Why minerals compete
Several minerals share the same absorption pathways in the gut, so a large dose of one can reduce how much of another you take up at the same time. This mostly matters for concentrated single-mineral supplements, not the modest amounts in food or a multivitamin [1].
The main interactions
- Calcium ↔ iron. Calcium can reduce the absorption of both nonheme and heme iron, so a calcium supplement taken with an iron supplement blunts iron uptake [1].
- Iron ↔ zinc. NIH notes that supplements containing 25 mg of iron or more can reduce zinc absorption and lower plasma zinc [1].
- Zinc ↔ copper. Long-term high-dose zinc can lower copper status — covered in detail in [zinc intake and copper balance](/learn/zinc-intake-and-copper-balance).
- Calcium ↔ magnesium. Very large doses taken together can compete, though typical doses are fine.
Helpers and inhibitors for iron
Iron absorption is especially sensitive to what it's taken with:
- Vitamin C enhances absorption of nonheme (plant) iron [1].
- Tea and coffee (polyphenols), phytates in grains and legumes, and calcium inhibit nonheme iron [1].
An important reality check
NIH adds a crucial caveat: in a typical varied Western diet, these enhancing and inhibiting effects are largely attenuated and have little effect on most people's iron status [1]. So this is mainly a concern if you take high-dose single minerals, have a diagnosed deficiency, or have been told to supplement.
Practical timing
- Separate iron and calcium supplements by a couple of hours if you take both.
- Take iron with a source of vitamin C, and away from tea or coffee, to aid absorption.
- Avoid stacking several high-dose single minerals at the same time; a balanced multivitamin spaces the load (see [stacking safely](/learn/supplement-stacking-safety)).
- For most people getting minerals from food, no special timing is needed [2].