The effect of calcium on iron absorption

The experimental and epidemiological evidence demonstrating that Ca inhibit s Fe absorption was reviewed, with the objectives of estimating the potenti al impact of variations in Ca intake on dietary Fe bioavailability and of p roviding some guidelines for predicting the effects on Fe status of recent recommendations for higher dietary Ca intake. In animal models Ca salts red uced both haem- and non-haem-Fe absorption, the effect being dependent on t he amount of Ca administered rather than the Ca:Fe molar ratio; dairy produ cts had a variable effect; factors other than Ca may have been important. I n single-meal human absorption studies, both haem- and non-haem-Fe absorpti on was inhibited by Ca supplements and by dairy products, the effect depend ing on the simultaneous presence of Ca and Fe in the lumen of the upper sma ll intestine and also occurring when Ca and Fe were given in the fasting st ate. The quantitative effect, although dose dependent, was modified by the form in which Ca was administered and by other dietary constituents (such a s phosphate, phytate and ascorbic acid) known to affect Fe bioavailability. The mechanism by which Ca influences Fe absorption has not been elucidated . The effects of factors that modulate Fe bioavailability are known to be e xaggerated in single-meal studies, and measurements based on several meals are more likely to reflect the true nutritional impact. The results of most multiple-meal human studies suggest that Ca supplementation will have only a small effect on Fe absorption unless habitual Ca consumption is very low . Outcome analyses showed that Ca supplements had no effect on Fe status in infants fed Fe-fortified formula, lactating women, adolescent girls and ad ult men and women. However it should be noted that the subjects studied had adequate intakes of bioavailable Fe and, except in one study, had relative ly high habitual Ca intakes. Although cross-sectional analyses in Europe ha ve shown a significant inverse correlation between Ca intake (derived prima rily from dairy foods) and Fe stores, the quantitative effect was relativel y small. The general conclusion is that dietary Ca supplements are unlikely to have a biologically significant impact on Fe balance in Western societi es unless Ca consumption is habitually very low; however, increased consump tion of dairy products may have a small negative effect that could be funct ionally important in pregnancy if Fe supplements are not taken. It is uncer tain whether the inverse relationship between consumption of dairy products and Fe status is due entirely to increased Ca intake; substitution of milk proteins for meat may also have negative effects on Fe balance.