controlled randomized trial assessing the effects of calcium supplementation on bone mass in adolescent girls ages 8 to 13 years (n = 354). A secondary analysis at year 4 of this 7-year trial found that girls in the supplemented group achieved a total calcium (food plus supplements) intake of 1,500 mg/day. When assessed for interactions between calcium and iron, measures of iron status—hemoglobin, hematocrit, and corpuscular indexes—were not significantly different from those of girls in the placebo group who reached a calcium intake of 800 mg/day. Ames et al. (1999) found no effect of a calcium intake of approximately 1,200 mg/day compared with 500 mg/day for 5 weeks on iron absorption in children 3 to 5 years of age.

With respect to zinc, McKenna et al. (1997) conducted a calcium and zinc balance study on a subset (n = 26) of participants in a longitudinal clinical trial of the effects of calcium supplementation on bone mass in girls with a mean age of 11 years. Trial participants received either 1,000 mg/day of supplemental calcium or a placebo. Mean calcium intake reached 847 ± 287 and 821 ± 224 mg/day from diet for placebo and intervention groups, respectively, at 6 months. With the additional supplement, the mean calcium intake in the intervention group exceeded 1,700 mg/day. The results of the balance study found no effect in the intervention group from intake of approximately 1,700 mg of calcium per day on net zinc absorption, zinc excretion, or zinc balance compared with intakes of approximately 800 mg/day in the placebo group.

Taken together, the studies suggest that calcium intakes of 1,500 to 1,700 mg/day do not interfere with iron or zinc absorption in adolescent girls. However, as calcium intakes among this age group could be higher than those studied, there is little evidence to shed light on the larger issue.

Calcium supplement intake has long been associated with constipation. In fact approximately 1 of every 10 participants in the WHI calcium–vitamin D supplementation trial reported moderate to severe constipation (Jackson et al., 2006). If a food source of calcium is the problem, the constipation is likely due to the components of dairy products (Anthoni et al., 2009) rather than to the calcium in food. Calcium supplements, which are regarded as “binding,” can cause side effects for some people, such as constipation and gas (Jackson et al., 2006; Prince et al., 2006), which varies greatly from person to person. Usually the constipation is alleviated by increasing intakes of water or fiber-rich foods, or by trying another form of supplement (calcium citrate may be less constipating than calcium carbonate, for example). Although such conditions warrant attention, the utility of constipation as an indicator for DRI development is doubtful.