Can Calcium Block Absorption of Other Vitamins and Minerals?

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Calcium, an essential element and the most abundant mineral found in the body, plays a key role in building bone density and maintain muscle and nerve function. Calcium may interfere with the absorption of iron and possibly zinc when taken at the same time, but studies have not always shown this to be significant.


Taking calcium at the same time as non-heme iron, which comes from plant sources, can cause decreased iron absorption. Absorption of heme iron, which comes from animal sources, does not significantly decrease when you eat foods high in calcium at the same time, according to the Office of Dietary Supplements. A Kansas University Medical Center study reported in the January 1991 issue of “The American Journal of Clinical Nutrition” found that taking calcium with iron supplements reduced iron absorption when taken with food and also decreased absorption of dietary non-heme iron.


The addition of dairy or other high-calcium foods to meals rich in iron can reduce iron absorption by as much as 50 to 60 percent, according to an article published in the May 1992 “European Journal of Clinical Nutrition” by Swedish researchers from Sahlgren Hospital. Researchers advised reducing diary intake of calcium in main meals high in iron, particularly for groups at risk of iron deficiency such as pre-menopausal women, adolescents and children.


While animal studies show that calcium interferes with zinc, human studies don’t always give the same results. A Tuft's University study reported in the June 1997 issue of “The American Journal of Clinical Nutrition” measured the effect of calcium supplementation in postmenopausal women on zinc absorption. Subjects took calcium doses of either 890 mg or 1,258 mg over a 36-day period. In both groups, zinc absorption and zinc levels in the body were reduced by approximately 2 mg per day. A separate study of men and women ages 21 to 69 assessed the effect of calcium given with or without zinc on zinc absorption. Giving calcium with the meal but not adding supplemental zinc reduced zinc absorption by 50 percent.


Calcium intake is considered extremely important in certain age groups, including adolescent girls. An Ohio State University study looked at the effects of calcium supplementation on zinc absorption in adolescent girls. The study, reported in the May 1997 issue of “The American Journal of Clinical Nutrition,” found that the addition of 1,000 mg of supplemental calcium decreased zinc absorption from 21 to 15 percent, an amount researchers considered not statistically significant.