Coffee and tea contain polyphenols that bind non-heme iron in the gut and form insoluble complexes that pass through unabsorbed. Hurrell and colleagues (1999, Br J Nutr) tested this directly using radio-labeled iron in adult humans eating a standardized bread meal with different beverages. Absorption was quantified by erythrocyte incorporation of the tracer. Compared to water, beverages containing 20 to 50 mg of polyphenols per serving reduced iron absorption by 50 to 70%. At 100 to 400 mg, the reduction was 60 to 90%. Black tea: 79 to 94%. Peppermint tea: 84%. Cocoa: 71%. Chamomile: 47%. Adding milk did not meaningfully change the effect. The mechanism is specific to galloyl structure, not total phenolic content. Brune, Rossander, and Hallberg (1989, Eur J Clin Nutr) showed that tannic acid inhibits in a dose-dependent manner that tracks galloyl content: 5 mg cut absorption 20%, 25 mg cut it 67%, and 100 mg cut it 88%. Gallic acid inhibited equivalently per mol of galloyl groups. Catechin, which lacks the galloyl ester, showed no inhibition. Chlorogenic acid, the dominant polyphenol in coffee, inhibits but less potently than tannins. Heme iron behaves differently. It is absorbed through a separate brush-border pathway that is still incompletely characterized, likely involving receptor-mediated endocytosis. Iron is liberated from the porphyrin ring inside the enterocyte by heme oxygenase 1. Because the iron stays shielded inside the porphyrin until intracellular release, polyphenols in the gut lumen don't reach it. Heme iron from meat is largely protected. Non-heme iron from plants, eggs, and fortified foods is the vulnerable pool. Vitamin C counteracts the interaction by reducing Fe³⁺ to Fe²⁺ and forming a soluble ascorbate-iron complex that resists polyphenol binding. Hallberg, Brune, and Rossander (1986, Hum Nutr Appl Nutr) measured the dose-response directly. Hallberg and Hulthén's later absorption algorithm (2000, Am J Clin Nutr) integrated those data and predicts that 50 mg of ascorbate added to an inhibitor-rich meal increases non-heme iron absorption roughly 3 to 6-fold. For anyone with borderline iron status, menstruation-related losses, a plant-based diet, or pregnancy, timing matters. A two-hour gap between the beverage and iron-rich foods, or pairing the meal with vitamin C, is the simplest fix. The mechanism has been in the literature for three decades. It's rarely in standard dietary counseling, rarely on any bottle, and almost never mentioned by the industry selling iron. pubmed.ncbi.nlm.nih.gov/10999016/ pubmed.ncbi.nlm.nih.gov/2598894/ pubmed.ncbi.nlm.nih.gov/3700141/ pubmed.ncbi.nlm.nih.gov/10799377/
