The subjects of this study were six normal and five maturity onset diabetics. On the first day of the study, subjects were given a control test meal, prepared by adding vitamin B12 3000r (mecobalamin) and vitamin E 500 mg (tocopherol acetate), after a 12-hr overnight fast. Then, on the second day in the case of the normal subjects, and on the seventh day in the case of the diabetic subjects, the subjects received a second test meal with 3.9 g konjac mannan (glucomannan). Venous blood samples were taken immediately before the test meal and again after 1, 3, 5, 8, 12 and 24 hr for analysis of vitamins. The absorption rate of vitamin E into the intestine was reduced when konjac mannan was added to the test meals, but that of vitamin B12 was not reduced in normal or diabetic subjects. It is suggested that konjac mannan reduces fat-soluble vitamin absorption removing bile acids, but does not reduce fat-insoluble vitamin absorption in the intestine.