Interaction fact sheet · Caution
Calcium and iron
Real interference at a single meal, little long-term impact in adults without a deficiency.
If you take iron to correct a deficiency, separate it from calcium. Otherwise, the evidence does not justify overhauling your meals.
The effect at a single meal
Hallberg and colleagues measured iron absorption in 126 people, with calcium doses of 40 to 600 mg added to bread rolls. The effect increased with the dose up to 300 mg: between 300 and 600 mg of calcium, iron absorption fell by 50 to 60%. The same reduction occurred with 165 mg of calcium supplied by milk, cheese or a calcium salt. Calcium also reduced the absorption of heme iron, the type found in meat.
Minihane and Fairweather-Tait confirmed the effect with supplements: in 14 adults, 400 mg of calcium carbonate at each meal reduced non-heme iron absorption from 15.8% to 4.7% over the day.
The effect on stores
The same team followed 11 adults without iron deficiency who took 1,200 mg of calcium per day with meals for 6 months, compared with a control group. Hemoglobin, hematocrit and ferritin did not change. Lönnerdal's review reaches the same conclusion across long-term studies: no decline in iron markers. The proposed hypothesis is an intestinal adaptation, observed in cell cultures, that could make calcium's effect temporary.
Cautious approach
If you take iron to correct a deficiency, keep it separate from calcium and dairy products, for example by taking iron at one end of the day and calcium at the other, following the package instructions or your doctor's advice. Iron supplementation should be justified by blood test results (ferritin), not taken by default: excess iron carries its own risks. For your dietary intake, see the table of iron-rich foods and our article on the low ferritin.
Limits
Single-meal measurements overestimate the effect on a whole diet. Conversely, long-term studies involve adults without a deficiency: they do not tell us what happens in someone correcting anemia, in adolescents, or during pregnancy, when iron needs are higher. The adaptation mechanism remains a hypothesis derived from cell cultures.
Main sources
- 01Hallberg L et al. Calcium: effect of different amounts on nonheme- and heme-iron absorption in humans. American Journal of Clinical Nutrition, 1991. PubMed 1984335
- 02Minihane AM, Fairweather-Tait SJ. Effect of calcium supplementation on daily nonheme-iron absorption and long-term iron status. American Journal of Clinical Nutrition, 1998. PubMed 9665102
- 03Lönnerdal B. Calcium and iron absorption: mechanisms and public health relevance. International Journal for Vitamin and Nutrition Research, 2010. PubMed 21462112
Frequently Asked Questions (FAQ)
How many hours between iron and calcium?
None of the studies cited here tested a specific interval. They show that the effect exists when both are taken with the same meal. In practice, take them at different times of day, and follow the leaflet or your doctor's advice if your iron is prescribed.
Do dairy products block iron?
In a single meal, yes: in the Hallberg trial, 165 mg of calcium from milk or cheese reduced iron absorption by 50 to 60%. Over several months, in adults without a deficiency, ferritin does not decrease. This matters mainly if you are correcting a deficiency.