Iron or Vitamin C for absorption: what is the difference

Iron and vitamin C are often mentioned in the same sentence, as if they were two versions of a single remedy for fatigue and anemia. In reality, one is a building material and the other only a helper in its absorption. The editorial team explains the mechanism, assesses the real size of the effect according to research, and describes what is important for athletes to know.

Why these two substances are compared at all

The query "iron or vitamin C" seems strange at first glance: one is a mineral, the other a vitamin. Yet in conversations about anemia and fatigue they are constantly mentioned together, and pharmacies sell combined preparations of iron with ascorbic acid. Because of this many people get the impression that vitamin C on its own "raises iron" or can replace an iron-containing supplement.

In reality the roles of these substances are fundamentally different. Iron is a substrate that the body uses for the synthesis of hemoglobin, myoglobin and the enzymes of the respiratory chain. Vitamin C contains no iron and cannot compensate for its deficiency. It only facilitates the absorption of a certain form of iron from food or a supplement.

This article explains how their functions differ, how exactly vitamin C affects absorption, how large this effect is in a real diet, and why for athletes other regulators are also important, above all the hormone hepcidin.

Understanding the mechanism helps to avoid two typical mistakes: taking vitamin C "for anemia" without iron and, conversely, taking iron without a confirmed deficiency merely because of fatigue.

Iron: functions and forms

In the human body iron is mostly part of the hemoglobin of red blood cells; the rest is in muscle myoglobin, in enzymes and in reserves as ferritin. For athletes iron is critical, because oxygen transport and mitochondrial function depend on it. Iron deficiency even without anemia can reduce performance and increase fatigue.

In food iron comes in two types. Heme iron is found in meat, liver and fish and is absorbed relatively well and consistently. Non-heme iron is found in plant products, eggs, fortified foods and in most supplements; its absorption is much lower and depends heavily on the composition of the food (Hurrell, Egli, 2010).

The body has no active mechanism for excreting iron, so it regulates its balance precisely at the level of absorption. The main regulator is the liver hormone hepcidin. When iron stores are sufficient or there is inflammation in the body, hepcidin levels rise and absorption drops sharply. That is why an excess of iron from supplements is not absorbed "in proportion to the dose."

Iron supplements exist in various forms: sulfate, fumarate, gluconate of iron (II), bisglycinate, iron (III) preparations. They differ in elemental iron content, tolerability and price. The review by Camaschella (2015) in NEJM emphasizes that ferrous iron salts remain the standard of oral therapy for deficiency.

Vitamin C: how it aids absorption

Non-heme iron in food is mostly in the trivalent form (Fe³⁺), which is poorly absorbed. The DMT1 transporter in the cells of the duodenum carries divalent iron (Fe²⁺). Vitamin C reduces Fe³⁺ to Fe²⁺ and forms with it soluble complexes that remain available for absorption even in the less acidic environment of the intestine.

The second function of ascorbic acid is to counteract inhibitors. Phytates of grains and legumes, polyphenols of tea and coffee, and calcium from dairy products bind iron and slow its absorption. Vitamin C partly neutralizes this effect. Classic work by the Hallberg group (1989) showed a dose-dependent enhancement of non-heme iron absorption from food with the addition of ascorbic acid.

Vitamin C has almost no effect on heme iron, because it is absorbed by a separate pathway as part of heme. So for a person who regularly eats red meat, the effect of additional ascorbic acid is less significant than for a vegetarian.

It is also important that vitamin C works only at the moment it is present together with iron in the intestine. It does not accumulate iron in the body and does not raise ferritin on its own. If there is little iron in the diet, there is simply nothing to improve absorption of.

How large the effect really is

Laboratory studies with single meals demonstrate a marked enhancement of iron absorption by vitamin C. However, in longer studies with a mixed diet the picture is more modest. Cook and Reddy (2001) showed that under conditions of a full, varied diet the effect of additional ascorbic acid on non-heme iron absorption was much smaller than in experiments with individual dishes.

For iron supplements the data are also ambiguous. The randomized study by Li and colleagues (2020) in JAMA Network Open, involving adults with iron-deficiency anemia, found no significant advantage of the combination of iron with vitamin C over taking iron alone in terms of the dynamics of hemoglobin and ferritin. That is, when taking therapeutic doses of iron, additional ascorbic acid is not mandatory.

So vitamin C is most useful where iron comes mainly from plant food and the diet is rich in inhibitors. In the case of taking iron supplements its role is auxiliary.

At the same time vitamin C should not be considered useless: for vegetarians and people with borderline iron stores, combining foods - for example legumes with peppers, cabbage or citrus - is a simple and safe way to slightly improve the balance.

Athletes and hepcidin

In athletes, especially runners and women, iron deficiency occurs more often than in the general population. The reasons are losses in sweat, microbleeding in the gastrointestinal tract, hemolysis under impact loads, and menstrual losses. The review by Peeling and colleagues (2008) drew attention to another factor - the inflammatory reaction after training.

Intense exercise causes the release of interleukin-6, which after a few hours raises hepcidin levels. In this "window" iron absorption is reduced regardless of how much vitamin C a person has taken. So the timing of iron intake relative to training may matter no less than the choice of supplement.

Vitamin C in this context does not cancel the action of hepcidin: the hormone blocks the exit of iron from the intestinal cells into the blood. So for an athlete with a deficiency the key questions are to confirm the deficiency with tests, to choose the form of iron and the right time of intake, while ascorbic acid remains only an auxiliary factor.

• iron deficiency is confirmed by tests: ferritin, hemoglobin, and if needed transferrin saturation;
• iron from food is better absorbed without simultaneous tea, coffee and dairy products;
• vitamin C is useful mainly for non-heme iron from plant food.

Editorial conclusions

Iron and vitamin C are not alternatives. Iron is an essential element whose deficiency can be eliminated only by the intake of iron itself. Vitamin C is a helper that reduces non-heme iron to a more easily absorbed form and partly overcomes the action of inhibitors.

The real effect of vitamin C in a full diet is moderate, and when taking iron supplements the clinical advantage of the combination is not confirmed in a modern randomized study. The greatest benefit from it is obtained by vegetarians and people with a predominantly plant-based diet.

For athletes it is more important to take hepcidin and the timing of intake into account than to look for the "ideal complex." And most importantly: iron supplements without a confirmed deficiency can be harmful.

For practical advice on choosing, read the article "Iron vs Vitamin C for absorption: what to choose and for whom." We also recommend our materials on the ferritin test for athletes and on iron in a plant-based diet.

References

1. Hallberg L, Brune M, Rossander L. The role of vitamin C in iron absorption. Int J Vitam Nutr Res Suppl. 1989;30:103–108.
2. Cook JD, Reddy MB. Effect of ascorbic acid intake on nonheme-iron absorption from a complete diet. Am J Clin Nutr. 2001;73(1):93–98.
3. Li N, Zhao G, Wu W, et al. The efficacy and safety of vitamin C for iron supplementation in adult patients with iron deficiency anemia: a randomized clinical trial. JAMA Netw Open. 2020;3(11):e2023644.
4. Hurrell R, Egli I. Iron bioavailability and dietary reference values. Am J Clin Nutr. 2010;91(5):1461S–1467S.
5. Camaschella C. Iron-deficiency anemia. N Engl J Med. 2015;372(19):1832–1843.
6. Peeling P, Dawson B, Goodman C, et al. Athletic induced iron deficiency: new insights into the role of inflammation, cytokines and hormones. Eur J Appl Physiol. 2008;103(4):381–391.
7. Institute of Medicine. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Washington (DC): National Academy Press; 2000.