Chitosan is a fiber extracted from the shells of crustaceans such as shrimp and crab or the cell walls of fungi. It is sometimes used in the food industry as a thickener and emulsifier. In a placebo-controlled trial, 96 subjects with overweight and obesity took 2.5 grams of chitosan per day or placebo for 90 days. Despite having similar calorie intake, those in the chitosan group lost an average of 3.1 kg (almost 7 pounds) while those in the placebo group had no significant change in body weight. In addition, the chitosan group had decreased body fat, abdominal fat, and waist circumference, had reductions in high HgA1c (a marker of glucose regulation), and reported improved quality of life.¹ Two meta-analyses, one that included data from 14 randomized controlled trials with a combined total of 1,101 participants and another that included data from 15 trials with 1,130 participants, showed chitosan mildly reduces body weight and body fat in individuals with overweight or obesity.^2,3 Findings from another trial suggest taking 2 grams per day of vitamin C along with 3 grams per day of chitosan may induce greater weight loss than chitosan alone.⁴ Chitosan has been shown to reduce fat absorption in laboratory animals, but controlled human trials have found chitosan has little to no impact on fat absorption.^5,6,7,8 Although more research is needed to understand how chitosan induces weight loss, one possibility is that, as a prebiotic fiber, it may alter the gut microbiome composition in ways that promote healthy metabolism.⁹

The fiber-like polysaccharide chitosan is found in the exoskeletons of insects, crabs, and shrimp, as well as the cell walls of fungi and yeast.¹⁰ Chitosan has properties similar to viscous fibers and is believed to reduce the absorption of dietary cholesterol and inhibit cholesterol synthesis. In one placebo-controlled trial, 116 subjects with obesity taking 3.2 grams of chitosan daily for 12 weeks resulted in a 4.7% drop in LDL-cholesterol levels relative to placebo; the effect could not be fully explained by shifts in cholesterol absorption and metabolism, suggesting an unknown mechanism may contribute to its benefits.¹¹ Furthermore, chitosan was found to be effective for lowering total and LDL-cholesterol levels in a meta-analysis of 14 randomized controlled trials.¹² Another meta-analysis included data from 14 randomized controlled trials in subjects with overweight and obesity and found treatment with chitosan, at doses of 1–3 grams per day for an average of 17 weeks, lowered total and LDL-cholesterol levels, increased HDL-cholesterol levels, and reduced triglyceride levels.¹³