Calcium and iron: the absorption problem nobody mentions
Adding milk to your iron-rich meal can cut absorption by half. Calcium is one of the strongest inhibitors of iron uptake, and most people have no idea.
If you take both calcium and iron, or you take iron supplements with dairy, there is something you should know. Calcium is one of the most potent inhibitors of iron absorption, and the evidence is not subtle.
How strong is the effect?
A landmark study by Hallberg et al measured iron absorption in 57 human subjects and found that adding dairy to common meals reduced iron absorption by 50-60%. A milkshake with a hamburger, cheese on a pizza, a glass of milk with dinner: all of these cut iron uptake roughly in half.
The effect applies to both heme iron (from meat) and non-heme iron (from supplements and plant foods). That makes calcium unusual. Most absorption inhibitors only affect non-heme iron. Calcium inhibits both.
Where the inhibition happens
The mechanism is different from other mineral conflicts. Zinc and iron compete for the same transporter (DMT1) in the intestinal lumen. Calcium appears to act at a later stage, inside the intestinal mucosal cells themselves, at what researchers call the "common intracellular transfer step."
This means that calcium's inhibition of iron is harder to work around with tricks like changing the chemical form of iron. Whether you take ferrous sulfate, ferrous bisglycinate, or get iron from a steak, calcium still interferes once it reaches the mucosal cell.
Does the dose matter?
Yes, but not in the way you might expect. The Hallberg study found that the inhibition depends on the absolute amount of calcium present, not the calcium-to-iron ratio. A very small amount of calcium (3 mg) had no measurable effect even at a high molar ratio. But typical supplemental doses (500-1000 mg) or a glass of milk (~300 mg calcium) are well above the threshold.
The practical implication: the calcium in a multivitamin (usually 100-200 mg) will have some effect. The calcium in a standalone supplement or a dairy-heavy meal will have a large effect.
What about vitamin C?
This is where it gets interesting. Vitamin C (ascorbic acid) is the strongest known enhancer of non-heme iron absorption. It works by converting ferric iron (Fe3+) to ferrous iron (Fe2+), the form your body absorbs more efficiently. A review in the International Journal for Vitamin and Nutrition Research confirmed that ascorbic acid is the most efficient enhancer of non-heme iron absorption, particularly when inhibitors are present.
Hallberg's study found that the relative increase in iron absorption from vitamin C was the same whether or not calcium was present. In other words, vitamin C helps, but it does not cancel out the calcium effect. They work through different mechanisms.
So pairing iron with vitamin C is smart. But it is not a substitute for separating iron from calcium.
What to do about it
The fix is timing:
- Take iron on an empty stomach, ideally in the morning. Iron absorption is highest when hepcidin levels are lowest, which is early in the day.
- Take calcium later, with a different meal. Evening is a common choice, and there is some evidence that calcium before bed supports sleep.
- Separate them by at least 2 hours. This gives iron time to be absorbed before calcium arrives.
- Pair iron with vitamin C, not dairy. A glass of orange juice or a 100 mg vitamin C tablet with your iron supplement helps absorption. A glass of milk does the opposite.
- Watch your multivitamin. If it contains both calcium and iron, the iron may not be well absorbed. Consider standalone supplements taken at different times instead.
Who should care most?
Anyone supplementing iron for a reason: low ferritin, heavy periods, pregnancy, plant-based diet, endurance athletes. These are the people for whom iron absorption efficiency matters most. Losing 50% of your supplemental iron to a timing mistake is significant when you are trying to rebuild stores.
If your iron levels are fine and you eat a varied diet, the interaction is less of a concern at dietary levels. But if you are actively supplementing, separate them.
This article is for educational purposes only. It is not medical advice. Always consult a qualified healthcare professional before making changes to your supplement regimen.