Iron is one of the few nutrients where what you take it with genuinely changes how much of it you absorb. Most of what circulates about this is half-right. Here is what the human studies actually found.
Tea and coffee reduce absorption, and it is not a small effect
Tea and coffee contain polyphenols, which bind non-haem iron in the gut and reduce how much gets across. This is well established, and it applies to iron from supplements as well as from food.
How much? In the one human study that measured this directly against a chelated form of iron, both a strong espresso and a tea infusion roughly halved the amount of iron absorbed — from 7.5 per cent down to 3.9 per cent in each case (Layrisse and colleagues, Journal of Nutrition, 2000). A weaker American-style coffee had no significant effect, so the strength matters.
Worth noting, because the opposite is sometimes claimed: that experiment had no ferrous sulfate comparison group, so it does not show that any particular form of iron is immune to the effect. If you want to be careful, leave an hour between your iron and a strong brew. That is the whole trick.
Phytates are the other big inhibitor
Phytate, found in wholegrains, legumes, nuts and seeds, binds iron in much the same way. In the same set of experiments, breaking down the phytate in a wheat meal roughly doubled the iron absorbed from ferrous sulfate and raised it by about two thirds from the chelated form.
The useful reading of that is not “avoid wholegrains” — they are good for you for other reasons — but that a very high-phytate meal is not the best moment to take an iron supplement.
Calcium and zinc compete with iron
Calcium and zinc use overlapping absorption machinery, so taking them at the same time as iron means they compete. The practical answer is spacing: leave roughly two hours between an iron supplement and a calcium or zinc supplement, and take iron at a different meal from a large serving of dairy.
Vitamin C: real mechanism, uncertain benefit
This is the one where the honest answer is more interesting than the marketing answer.
The mechanism is real and long established. Ascorbic acid improves non-haem iron absorption by reducing iron to the form the gut takes up and by countering phytates and tannins. The foundational work, across 299 subjects, identified about 50 mg per main meal as desirable for the effect (Hallberg, Brune and Rossander, Human Nutrition: Applied Nutrition, 1986).
But the clinical benefit has not been demonstrated. Two findings matter here. First, the effect largely disappears when you measure across a whole diet rather than a single meal — one study varied vitamin C intake from 51 to 247 mg a day across whole-diet periods and found no significant difference in iron absorbed (Cook and Reddy, American Journal of Clinical Nutrition, 2001).
Second, and more decisively: a randomised trial of 440 adults compared oral iron with and without 200 mg of vitamin C three times daily. Haemoglobin recovery was equivalent, and the authors concluded that vitamin C supplements are not essential alongside oral iron (Li and colleagues, JAMA Network Open, 2020). A 2023 meta-analysis of seven trials found the same non-significant result.
So: a glass of orange juice with your iron will not hurt, and the reasoning behind it is sound. Anyone telling you it is the thing that makes a supplement work is going further than the evidence does.
The thing that matters more than any of this
Absorption percentages are interesting, and they are also a rounding error next to whether you take the thing at all. A supplement you take every morning at 60 per cent of its theoretical absorption beats a perfectly optimised one sitting in a cupboard.
If you are going to act on one paragraph of this article, make it this one: pick the time of day you will actually remember, and keep the spacing rules to the two that matter — an hour from strong tea or coffee, two hours from calcium or zinc.
This article is general information, not medical advice. Whether an iron supplement is right for you is a question for your doctor. Always read the label and follow the directions for use.