Table 4.

Single-meal bioavailability studies showing reductions in iron bioavailability with tannin consumption¹

Reference	Subjects, n	Iron status	Population	Intervention	Tannin type	Outcome
20	10	Replete	Women	Control meal with water vs. meal with tea	Black tea	Significant reduction in iron bioavailability by 20%.
56	10	Replete and depleted	Men and women	Oregano, spinach, coffee, tea, or tannic acid vs. control	Black tea, tannic acid, polyphenols/condensed tannin	Tannic acid significantly reduced iron bioavailability; oregano, tea, and coffee percentage inhibited bioavailability by >60%, which was less than their respective equivalent tannic acid doses. Spinach reduced bioavailability by 30% despite tannic acid equivalents similar to its tannic acid, tea, coffee, and oregano counterparts.
57	6 (C), 13 (I)	Replete	Men and women	High- vs. low-availability meal in vegetarians vs. nonvegetarians	Polyphenols/condensed tannin	Significant impairment of iron absorption from low-bioavailability meals in vegetarian and nonvegetarian consumers. Similar iron bioavailability between vegetarians (1.4% vs. 14.9% in bran vs. whole- wheat rolls, respectively) and nonvegetarians (22.3% vs. 2.2%) despite higher average phytate intake in vegetarian group.
22	10	Replete and depleted	Women	Meal with black tea or ascorbic acid, or control meal	Black tea	Significant reduction in iron bioavailability with tea consumption (18.2% vs. 7.1% in control vs. 150 mL tea drinkers, respectively, and 19.7% vs. 5.6% in control vs. 300 mL tea drinkers, respectively), not dependent on polyphenol burden (1 vs. 2 cups tea).
58	8	Replete and depleted	Men and women	10 different beverages	Black tea, herbal tea, cocoa, or coffee	Significant reduction in iron bioavailability with tannin consumption (tea); range in reductions for tea vs. water: 3–27%, dependent on whether food consumed.
59	13	Replete and depleted	Men and women	Control breakfast vs. coffee or tea	Black tea, polyphenols	Significant reduction in bioavailability with tea or coffee consumption vs. control (60–90% reduction vs. control; average 10% less iron absorbed).
60	22	Replete	Women	Control meal vs. meal with tea	Black tea	Nonsignificant differences in iron absorption with tea consumption (reduction 1.7%), although effects of tea noted with ascorbic acid consumption (20% reduction vs. ascorbic acid alone).
61	8	Replete	Women	Broccoli with tannic acid (500 mg) or broccoli alone	Tannic acid	Significant reduction (10% vs. 0.3%) in iron absorption for broccoli meal vs. broccoli + tannic acid meal.
31	14 (C), 15 (I)	Replete	Women	Control meal with 5 mg FeSO4 vs. tannic acid, phytic acid, or pectin	Tannic acid	16–25% significant reduction in iron absorption with tannic acid vs. no tannin consumption.
62	10 (C), 16 (I)	Replete	Women	Bread baked with tannic acid (12–884 mg) vs. control	Tannic acid	Significant reduction in iron absorption ratio and serum iron with consumption of tannic acid in bread (average reduction of iron absorbed, 3–10%).
16	10 (C), 11 (I)	Replete	Men	Meal with varying amounts of tannin-rich condiment [yod kratin (a vegetable); 0–584 tannic acid equivalents] to control	Polyphenols/condensed tannins	10% significant reduction in iron bioavailability with highest tannin content meal vs. control.

¹C, control; I, intervention.