We investigated the effect of calcium on iron absorption in 126 human subjects. Addition of calcium chloride to wheat rolls significantly reduced iron absorption. Doses between 40 and 600 mg Ca were studied. The inhibition was clearly dose related up to 300 mg Ca. Calcium added to the dough when making the rolls reduced phytate degradation during fermentation and baking. As little as 40 mg Ca added to 80 g flour reduced phytate degradation by 50%, thus increasing the phytate content of the rolls to levels interfering with iron absorption. Calcium also had a direct dose-related inhibiting effect on iron absorption, noted by adding calcium to the rolls after they had been baked instead of to the dough. Iron absorption was reduced by 50–60% at doses of 300–600 mg Ca. Giving 165 mg Ca as milk, cheese, or calcium chloride reduced absorption by 50–60%. The same amount of calcium also significantly reduced heme-iron absorption, suggesting that the effect of calcium is related to the mucosal transfer of iron. The observed marked inhibitory effect on iron absorption of calcium in amounts frequently encountered in normal meals has important nutritional implications.

The mineral content of legumes is generally high, but the bioavailability is poor due to the presence of phytate, which is a main inhibitor of Fe and Zn absorption. Some legumes also contain considerable amounts of Fe-binding polyphenols inhibiting Fe absorption. Furthermore, soya proteinper sehas an inhibiting effect on Fe absorption. Efficient removal of phytate, and probably also polyphenols, can be obtained by enzymatic degradation during food processing, either by increasing the activity of the naturally occurring plant phytases and polyphenol degrading enzymes, or by addition of enzyme preparations. Biological food processing techniques that increase the activity of the native enzymes are soaking, germination, hydrothermal treatment and fermentation. Food processing can be optimized towards highest phytate degradation provided that the optimal conditions for phytase activity in the plant is known. In contrast to cereals, some legumes have highest phytate degradation at neutral or alkaline pH. Addition of microbial enzyme preparations seems to be the most efficient for complete degradation during processing. Fe and Zn absorption have been shown to be low from legume-based diets. It has also been demonstrated that nutritional Fe deficiency reaches its greatest prevalence in populations subsisting on cereal- and legume-based diets. However, in a balanced diet containing animal protein a high intake of legumes is not considered a risk in terms of mineral supply. Furthermore, once phytate, and in certain legumes polyphenols, is degraded, legumes would become good sources of Fe and Zn as the content of these minerals is high.

Iron absorption was measured from five kinds of bread made from various types of flour and fermented in different ways in order to obtain a wide variation in the content of fiber, phytate (inositol hexaphosphate) and its degradation products, inorganic phosphate and inositol phosphates with fewer numbers of phosphate groups (inositol pentaphosphate through monophosphate). Each experiment had 9–10 subjects and, in each subject, iron absorption was measured from control rolls made from low extraction wheat flour and one kind of test roll using two different radioiron tracers: ⁵⁵Fe and ⁵⁹Fe. The inhibition of iron absorption was closely related to the content of phytate-phosphorous as determined using the AOAC method, and to the sum of the tri- through hexaphosphate groups as determined using the HPLC method. As an example, prolonged fermentation of whole-rye bread reduced total inositol phosphates to the same amount as in the control rolls and increased fractional iron absorption to the same high level, in spite of a fiber content five times as great. The results strongly suggest that the inhibitory effect of bran on iron absorption is due to its content of phytate and other inositol phosphates present after fermentation, rather than to its content of fiber or other constituents. Thus, effective fermentation will increase the bioavailability of iron in whole-meal bread.

Background: Inositol hexaphosphate (IP₆) is a well-known inhibitor of iron absorption, whereas the effects of the less-phosphorylated derivatives of IP₆ are less known. Objectives: The objective was to investigate the effects of inositol tri-, tetra-, and pentaphosphates (IP₃, IP₄, and IP₅, respectively) on iron absorption in humans. Design: Iron absorption was measured in 5 experiments from single meals by extrinsic labeling with ⁵⁵Fe and ⁵⁹Fe and determination of whole-body retention and the erythrocyte uptake of isotopes. In experiments 1–3 the meals contained white-wheat rolls to which 10 mg P as IP₅, IP₄, or IP₃ , respectively, was added. Inositol 1,2,6-triphosphate [Ins(1,2,6)P₃] and a mixture of isomers of IP₄ and IP₅ were studied. White-wheat rolls contained 10 mg P as IP₃ + IP₄ and 2 mg P as IP₅ + IP₆ in experiment 4 and 20 mg P as IP₃ + IP₄ and 3 mg P as IP₅ + IP₆ in experiment 5; inositol phosphates were obtained via fermentation of sodium phytate. Each experiment had 8–11 subjects. Results: In experiment 1, iron absorption was reduced by 39%, whereas there was no significant effect on iron absorption in experiments 2 and 3. In experiments 4 and 5, iron absorption was reduced by 54% and 64%, respectively, suggesting that IP₃ and IP₄ contributed to the inhibitory effect. Conclusions: IP₅ has an inhibitory effect on iron absorption, whereas IP₃ and IP₄ in isolated form have no such effect. IP₃ and IP₄ in processed food contribute to the negative effect on iron absorption, presumably by binding iron between different inositol phosphates. To improve iron absorption from cereals and legumes, degradation of inositol phosphates needs to be to less-phosphorylated inositol phosphates than IP₃.

While it is known that phytic acid, inositol hexaphosphate, has a negative effect on zinc and calcium absorption, the effects of inositol which is phosphorylated to a lesser extent are less known. We have prepared inositol triphosphate (IP-3), tetraphosphate (IP-4), pentaphosphate (IP-5) and hexaphosphate (IP-6) by hydrolysis of sodium phytate and separation by ion-exchange chromatography and have studied their effect on zinc and calcium absorption. Using a suckling rat pup model, we found that liver uptake of ⁶⁵Zn after 6 h was 5% of the total dose from solutions of IP-6, 19% from IP-5, 28% from IP-4, 29% from IP-3 and 31% from ZnCl₂ (control). Non-absorbed calcium was 17%, 1.4%, 0.5%, 0.5% and 0.5% of the given dose of ⁴⁵Ca, respectively. Thus, at a high degree of phosphorylation (IP-6, IP-5), zinc and calcium uptake was inhibited, while no effect was observed for the other phosphates. Consequently, total “phytate” analysis, which includes inositol phosphates with varying degrees of phosphorylation, can give misleading information with regard to mineral availability. In addition, even limited dephosphorylation of inositol hexaphosphate can have a positive effect on mineral absorption.