Zinc and iron: why timing matters more than you think
Both minerals use the same absorption pathway. Taking them together can cut absorption by up to 50%. Here's what the research says and what you can do about it.
If you take both zinc and iron, you have probably seen conflicting advice about whether they can go together. The short answer: they compete for absorption. The longer answer is more useful.
The mechanism
Zinc and iron are both divalent cations. They share a transporter called DMT1 (divalent metal transporter 1) in the small intestine. When both minerals arrive at the same time, they compete for the same seats. The result is that your body absorbs less of each.
A 1981 study in the American Journal of Clinical Nutrition found that nonheme iron at Fe:Zn ratios of 2:1 and 3:1 substantially inhibited zinc uptake in humans. The effect goes both ways: a 2012 review in Biometals confirmed that zinc also inhibits iron bioavailability via competitive binding to DMT1 and Zip14 transporters.
How much does it actually matter?
The degree of competition depends on several factors:
- Dose ratio. At typical supplement doses (15-30 mg zinc, 18-36 mg iron), the interaction is moderate but measurable. Higher doses worsen it.
- Form. Heme iron (from food) is less affected than non-heme iron (most supplements). Chelated forms may behave differently.
- Whether food is present. Taking both with a meal blunts the competition somewhat, because food slows absorption of both minerals and distributes them over time.
- Individual iron status. People with low ferritin upregulate DMT1 expression, which may partially compensate.
A review in the Journal of Nutrition concluded that competitive inhibition is measurable at Fe:Zn ratios of 2:1 or greater when the total ionic load exceeds 25 mg, which is common with standalone supplements. At food-fortification levels, the effect is smaller.
What to do about it
The practical fix is simple: separate them by time.
- Take iron in the morning on an empty stomach. This is when absorption is highest anyway, because hepcidin (the hormone that regulates iron absorption) follows a circadian pattern and is lowest in the morning.
- Take zinc in the evening, with or after dinner. Zinc is well absorbed with food, and evening dosing avoids the morning iron window.
- Aim for at least 2 hours of separation. This gives the first mineral time to clear the transporter.
If you take a multivitamin that contains both, the doses are usually low enough that the interaction is minor. The concern is mainly with standalone iron and zinc supplements at therapeutic doses.
The vitamin C trick
If you take iron, consider pairing it with vitamin C instead. Ascorbic acid converts ferric iron (Fe3+) to ferrous iron (Fe2+), the form DMT1 actually transports. A review in the International Journal for Vitamin and Nutrition Research found that ascorbic acid is the most efficient enhancer of non-heme iron absorption, particularly in the presence of absorption inhibitors.
This is a case where the right pairing matters as much as avoiding the wrong one.
The bottom line
Zinc and iron are not dangerous together. They are just inefficient together. If you are supplementing both, separate them by a couple of hours and you get the full benefit of each.
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.