Iron is an essential trace mineral that is required for the formation of haemoglobin and is used in oxidoreduction reactions such as in the electron transport chain. Dietary iron can be in the form of haem iron, or non-haem iron, the former being part of the haemoglobin or myoglobin molecules in animal tissues, while the latter is present in plant tissues. Generally, haem iron has a higher absorption than non-haem iron. Iron deficiency is the single most common nutritional deficiency in the world, something that is reflective of the widespread animal protein deficient diets of many areas in the third world. Calcium is thought to adversely affect the absorption of a number of minerals including iron. However, the evidence that calcium is able to inhibit iron absorption was derived from studies using food, that did not isolate the effects of calcium from that of the food matrix.
Research is therefore lacking regarding the effects of isolated calcium on iron absorption. To these ends, researchers1 have used a dose response study design to evaluate the effects of calcium on iron absorption when co-administered without the confounding variable of the food matrix. Radioactive iron tracers were fed to 54 healthy non-pregnant women in four trials which studied calcium (as calcium chloride) doses of between 200 and 1500mg on the absorption of both 5mg of non-haem (as ferrous sulfate) and 5mg of haem (as concentrated red blood cells) iron. All studies were performed on an empty stomach to remove the effects of the food matrix. The results showed that doses of calcium above 1000mg reduced the absorption of non-haem iron by 49.6%, and doses of calcium above 800mg reduced haem iron absorption by 37.3%. However, calcium doses below these values did not affect the absorption of iron.
Previous studies using calcium in a food matrix have shown inhibitory effects on iron absorption, but this effect in not present if the nutrients are given in isolation on an empty stomach. The food matrix is therefore able to interact with mineral absorption, something that has been shown in other studies involving other minerals, such as zinc and magnesium. Conclusions from studies involving calcium administered with food suggest that calcium can inhibit the enzymatic degradation of phytate, and that the higher phytate levels associated with calcium are then able to inhibit haem iron absorption. As has been noted in previous studies investigating mineral absorption, large variations exist between subjects that can affect the significance of results if small sample sizes are used. Interestingly, In the absence of calcium, haem-iron absorption was 13.9 ands 11.1% in this study, which is lower than has been reported in previous studies.