You can get sufficient amounts of both iron and calcium by eating whole, natural foods that are rich in these two minerals and part of a balanced, consistent Healthiest Way of Eating. Fortunately for all of us, whole, natural foods have evolved in such a way as to meet our nutrient needs provided that we do our part to consume them in a balanced and responsible way.
With respect to interactions between calcium and iron, let me address possible absorption problems from one perspective at a time. First, when it comes to calcium inhibition of iron absorption, I've seen several research studies showing clear, dose-dependent reductions in iron absorption following addition of calcium to a meal or to foods themselves. These reductions in iron absorption involved both plant (non-heme) and animal (heme) forms of iron, and were in the general range of 50%. While these studies definitely warrant more follow-up, I am reluctant to draw major conclusions from them about the interaction between these two minerals when it comes to whole, natural foods.
From my perspective, it is very different to add supplemental calcium to foods and examine the effects on iron absorption than it is to start with naturally high-calcium, non-supplemented foods and look at their impact on iron. In studies that I have seen on natural foods and long-term iron status, there does not appear to be any significant lowering of iron absorption when iron-rich foods and calcium-rich foods are consumed simultaneously. Over the long run, our bodies seem well equipped to handle both minerals simultaneously from the standpoint of iron absorption from whole, natural foods.
With respect to iron inhibiting calcium absorption, I have seen in vitro laboratory studies that show calcium absorption to be blocked when excessive amounts of iron have been allowed to accumulate in the spaces surrounding our cells. These lab studies, while extremely important, do not address the issue of iron and calcium that have been naturally stored up inside of food and are trying to make their way from food up into the body. The only studies that I have seen in this area of iron impact of calcium absorption have involved mineral supplements rather than whole foods. That being said the evidence in these studies points consistently in the direction of very little impact of iron on calcium absorption.
There is active transport of calcium from the upper parts of the small intestine into the cells lining the intestinal tract when calcium intake is very low, and this process is very carefully regulated. There is also passive transport of calcium lower down in the digestive tract that takes place under conditions of normal or high intake of calcium. I would expect this passive process to be influenced by a variety of minerals present in the lower small intestine, but I once again have found no evidence that separation of high-iron foods and high-calcium foods (or high-calcium foods and iron supplements) would be necessary to obtain beneficial amounts of both minerals.
For more information on this topic, please see:
Abrams SA, Griffin IJ, Davila P, et al. Calcium Fortification of Breakfast Cereal Enhances Calcium Absorption in Children Without Affecting Iron Absorption. J Pediatr. 2001;139(4):522-6.
Grinder-Pedersen L, Bukhave K, Jensen M, et al. Calcium From Milk or Calcium-Fortified Foods Does Not Inhibit Nonheme-Iron Absorption From a Whole Diet Consumed Over a 4-d Period. Am J Clin Nutr. 2004;80(2):404-9.
Hallberg L. Does Calcium Interfere With Iron Absorption? Am J Clin Nutr. 1998;68(1): 3-4.
Harris SS. The Effect of Calcium Consumption on Iron Absorption and Iron Status. Nutr Clin Care. 2002;5(5):231-5.
Roughead ZK, Zito CA, Hunt JR. Initial Uptake and Absorption of Nonheme Iron and Absorption of Heme Iron in Humans Are Unaffected by the Addition of Calcium As Cheese to a Meal With High Iron Bioavailability. Am J Clin Nutr. 2002;76(2):419-25.
Wauben IP, Atkinson SA. Calcium Does Not Inhibit Iron Absorption or Alter Iron Status in Infant Piglets Adapted to a High Calcium Diet. J Nutr. 1999;129(3):707-11.