Iron supplements and constipation: what's the link?

Iron deficiency is the most common nutrient insufficiency worldwide. Unfortunately, treating with iron supplementation may come with a side effect: constipation. 

Let’s take a look at the link between iron supplements and constipation. 

Iron isn't easy to absorb, even in supplement form. It needs to have a particular charge, in particular amounts, to be absorbed by the cells of the small intestines, and then taken through a series of reactions to make it available for use in the body. Anything unabsorbed moves to the large intestine.

Iron absorption and gut health

The colon is full of bacteria that regulate the gut environment, support bowel motility, stool quality, and prevent constipation. A healthy gut microbiome, with more “good bacteria” than pathogenic bacteria, means healthy bowel motions. Constipation suggests that the biome is not in balance. 

Selecting an iron supplement

When selecting an iron supplement for adequate absorption there are a few things to consider:

• ionic charge
• form of iron
• dosage

Ionic Charge 

The ingredients label on iron supplements will list what type of iron is used in the formulation. Part of that name will either be “ferrous” or “ferric”. This refers to what kind of ionic charge the iron carries: ferrous iron is Fe2+ and ferric iron is Fe3+. Ferric iron is more likely to cause constipation for some individuals.

Here's why: 

Iron will only be absorbed in the small intestine if the iron is in its ferrous state. Ferric iron can be converted to ferrous iron, but only through a chemical reaction that requires vitamin C and other cofactors; it appears unlikely that all of the ferric iron in a supplement could be converted before it reaches the small intestines. 

Without conversion to its ferrous charge, ferric iron cannot be absorbed, so it will travel to the colon and may contribute to dysbiosis and constipation. 

Form of iron

Supplemental iron comes in many forms. Getting the most absorbable form is the key to preventing dysbiosis and constipation. Common forms are ferrous sulphate and ferrous fumarate.

“Ferrous” refers to iron (latin: ferric), and sulphate is exactly what it says: sulphate. This is a sulphur atom surrounded by four oxygen atoms, attached to a charged iron atom. 

Ferrous fumarate is a type of acid (fumaric acid) attached to an iron atom. These forms of iron are found in the most commonly prescribed iron supplements, and are readily available in most pharmacies. They're also the least absorbable forms, and they are why iron supplementation has a reputation for slowing the bowels.

Supplementation with ferrous sulphate will more often than not cause gastrointestinal symptoms including constipation, but other symptoms may include:

• black stools
• gas
• bloating 
• heartburn

The good news is that there are alternatives!

Dosage

The body can only absorb so much of any mineral at a given time. Even with highly absorbable forms of iron such as ferrous diglycinate, a large dose may cause some iron to remain in the gut and contribute to constipation.

 It’s recommended to start with a small dose, and gradually increase it to let your body adjust. If you are on a moderate to high dose of iron, speak to your healthcare professional about the best way to split your dose across the day to maximise absorption and reduce side effects.

Supporting your body when taking an iron supplement

• Taking a vitamin C supplement alongside your iron supplement may help to increase its absorption.
•  Taking iron away from food and other supplements may prevent other nutrients inhibiting its absorption. 
• Taking a probiotic may be beneficial for promoting good bacteria in the colon and preventing dysbiosis.

By taking a high quality, bioavailable form of iron in split doses, you may reduce the risk of dysbiosis and constipation. 

Speak to a healthcare professional if you continue to experience constipation while taking an iron supplement.

Related Reads:

• Why do I always feel so tired and what do I do about it?
• The best sources of iron for vegetarians
• What are probiotics and how do they work?

References

1. Lal, A. (2019). Iron in Health and Disease: An Update. The Indian Journal of Pediatrics, 87(1), 58–65. https://pubmed.ncbi.nlm.nih.gov/31520313/ 
2. Gagliardi, A., Totino, V., Cacciotti, F., Iebba, V., Neroni, B., Bonfiglio, G., Trancassini, M., Passariello, C., Pantanella, F., & Schippa, S. (2018). Rebuilding the Gut Microbiota Ecosystem. International Journal of Environmental Research and Public Health, 15(8), 1679.  https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6121872/ 
3. Rusu, I. G., Suharoschi, R., Vodnar, D. C., Pop, C. R., Socaci, S. A., Vulturar, R., Istrati, M., Moroșan, I., Fărcaș, A. C., Kerezsi, A. D., Mureșan, C. I., & Pop, O. L. (2020). Iron Supplementation Influence on the Gut Microbiota and Probiotic Intake Effect in Iron Deficiency—A Literature-Based Review. Nutrients, 12(7), 1993. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7400826/ 
4. Stoffel, N. U., Zeder, C., Brittenham, G. M., Moretti, D., & Zimmermann, M. B. (2019). Iron absorption from supplements is greater with alternate day than with consecutive day dosing in iron-deficient anemic women. Haematologica, 105(5), 1232–1239.
5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7193469/#:~:text=Oral%20iron%20supplementation%20with%20ferrous,iron%20deficiency%20anemia%20(IDA).&text=Because%20iron%20absorption%20from%20oral,2%20or%203%20daily%20doses