It is known that phytic acids contained in soybeans etc. react with iron ions to produce an insoluble iron polymer.
Sodium Ferrous Citrate (SFC), the ingredient of Sanferol^®, or ferrous sulfate was dissolved in a 0.6% sodium phytate solution to prepare 0.1% iron solutions. To these solutions, hydrochloric acid was added to adjust the pH to 1, 3, 4 or 8. Comparison of the permeability of iron from the SFC and ferrous sulfate solutions through a cellulose filter demonstrated that the permeability of iron from the SFC solutions was constant, indicating that SFC reacts less with phytic acids than does ferrous sulfate as shown Figure 3. With the ferrous sulfate solutions, the more the pH increased and approached neutral pH, the more the permeability of the iron decreased.⁵⁾

Cited from Terashima C et al. (1992). II. Sodium Ferrous Citrate: Iron Fortifier Materials: Tokyo, Japan: Japan Confectionery and Innovative Food Ingredients Research Center, pp.1-10.

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