The Antagonistic Influence of Phytic Acid on Zinc Absorption: An In Vitro Comparison of Inorganic and Chelated Trace Mineral Sources

<b>Background/Objectives:</b> Zinc, an important trace metal, requires daily intake but dietary antagonists including phytic acid reduce its absorption. It is unclear if phytic acid affects zinc absorption at the level of bioaccessibility (how much soluble zinc is available from digestion) or bioavailability (how much zinc is absorbed by the intestine). This study investigates at which level this occurs at and if the zinc source alters the response. <b>Methods:</b> Following a standardised <i>in vitro</i> digestion (INFOGEST), the yield of soluble zinc was measured as the bioaccessible fraction from inorganic and chelated zinc sources, with and without phytic acid. Bioavailability was assessed by measuring cellular zinc uptake in intestinal cell lines (Caco-2 and IPEC-J2). <b>Results:</b> Phytic acid affected the bioaccessibility of zinc, with varying impacts depending on the zinc source. Zinc proteinate had the highest bioaccessibility (42%) without phytic acid, while inorganic zinc sulphate (24%) and zinc bisglycinate (27%) were lower. ZnSO₄ was more susceptible to phytic acid antagonism than chelated zinc sources (from 2:100 molar ratio of phytic acid: zinc), while the chelated zinc sources were only affected at a molar ratio of 4:100, with zinc bisglycinate being more susceptible than zinc proteinate. Cellular zinc uptake (bioavailability) and toxicity at equimolar concentrations were unaffected by phytic acid. <b>Conclusions:</b> This study found that phytic acid affected bioaccessibility, not bioavailability. The zinc source impacts the response. Zinc proteinate was consistently more bioaccessible while both chelated zinc sources were less susceptible to phytic acid than inorganic zinc.