Calcium and iron absorption: two reports

Nutrition Research Newsletter, March, 1991

CALCIUM AND IRON ABSORPTION Two Reports ron and Calcium Supplements: Adult women are often advised to increase their intakes of both iron and calcium, by supplementation if necessary. Recent clinical observations indicate, however, that if these minerals are consumed together, calcium may inhibit the absorption of iron, both from supplements and from food. In studies conducted at Kansas University Medical Center, a double-radiosotope technique was used to assess the effect of three types of calcium supplements (calcium carbonate, citrate, and phosphate) on absorption of either a ferrous sulphate supplement or nonheme food iron. Supplements were administered in several doses and taken either with or without food. The subjects were 28 women and 33 men aged 20-39 years.

When taken without food, calcium carbonate did not inhibit the absorption of ferrois sulphate; however, at relatively high doses (600 mg), both calcium citrate and calcium phosphate significantly reduced iron absorption, by 49% and 62%, respectively. When taken with food, all three calcium supplements inhibited the absorption of the iron supplement, by 40-57%. They also inhibited the absorption of nonheme food iron. The degree of inhibition varied with the composition of the test meal.

These findings suggest that individuals who need both calcium and iron supplements should choose calcium carbonate and take it between meals. If taken with meals, "calcium supplements used regularly may make it more difficult to meet daily iron requirements." James D Cook, Sandra A Dassenko, and Paul Whittaker, Calcium Supplementation: Effect on Iron Absorption, American J Clinical Nutrition 53(1):106-111 (Jan 1991) [Reprints: JAmes D Cook, Division of Hematology, Kansas University Medical Center, 39th and Rainbow, Kansas City KS 66103] Calcium Supplements and Dietary Iron: As discussed above, calcium supplements can interfere with absorption of dietary or supplemental iron. The same problem can occur when calcium from foods is consumed with an iron source. Researchers from the University of Goteborg, Sweden, evaluated the effects of various sources of dietary calcium on iron absorption in normal adult volunteers using a double-radiosotope technique. The researchers initially planned to use wheat rolls as the iron-containing test meal, and to add calcium to the four before baking. However, it was found that the addition of calcium reduced phytate degradation during fermentation and baking, thus interferring indirectly with iron absorption.

To avoid this problem, a second set of experiments was conducted in which calcium was added to rolls after baking. It was found that calcium accded in this way had a direct dose-related inhibiting effect on iron absorption, reducing it by 50-60% at calcium doses of 300-600 mg. When 165 mg calcium was given as milk, cheese, or calcium chloride, iron absorption was again reduced by 50-60%. Absorption of heme iron from a hamburger meal was also reduced by 165 mg calcium.

The observed marked inhibitory effect on iron absorption of calcium in amounts frequently encountered in normal meals has important nutritional implications." The unexpected effect on phydate degradation is also of practical importance, since calcium or milk is added to bread dough before baking in some countries. Leif Hallberg et al, Calcium: Effect of Different Amounts on Nonheme- and Heme-Iron Absorption in Humans, American J Clinical Nutrition 53(1):112-119 (Jan 1991) [Reprints: Leif Hallberg, Department of Medicine II, University of Goteborg, Sahlgren Hospital, S-413 45 Goteborg,, Sweden]