Calcium and iron are both essential minerals that play vital roles in the human body. Calcium is responsible for building and maintaining strong bones, while iron helps transport oxygen in the blood. Although both minerals are crucial, a common recommendation is to avoid taking iron and calcium supplements together. Here is why.
Key takeaways
- Calcium and non-haem iron compete for the same transporters in the intestines.
- Calcium can bind with iron, forming insoluble complexes that cannot be absorbed.
- Weight loss surgery patients, pregnant women and vegetarians are particularly affected.
- Separate calcium-rich foods or supplements from iron-rich ones.
Why calcium and iron should not be taken together
- Competitive absorption: calcium and iron compete for absorption. When consumed together, calcium can inhibit iron absorption, which is particularly concerning for people at risk of iron-deficiency anaemia.1 Both calcium and non-haem iron (the type found in plant-based foods and supplements) use divalent metal transporters for absorption in the intestines. When both are present in high concentrations, they compete for these transporters, so absorption of one or both can be reduced.2
- Digestive issues: high doses of calcium, especially when taken with iron supplements, can cause digestive issues such as constipation, while some iron supplements can cause stomach cramps, nausea or diarrhoea.3 The stomach's acidic environment is essential for iron absorption. Calcium carbonate, a common form of calcium supplement, can act as an antacid and raise the pH in the stomach, potentially decreasing the solubility and absorption of non-haem iron.4
- Optimal absorption: to get the most out of your supplements, optimise absorption. Separating calcium and iron supplements ensures the body absorbs both minerals as well as possible.5 In the intestines, calcium can bind with iron to form insoluble complexes that cannot be absorbed. So even if there is plenty of iron in the digestive system, it might not be absorbed efficiently if there is also a high concentration of calcium.6
- Special populations: pregnant women, vegetarians, weight loss surgery patients and others at higher risk of iron deficiency might be particularly affected by the competition between calcium and iron.7
- Meal considerations: when planning meals, especially if you rely on fortified foods for iron, it might help to separate foods high in calcium from those rich in iron – for example, having dairy products at a different time from iron-fortified cereals.8
- Haem vs non-haem iron: haem iron (found in animal foods) is absorbed differently from non-haem iron. While calcium can affect non-haem iron absorption, its impact on haem iron absorption is less significant, because the two types of iron take distinct pathways during absorption.9
- Regulation of iron homeostasis: the body has a sophisticated system to regulate iron levels. When iron stores are low, the intestines increase iron absorption; when levels are adequate or high, absorption decreases. The presence of other minerals, including calcium, can further influence this system.10
Be strategic about calcium and iron
While both iron and calcium are vital for our health, it is important to be strategic about their intake. By understanding their interactions, you can make informed decisions about diet and supplementation to ensure optimal absorption and health benefits. When planning a diet or supplement routine, separate calcium-rich foods or supplements from iron-rich ones.
After bariatric surgery, take calcium and iron at least two hours apart, and follow the supplement schedule from your bariatric team.
Sources
- Hallberg L, Brune M, Rossander L. Effect of ascorbic acid on iron absorption from different types of meals. Human Nutrition. Applied Nutrition. 1986;40(2):97-113.
- Gunshin H, Mackenzie B, Berger UV, et al. Cloning and characterization of a mammalian proton-coupled metal-ion transporter. Nature. 1997;388(6641):482-488.
- Fine KD, Santa Ana CA, Porter JL, Fordtran JS. Intestinal absorption of magnesium from food and supplements. Journal of Clinical Investigation. 1991;88(2):396-402.
- Diaz M, Rosado JL, Allen LH, Abrams S, Garcia OP. The efficacy of a local ascorbic acid-rich food in improving iron absorption from Mexican diets: a field study using stable isotopes. American Journal of Clinical Nutrition. 2003;78(3):436-440.
- Cook JD, Monsen ER. Vitamin C, the common cold, and iron absorption. American Journal of Clinical Nutrition. 1977;30(2):235-241.
- Hallberg L, Rossander-Hulten L. Calcium and iron absorption: mechanism of action and nutritional importance. European Journal of Clinical Nutrition. 1991;45(5):225-233.
- Milman N, Byg KE, Bergholt T, Eriksen L. Iron status and iron balance during pregnancy. A critical reappraisal of iron supplementation. Acta Obstetricia et Gynecologica Scandinavica. 1999;78(9):749-757.
- Lynch SR. Interaction of iron with other nutrients. Nutrition Reviews. 1997;55(4):102-110.
- Hallberg L, Hulthen L. Prediction of dietary iron absorption: an algorithm for calculating absorption and bioavailability of dietary iron. American Journal of Clinical Nutrition. 2000;71(5):1147-1160.
- McKie AT, Barrow D, Latunde-Dada GO, et al. An iron-regulated ferric reductase associated with the absorption of dietary iron. Science. 2001;291(5509):1755-1759.
This article is for general information only and does not replace personal medical advice. Always discuss your situation, blood test results and supplementation with your bariatric team or healthcare provider.