Factors Influencing Iron Dialyzability and Uptake in In Vitro Models

Doctoral thesis, 2003

The availability or iron from a meal is influenced by numerous factors. This thesis deals with three of them: organic acids, plant proteins (pea and soy), and transit time through the stomach and small intestine. These factors were studied by employing an in vitro dynamic gastrointestinal model and a human epithelial cell line (Caco-2) to estimate iron availability. When organic acids were applied directly on the differentiated Caco-2 cells, a distinct relationship was observed between the chemical structure of the organic acid and its influence on ferric and ferrous iron uptake. The effect of the acids could be explained in two ways: a decrease in the pH of the solution and the formation of complexes with iron ions. For ferrous iron, the observed positive influence by organic acids on iron absorption was entirely related to the change in pH, while for ferric iron, an additional effect by the presence of the anion was shown. The combination that offered the greatest enhancement of iron uptake was with ferric iron and tartaric acid, although these had an inhibitory effect on iron absorption. Results from experiments in which an in vitro digestion step preceded the Caco-2 cell uptake assay confirmed the enhancing effect of tartaric acid on iron absorption, and the inhibitory effect of citric acid. The influence of transit time through the stomach and small intestine was investigated in the in vitro gastrointestinal model with three different meals consisting of lactic-fermented vegetables with white or wholemeal bread and sourdough-fermented wholemeal rye bread. In two of the meals the amount of dialyzable iron increased with prolonged transit time. For the third meal, with vegetables and wholemeal bread, the level of dialyzable iron was too low to distinguish any differences between the transit times. Additional experiments in which dialysates were applied on the Caco-2 cells showed that the iron uptake by the epithelial cells improved with prolonged transit time. Phytate-free isolates of pea and soy proteins were studied in the in vitro digestion/Caco-2 cell model. The results showed a higher iron uptake in the pea protein isolate than in the soy protein isolate, suggesting a difference between the protein sources per se.