The Impact of Tannin Consumption on Iron Bioavailability and Status: A Narrative Review

Iron deficiency remains a global health issue, and antinutritional factors, such as tannins, are often cited as contributors to the high prevalence of deficiency. Despite this, tannin-rich diets may have potential beneficial cardiovascular and cancer fighting properties due to tannins9 antioxidant activity. Further, epidemiological studies and long-term trials involving participants who consumed diets rich in antinutritional factors, particularly tannins, conflict with single-meal bioavailability studies. The purpose of this narrative review is to determine tannins9 effects on iron bioavailability and status, and establish whether adaptation to tannins reduces tannins9 antinutritional effects over time. We also aimed to compare tannins used in iron studies. Common themes related to iron bioavailability and iron status with tannin consumption were collected and collated for summary and synthesis based on models and subjects used. Overall, there was dissonance among iron bioavailability and status in studies. Single meal studies with hydrolyzable and oligomeric catechin/epicatechin tannins (tea and tannic acid) generally support reductions in bioavailability with tannin consumption compared to condensed tannin9s consumption, although condensed tannins are more commonly found in food. Long term animal models, epidemiological data, and multi-meal studies generally do not support changes in iron status related to tannin intake. Studies suggest that long-term tannin consumption may reflect iron status differently than single meal studies or bioavailability iron models predict. Further, iron bioavailability studies employing condensed tannins, which are more commonly consumed, may better predict mealtime iron bioavailability. More research is needed to develop representative antinutritional-iron studies, as well as the possibility of, and mechanisms underlying the adaptation to tannins and other antinutritional factors that occurs over time.