Calcium and iron are nutrients that are vital for good health. Both should be included in the diet every day. Poor iron status is particularly common in teenage girls and in young children. For example, The National Diet and Nutrition Survey of children aged 1 – 4 years indicated that 1 in 12 young children were anaemic and 16% of the under four-year-olds and 4% of the over four-year-olds sampled consumed less iron than the lower reference nutrient intake (LRNI), the amount judged to be enough for only a small number of individuals with low needs (Gregory et al, 1995). Because the vast majority of the children consumed milk regularly, calcium intakes were adequate. Only 1% of children aged under 4 years and 2% of children aged over 4 years had intakes below the LRNI.

Short-term studies indicate that adding calcium salts or food that is rich in calcium to a meal has an adverse effect on iron absorption. As a result, it has sometimes been suggested that a high intake of milk is a contributory factor in the development of poor iron status. However, this conclusion is derived from the results of studies in which iron absorption was measured after a single meal or over just a few days. During the past few years, a number of long-term studies investigating the effect of calcium on iron absorption have reached a different conclusion.

Ames and colleagues (1999) placed pre-school children on each of two different diets for five weeks. One diet contained usual amounts of calcium (about 500mg/day) and the other diet was high in calcium (1180mg/day). (The additional calcium was derived from dietary sources as opposed to supplements.) Stable isotope techniques were used to compare iron absorption from each diet. Contrary to short-term studies, the high calcium diet did not cause a reduction in iron absorption; a similar amount of iron was absorbed from both diets (Ames et al, 1999).

In another study, a group of full-term infants, less than 6 months old, were randomly assigned to consume iron-fortified infant formula (465mg Ca/l) or the same formula with added calcium (1800mg Ca/l) (Dalton et al, 1997). Tests after four and nine months indicated that there was no difference in the iron status or the incidence of iron deficiency between the two groups.

Illich-Ernst and colleagues (1998) undertook a randomised, double-blind, placebo-controlled intervention trial in a group of girls at the beginning of puberty. For four years, half the group received a calcium supplement (1000mg/day) and the other half received a placebo. Iron status was assessed annually. While the pubertal growth spurt and menstruation affected iron status, long-term calcium supplementation did not affect serum ferritin or red blood cell indexes. Similarly, Minihane and Fairweather-Tait (1998) monitored the effect of calcium supplementation (1200mg/day) in a group of adults over a six-month period. They showed that calcium did not have any adverse effects on functional iron or iron stores.

At present, it is unclear why short-term and longer-term studies investigating the effect of calcium on iron absorption provide such different results. One suggestion is that there is an adaptive response in the intestinal mucosal cells. In the meantime, advice about good dietary sources of iron is a more straightforward strategy to tackle the important issue of poor iron status. The attention focused on poor iron status should not obscure advice about the importance of obtaining adequate dietary calcium, particularly to young people who have yet to achieve peak bone mass.

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References
Ames, S. K. et al (1999) Effects of high compared with low calcium intake on calcium absorption and incorporation of iron by red blood cells in small children. American Journal of Clinical Nutrition 70, 44-48.

Dalton, M. A. et al (1997) Calcium and phosphorus supplementation of iron-fortified infant formula: no effect on iron status of healthy full-term infants. American Journal of Clinical Nutrition 65, 921-926.

Gregory, J. et al (1995) National Diet and Nutrition Survey: children aged 1_ to 4_ years. Volume 1: Report of the diet and nutrition survey. London: HMSO.

Illich-Ernst, J. Z. et al (1998) Iron status, menarche, and calcium supplementation in adolescent girls. American Journal of Clinical Nutrition 68, 880-887.

Minihane, A. M. and Fairweather-Tait, S. J. (1998) Effect of calcium supplementation on daily nonheme-iron absorption and long-term iron status. American Journal of Clinical Nutrition 68, 96-102.