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Availability of iron from a diet of corn tortillas and cooked beans with different levels of fiber was determined by hemoglobin repletion. Neutral detergent fiber (NDF) in corn tortillas and cooked beans and soluble, ionizable, and total iron the the test foods were determined. Weanling male rats were fed an iron-free diet until anemic and then divided into groups. The rats were fed diets with 6.5,10, and 15% NDF and 25, 30, and 35 ppm of total iron. Iron availability was significantly reduced by 15% NDF. The amount of iron in the diet affected iron repletion. A significant correlation between soluble iron at pH 1.35 in vitro and percent efficiency of hemoglobin repletion was obtained. Iron from com tortillas and cooked beans is approximately 50% less available than ferrous sulfate.

The effect of tannin content on iron (Fe) bioavailability from several legumes was evaluated. Absorption of Fe from a casein (C), isolated soy protein (ISP), chickpea (CP) or red kidney bean (RKB) test meal was tested in marginally Fe-deficient rats [9.9 +/- 0.2 g Hb/100 mL (mean +/- SD)] using the extrinsic tag technique. Also, the effects of a casein habitual diet or of one of three casein-legume habitual diets fed before and after the test meal were investigated. Weanling male rats were fed the habitual diets containing 23 +/- 4 mg Fe/kg of diet (range 17-28) for 13 d. On d 14, after an overnight fast, rats were fed the test meal (1.5 g + 0.1 microCi 59Fe), and whole-body radioactivity was determined. The same habitual diet was refed for seven additional days, and whole-body radioactivity was determined again. Rats retained more iron from C (86%) than from ISP, CP or RKB test meals (73%, 75% and 67%, respectively) when the respective casein-legume habitual diets were fed before and after the test meals. With the casein habitual diet, there was no difference in retained iron from C, ISP, CP or RKB test meals (86%, 87%, 83% and 82%, respectively). Retention of iron from an RKB test meal was increased from 69.6 to 73% when about 90% of the extractable tannins were removed, but the difference was not statistically significant. Thus, feeding a casein-legume diet but not a casein diet prior to a test meal apparently predisposes the rat to lower iron absorption. However, in these studies, tannins per se did not significantly depress iron bioavailability.

Iron, copper and zinc but not magnesium were bound by neutral (NDF) and acid (ADF) detergent fiber obtained from cooked pinto beans. Iron binding increased with higher pH, higher iron concentration, higher fiber concentration and smaller fiber particle size. Maximum binding of iron was at pH 6.5 and minimal binding at pH 4.0 for both NDF and ADF. A 50% decrease in particle size increased iron binding by 8%. Binding of copper and zinc increased with higher copper and zinc concentrations. Both NDF and ADF had a greater affinity for copper than for either iron or zinc. Scatchard plots indicated the presence of two types of binding sites for zinc, one type for iron, and one type for copper.