BACKGROUND:The use of dietary supplements, especially those for weight control, is increasing. Advertising claims for some of these supplements may give consumers unrealistic expectations. For example, it is claimed that the supplement chitosan can block the absorption of up to 120 g of dietary fat per day. OBJECTIVE: To test the effect of a chitosan supplement used as directed, on the absorption of dietary fat. DESIGN: Seven healthy male volunteers with average age 28, range 23 -30, average body mass index (BMI) 26, range 23 -31, maintained a high fat intake ( > 120 g=day) for 12 days. On days 6 -9, chitosan was taken as directed by the manufacturer prior to meals and snacks and totaled 15 capsules or 5.25 g of chitosan daily. A charcoal marker was consumed on days 2, 6 and 10 to mark the baseline and supplement periods. All feces were collected on days 2 -12, and were divided based on the appearance of charcoal in the feces. Fecal fat content corresponding to the four baseline and four chitosan-supplemented days was measured. RESULTS: Average dietary fat intake did not differ between baseline (135 AE 5 g), and chitosan (135 AE 3 g) periods. Fecal fat excretion did not differ between the two periods (6.9 AE 1.0 g=day baseline; 6.8 AE 0.4 g=day chitosan). The fecal fat content as a percentage of fat consumed did not differ (5.1 AE 0.7% baseline, 5.0 AE 0.3% chitosan). Fecal mass did not differ (176 AE 26 g=day baseline, 182 AE 14 g=day chitosan). CONCLUSION: Consumption of this chitosan supplement did not increase fecal fat content and therefore did not block fat absorption.