controlled protocol.
Kaats GR, Michalek JE, Preuss HG.
3. J Am Diet Assoc. 2005 Jan;105(1):72-7.
Chitosan supplementation and fat absorption in men and women.
Gades MD, Stern JS.
4. Int J Obes Relat Metab Disord. 2004 Sep;28(9):1149-56.
The effect of the dietary supplement, Chitosan, on body weight: a randomised controlled trial in 250 overweight and obese adults.
Mhurchu CN, Poppitt SD, McGill AT, Leahy FE, Bennett DA, Lin RB, Ormrod D, Ward L, Strik C, Rodgers A.
5. Asia Pac J Clin Nutr. 2007;16 Suppl 1:313-7.
Mechanism study of chitosan on lipid metabolism in hyperlipidemic rats.
Xu G, Huang X, Qiu L, Wu J, Hu Y.
6. Int J Food Sci Nutr. 2011 Dec 14. [Epub ahead of print]
Use of chitosan in the treatment of obesity: evaluation of interaction with vitamin B12.
Rodrigues MR, de Oliveira HP.
7. J Am Coll Nutr. 2008 Feb;27(1):22-30.
Safety aspects and cholesterol-lowering efficacy of chitosan tablets.
Tapola NS, Lyyra ML, Kolehmainen RM, Sarkkinen ES, Schauss AG.