Calcium in drinking water: effect on iron stores in Danish blood donors—results from the Danish Blood Donor Study

BACKGROUND: Studies confirm that calcium inhibits iron absorption. Danish tap water comes from groundwater, which contains varying amounts of calcium depending on the subsoil. We investigated the association of calcium in drinking water with iron levels in Danish blood donors. STUDY DESIGN AND METHODS: We used data on Danish blood donors including dietary and lifestyle habits, blood donation history, and physiologic characteristics including measures of ferritin levels along with information on area of residence from The Danish Blood Donor Study. Data on calcium levels in groundwater (“water hardness”) were obtained through the Geological Survey of Denmark and Greenland. We performed multiple linear and logistic regression analyses to evaluate the effect of water hardness on ferritin levels and risk of having iron deficiency (defined as ferritin levels <15 ng/mL), stratified by sex. RESULTS: There was a significant negative association between water hardness and ferritin levels in both men and women. Risk of iron deficiency was correspondingly increased in both men (odds ratio [OR], 1.55; 95% confidence interval [CI], 1.14-2.12) and women (OR, 1.20; 95% CI, 1.03-1.40) with increasing water hardness. In analyses restricted to individuals who received supplemental iron tablets no significant association between groundwater hardness and ferritin levels was observed. CONCLUSIONS: As measured by ferritin levels, residential drinking water calcium content is associated with blood donors- iron levels and risk of iron deficiency. However, effect sizes are small.