Twelve healthy young men added bran 15 g twice daily to their diet and underwent aspiration of bile-rich duodenal contents at 0, 1, 2, 4 and 6 weeks. In the 6-week sample, but not earlier, there was a significant decrease in deoxycholate and increase in chenodeoxycholate. Bile was unsaturated with cholesterol at the beginning and remained so throughout. The delay in the action of bran may be because it reduces the formation of deoxycholate in the colon and does not merely impede its absorption.