Chitosan treatment results in significantly lower serum LDL cholesterol concentrations. To assess the working mechanism of chitosan, we measured serum surrogate markers of cholesterol absorption (campesterol, sitosterol, cholestanol), synthesis (lathosterol, lanosterol, desmosterol), and degradation to bile acids (7α-hydroxy-cholesterol, 27-hydroxy-cholesterol) corrected for cholesterol concentration (R_sterols). Over 12 weeks, 116 obese subjects (BMI 31.7, range 28.1 – 38.9 kg/m²) were studied under chitosan (n=61) and placebo treatment (n=55). The participants were briefly educated regarding improvement of nutrition quality and energy expenditure. Daily chitosan intake was 3200 mg. Serum LDL cholesterol concentration decreased significantly more (P=0.0252) under chitosan (-8.67 ± 18.18 mg/dl, 5.6%) than under placebo treatment (-1.00 ± 24.22 mg/dl, 0.9%). This reduction was not associated with the expected greater decreases in markers of cholesterol absorption under chitosan treatment. Also, increase in markers of cholesterol synthesis and bile acid synthesis under chitosan treatment was not any greater than under placebo treatment. In conclusion, a significant selective reduction of serum LDL cholesterol under chitosan treatment is neither associated with a reduction of serum surrogate markers of cholesterol absorption nor with an increases of markers for cholesterol and bile acid synthesis.