What to Combine Iron With

Iron supplements are notoriously fussy. The same tablet can be absorbed well or poorly depending on what you take it with, when you take it and even when you last trained. For athletes, especially women and endurance competitors, getting this right can be the difference between restoring iron stores in a few months or struggling for a year.

Why iron absorption is so variable

Iron exists in two dietary forms. Haem iron, from meat and fish, is absorbed relatively efficiently and is less affected by other foods. Non-haem iron, found in plants, fortified foods and nearly all supplements, is absorbed much less predictably.

The body has no active way to excrete excess iron, so it controls intake at the gut. The main regulator is hepcidin, a hormone produced by the liver. When hepcidin is high, iron absorption from the gut and release from stores are reduced.

Hepcidin rises in response to inflammation, to high iron stores and, importantly, to iron intake itself. This explains why taking more iron does not always lead to proportionally more absorption.

Before supplementing, it is worth confirming deficiency with a blood test including ferritin, haemoglobin and, ideally, transferrin saturation. Iron supplements without a confirmed need offer no performance benefit and can cause harm.

Good partners: what helps absorption

Vitamin C is the best-known enhancer. Hallberg and colleagues showed that ascorbic acid counteracts the inhibitory effect of phytate and increases non-haem iron absorption in a dose-dependent way. Taking iron with a glass of orange juice or a vitamin C-rich food is a simple, inexpensive step.

Meat, fish and poultry contain a so-called "meat factor" that enhances absorption of non-haem iron eaten at the same meal. For omnivores, adding a little meat to a bean-based dish improves iron uptake.

Other helpful combinations include:

• fruit and vegetables rich in vitamin C, such as peppers, kiwi, broccoli and citrus;
• fermented foods, in which fermentation reduces phytate content;
• an empty stomach, or at least a light meal, when tolerated;
• water, rather than tea, coffee or milk, as the drink of choice.

Poor partners: what gets in the way

Tea and coffee contain polyphenols that bind iron in the gut. The effect is large enough to matter, so a common piece of NHS advice is to avoid drinking them for a while before and after an iron tablet.

Calcium inhibits both haem and non-haem iron absorption. Hallberg and colleagues demonstrated a dose-dependent effect. Taking a calcium supplement, a glass of milk or a dairy-based protein shake at the same time as iron reduces its value.

Phytates in wholegrains, bran, legumes and nuts bind iron strongly. This does not mean these foods should be avoided, but iron tablets are best taken away from high-fibre breakfast cereals and bran.

Zinc and iron compete for absorption when taken in high doses together. Multimineral products contain modest amounts, but standalone high-dose zinc and iron supplements are better separated.

Timing, hepcidin and training

Studies by Moretti and colleagues found that a dose of oral iron raises hepcidin for up to 24 hours, reducing absorption from a second dose taken the same or next day. Stoffel and colleagues then showed in iron-depleted women that giving iron on alternate days led to greater total absorption than daily dosing.

Exercise also raises hepcidin. Peeling and colleagues observed a hepcidin peak roughly three to six hours after a running session. For athletes, this suggests that iron may be better taken in the morning before training or on rest days, rather than in the hours after a hard session.

Hepcidin also follows a daily rhythm, being lower in the morning. Combining these findings, many sports dietitians recommend a single morning dose, on alternate days, with vitamin C and water, separated from coffee and dairy. A narrative review by Sim and colleagues summarises this approach for athletes.

Common NHS supplements contain around 65 mg of elemental iron per tablet as ferrous sulfate. The appropriate dose and duration depend on the degree of deficiency and should be agreed with a GP, who will also arrange repeat blood tests.

Medicines and safety

Iron interacts with several medicines. It reduces the absorption of levothyroxine, some antibiotics (tetracyclines and quinolones such as ciprofloxacin), bisphosphonates and some drugs for Parkinson's disease. These should usually be separated from iron by several hours.

Proton pump inhibitors and antacids reduce stomach acid, which is needed to keep iron in a soluble form. People on long-term acid suppression may absorb oral iron less effectively.

Gastrointestinal side effects such as constipation, nausea and dark stools are common. Alternate-day dosing, taking iron with a small amount of food or switching to a different salt can help.

Iron overdose is dangerous, especially for children. Supplements should be stored securely. People with haemochromatosis or other iron-overload conditions should not take iron supplements at all.

Conclusions of the editorial team

Iron is best combined with vitamin C and water, taken in the morning and, for many people, on alternate days. Tea, coffee, calcium, dairy and high-phytate foods should be kept apart.

Athletes should consider training timing, because exercise raises hepcidin for several hours afterwards.

Medicines such as levothyroxine and certain antibiotics need to be separated from iron. You may also wish to read our articles on vegetarian diets and strength training, on ferritin testing for athletes, and on vitamin C supplements.

References

1. Hallberg L, Brune M, Rossander L. Iron absorption in man: ascorbic acid and dose-dependent inhibition by phytate. Am J Clin Nutr. 1989;49(1):140–144.
2. Hallberg L, Brune M, Erlandsson M, et al. Calcium: effect of different amounts on nonheme- and heme-iron absorption in humans. Am J Clin Nutr. 1991;53(1):112–119.
3. Moretti D, Goede JS, Zeder C, et al. Oral iron supplements increase hepcidin and decrease iron absorption from daily or twice-daily doses in iron-depleted young women. Blood. 2015;126(17):1981–1989.
4. Stoffel NU, Cercamondi CI, Brittenham G, et al. Iron absorption from oral iron supplements given on consecutive versus alternate days and as single morning doses versus twice-daily split dosing in iron-depleted women: two open-label, randomised controlled trials. Lancet Haematol. 2017;4(11):e524–e533.
5. Peeling P, Dawson B, Goodman C, et al. Effects of exercise on hepcidin response and iron metabolism during recovery. Int J Sport Nutr Exerc Metab. 2009;19(6):583–597.
6. Sim M, Garvican-Lewis LA, Cox GR, et al. Iron considerations for the athlete: a narrative review. Eur J Appl Physiol. 2019;119(7):1463–1478.