Salivary Cystatin SN Binds to Phytic Acid In Vitro and Is a Predictor of Nonheme Iron Bioavailability with Phytic Acid Supplementation in a Proof of Concept Pilot Study

BACKGROUND: Acute phytic acid intake has been found to decrease iron bioavailability; however, repeated phytic acid consumption leads to iron absorption adaptation. Salivary proline-rich proteins (PRPs) have been shown to inhibit iron chelation to tannins and may mediate similar iron absorption adaptation with phytic acid intake. OBJECTIVES: The objectives of this study were to determine whether salivary proteins bind to phytic acid in vitro, and to explore a proof of concept in a pilot study that examined the impact of 4-wk, daily phytic acid supplementation on individuals’ iron status, bioavailability, and salivary PRP concentrations. METHODS: High-performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization–time of flight were used to characterize in vitro salivary protein–phytic acid interactions. Nonanemic women (n = 7) consumed 350 mg phytic acid supplements 3 times daily for 4 wk, and meal challenges were employed to determine iron bioavailability, iron status, and salivary protein concentrations before and after supplementation periods. Enzyme-linked immunosorbent assay (ELISA) analysis of purified protein fractions and participant saliva identified proteins bound to phytic acid. RESULTS: In vitro salivary protein–phytic acid interaction identified cystatin SN, a non–proline rich salivary protein, as the specific bound protein to phytic acid. Iron bioavailability (P = 0.32), hemoglobin (P = 0.72), and serum ferritin (P = 0.08) concentrations were not reduced from week 0 to week 4 after phytic acid supplementation. Basic PRPs and cystatin SN concentrations were positively correlated with iron bioavailability at week 4. CONCLUSIONS: Overall, results suggest that phytic acid binds to the non-PRP cystatin SN and that salivary protein production may improve iron bioavailability with phytic acid consumption.