Effect of fiber on nonheme iron absorption

The effect of fiber on the absorption of food iron was examined by performing multiple radioiron absorption tests in normal male and female subjects. In an initial study, we added bran, pectin, or cellulose to muffins prepared with wheat flour. Because of the low values obtained in this study, another study was done in which apple juice containing 50 mg of ascorbic acid was added to the meal to increase iron absorption averaged 2.26% from plain muffins, 1.07% from bran muffins, 1.89% from pectin muffins, and 2.26% from cellulose muffins. Only the effect of bran was statistically significant. We next designed two complex meals that differed maximally in their content of naturally occurring fiber but that were matched with respect to macronutrient, mineral, iron, and ascorbic acid contents. Mean absorptions from the low- and high-fiber meals were significantly different, averaging 6.07% and 2.96%, respectively. These results demonstrate that inhibition of iron absorption is not a universal property of all fiber sources. Moreover, the relatively modest effect of maximally altering the natural fiber content of a meal suggests that fiber is not a major determinant of food iron availability in humans.