Evidence for the usefulness of in vitro dialyzability, Caco-2 cell models, animal models, and algorithms to predict zinc bioavailability in humans

Low bioavailability of zinc in certain diet types may contribute to zinc deficiency and its consequences in populations. As a result, several experimental models including animal models, in vitro dialyzability models, and Caco-2 cells, have been used to study these factors and estimate their impact on human zinc absorption. For the most part, consistency has been observed between the latter models and human absorption studies to identify factors that enhance or inhibit zinc bioavailability. However, dialyzability methods are limited to modeling luminal interactions among the factors as they affect zinc availability while Caco-2 cells can model luminal effects and uptake by intestinal cells. Neither animal nor in vitro methods can predict the magnitude of zinc absorption at the level of the human organism. Caco-2 cells will be useful models for understanding the mechanisms of intestinal zinc absorption. The in vitro methods are also limited to modeling absorption and the interactions that occur. Algorithms to estimate zinc absorption, based on dietary content of zinc absorption modifiers, have been derived from human studies. An algorithm derived from studies of zinc retention from radioactive zinc-labeled test meals underestimates zinc absorption compared to that derived from measurement of true zinc absorption from total diets using isotopic tracer methods. Based on the latter, phytate appears to be the only major inhibitor of zinc absorption from typical diets. Ultimately, population-based studies are needed to determine the impact of dietary factors that modulate zinc absorption on the adequacy of zinc status.