How does coffee block iron absorption?

How does coffee block iron absorption?

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  1. More appears to be known about the effect of caffeine (not just coffee, tea appears to have a worse effect than than coffee) on nutrient absorption than the actual causes.
    If you drink coffee while eating a hamburger, 39% of the iron will be blocked. The blocking affect only lasts for about an hour, though.
    The livestrong article indicates that only plant based iron is blocked, but actual tests show both are, though it’s possible that the effect is worse for nonheme iron since it is more difficult to absorb anyway.
    http://www.livestrong.com/article/427404-caffeines-effect-on-iron-absorption/
    http://m.ajcn.nutrition.org/content/37/3/416.abstract
    http://www.fitday.com/fitness-articles/nutrition/vitamins-minerals/the-effect-of-caffeine-on-vitamin-absorption.html

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  2. By how much does caffeine inhibit iron absorption?
    It’s not clear.
    Several studies have found that drinking coffee (or tea) with a meal can significntly reduce iron absorption from the foods in that meal.
    In one study, where the iron came mostly from beef (a source of heme iron), its absorption was inhibited by 39%. In another where the iron came from bread (non-heme iron), absorption was decreased by more than 60%.
    But it’s unclear how big a role caffeine plays in the process. In yet another study, the drug caffeine itself was fund yo reduce iron absorption by only about 6%. This suggests that other coffee components—perhaps its polyphenols, antioxidants that may be the source of coffee’s health benefits—are the main culprit.
    This possibility is bolstered by the fact that tea (which has more but different polyphenols than coffee and less caffeine) appears to inhibit iron absorption even more strongly than coffee.
    In any case this inhibition seems to require that the coffee or tea be consumed at about the same time as the iron source. Drinking it an hour ahead of food does not affect iron absorption.
    (The revised question asks how this happens: It’s known than the polyphenols in coffee—like chlorogenic acid—and the tannins in tea can bind to caffeine, making the caffeine less bioavailable.)

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  3. 1. Iron uptake seems to be blocked right at the lining of the gut:
    The Effect of Caffeine on Vitamin Absorption [ http://www.fitday.com/fitness-articles/nutrition/vitamins-minerals/the-effect-of-caffeine-on-vitamin-absorption.html ]
    . But this affects not only iron absorption, but also absorption of calcium, manganese, zinc and copper. Coffee also
    increases the excretion of the minerals magnesium, potassium, sodium and phosphate.
    2. If you drink coffee one hour before a meal nutrients from the …

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