The effect of inulin, fructooligosaccharides, and galactooligosaccharides on true intestinal absorption of iron and calcium was measured in men by using double stable-isotope techniques (oral 57Fe and 44Ca and intravenous 58Fe and 48Ca). The incorporation of iron isotopes into erythrocytes and the urinary excretion of calcium isotopes was measured with an inductively coupled plasma mass spectrometer. Twelve healthy, nonanemic, male subjects aged 20-30 y received four treatments consisting of a constant basal diet supplemented with 15 g/d inulin, fructooligosaccharide, or galactooligosaccharide, or not supplemented (control treatment). These four treatments were given for 21 d each according to a randomized, crossover design. Iron absorption was measured over the last 7 d of treatment (days 15-21) and calcium absorption was measured on day 21 of each treatment period. Mean (+/- SEM) iron absorption was 5.5 +/- 1.6%, 6.1 +/- 1.9%, 5.3 +/- 1.9%, and 5.1 +/- 1.5%, respectively, during treatment with inulin, fructooligosaccharide, galactooligosaccharide, or the control; mean ( SEM) calcium absorption was 25.8 +/- 2.3%, 26.3 +/- 1.9%, 26.3 +/- 2.6%, and 28.1 +/- 4.3%, respectively. None of the differences between treatments was significant. It is concluded that 15 g/d inulin, fructooligosaccharide, or galactooligosaccharide did not have a negative effect on iron and calcium absorption in young healthy men.