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Several plant-derived compounds have the ability to reduce iron absorption secondary to forming bonds with the iron molecule and sequestering it in the intestines, and coingestion of these compounds with iron reduces the efficacy of iron supplementation.
Curcumin (the most active component of Turmeric) has shown this potential in mice only when high doses of curcumin (estimated human dose of 8-12g) are paired with low dietary iron levels; diets with adequate levels of iron not being significantly hindered and dietary turmeric at 500mg not having any effect in humans.
The addition of 4.2g ground chili (capsicum annuum) to a meal fortified with 4mg non-heme iron showed a moderate inhibitory effect on iron absorption by 38%; this test meal being relatively high in phytic acid due to the addition of chili. Rosemary (source of Rosmarinic Acid) has also been noted to reduce nonheme iron absorption.
In regards to spices, those with high phenolic contents or those with a high phytate content may reduce iron absorption when both are ingested at the same time
Psyllium is a dietary fiber comprised of roughly equal parts soluble and insoluble fiber. While one study has noted a reduction in iron accumulation when coingested with non-heme iron while other studies using prolonged supplementation of around 10g fail to find alterations in iron metabolism. Psyllium has the potential to both reduce absorption of minerals via direct binding to nonheme in a manner not influenced by Vitamin C while in the colon the increase in pH from psyllium has been noted to increase resorption of calcium; thought to apply to other minerals as well.
While there may be an acute inhibitory effect on dietary fibers on nonheme iron absorption, the long term relevance of this is not known since fermentable dietary fibers may also increase mineral resorption from the colon
Coffee is known to inhibit iron absorption when coingested with nonheme iron, potentially due to the activities of Chlorogenic Acid which is a known iron chelator which would extend this inhibition to the dietary supplement known as Green Coffee Extract (a higher source of chlorogenic acid).
Other beverages that have been noted to inhibit iron absorption in a test meal include Peppermint tea, green tea (due to catechins), black tea (additional Theaflavins component may play a role), vervain tea, lime flower tea, pennyroyal tea, and chamomile tea.
A wide variety of beverages that are known to have a high antioxidant content, including coffee and teas from camellia sinensis such as green or black teas have some acute inhibitory effect on iron absorption
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