Iron is an essential component of hemoglobin, the protein in red blood cells that carries oxygen. Iron deficiency anemia occurs when there is not enough iron in the body to make hemoglobin and red blood cells. Iron supplements are prescribed to replace iron stores. The absorption of iron in the small intestine is greatest from an empty stomach. A variety of foods reduce the absorption and effectiveness of iron pills.
Grains and Fiber
Whole-grain cereals and breads reduce the absorption of iron from food and iron pills. Bran, oats and other grains contain phytic acid, which binds with iron in the intestine and prevents it from being absorbed. Bran has a particularly high level of phytic acid. A study in the American Journal of Clinical Nutrition in 1987 showed that adding bran to wheat flour reduced iron absorption by 75 percent. High-fiber diets limit iron absorption by increasing the speed at which iron pills pass through the intestine.
Soy
Soy protein also contains substantial amounts of phytic acid, which interferes with iron absorption. A study in the American Journal of Clinical Nutrition in 1992 showed that iron absorption was reduced by 75 percent when it was given with soybean milk.
Tea
Drinking one cup of tea immediately after taking iron pills reduces the amount of iron absorbed by more than 70 percent. Black tea has the greatest inhibitory effect, reducing absorption by 80 to 95 percent. However, herbal teas such as peppermint and chamomile also decrease iron absorption by 50 percent to 80 percent. Drinking tea more than two hours before taking iron minimizes the effect on iron absorption. Tea contains high levels of polyphenols, which are substances that bind with iron, preventing its absorption. There are many kinds of polyphenols with variable effects on iron absorption. Tannic acid, a polyphenol found in tea, is one of the most potent inhibitors of iron absorption.
Coffee
A single cup of coffee reduces iron absorption by at least 40 percent. The greatest effect occurs within one hour after taking iron pills. In contrast, drinking coffee more than one hour before taking iron does not block absorption. The decrease in iron absorption is greater with stronger coffee. However, even strong coffee has only 1/2 the inhibitory effect of tea. This is because coffee contains different types of polyphenols with weaker effects on iron absorption.
Other Beverages
Several other beverages that contain high levels of polyphenols prevent iron absorption. Cocoa reduces the absorption of iron by more than 70 percent. Prune and red grape fruit juices have a milder but still substantial effect, decreasing iron absorption by 30 to 60 percent. The amount of iron absorbed after drinking red wine is less than 1/2 that absorbed after white wine. Red wine contains a 10-fold higher concentration of polyphenols.
Spices
Several common spices, such as rosemary, oregano and cinnamon, contain different polyphenols that may reduce iron absorption. A study in the Journal of Nutrition in 2006 reported that adding dried ground chili pepper to food fortified with an iron supplement reduced the amount of iron absorbed by nearly 40 percent.
Dairy Products
Milk, cheese and other dairy products markedly interfere with iron absorption. The calcium in dairy products impairs the absorption of iron by blocking its transport across the cells lining the small intestine. A glass of milk or a slice of cheese containing 125mg of calcium reduces the absorption of iron pills by 50 percent. Servings with 300mg of calcium decrease iron absorption by as much as 80 percent. In contrast, calcium added to orange juice does not reduce iron absorption. This is because orange juice contains vitamin C and citrate, which enhance iron absorption and counterbalance the inhibitory effect of calcium. Dairy products consumed at least two hours before taking iron pills also do not affect iron absorption.
References
- "American Journal of Clinical Nutrition"; Phytates and the Inhibitory Effect of Bran on Iron Absorption in Man; Leif Hallberg, Lena Rossander and Ann-Britt Skanberg; Volume 45 pg. 988-996; 1987
- "American Journal of Clinical Nutrition"; Soy Protein, Phytate, and Iron Absorption in Humans; Richard Hurrell, Marcel-A Juillerat, Manju Reddy, Sean Lynch, Sandra Dassenko and James Cook; Volume 56 pg. 573-578; 1992
- "British Journal of Nutrition"; Inhibition of Non-Haem Iron Absorption in Man by Polyphenolic-Containing Beverages; Richard Hurrell, Manju Reddy and James Cook; Volume 81 pg. 289-295; 1999
- "The Journal of Nutrition"; Chili, but Not Turmeric, Inhibits Iron Absorption in Young Women from an Iron-Fortified Composite Meal; Siriporn Tuntipopipat, Kunchit Judprasong, Christophe Zedet, Emorn Wasantwisut, Pattanee Winichagoon, Somsri Charoenkiatkul, Richard Hurrell and Thomas Walczyk; Volume 136 pg. 2970-2974; 2006
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