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So why does my iron supplements give me constipation?

Women's Health | August 25, 2016 | Author: Naturopath

women's health, minerals

So why does my iron supplements give me constipation?

Iron deficiency is the most common nutrient insufficiency worldwide. Unfortunately, treating with iron supplementation often comes with an uncomfortable side effect: constipation. If you've ever stopped taking iron because of this adverse reaction, you're not alone. It is the most commonly reported reason that people give up taking iron.

Gut Bugs and Iron

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.

The colon is full of bacteria that regulate the gut environment, support bowel motility and stool quality, and prevent constipation. A healthy gut biome, with more “good bacteria” than pathogenic bacteria, means healthy bowel motions.

Constipation suggests that the “bad guys” in the gut have outnumbered the “good guys”.

Here's the connection to iron supplementation

You may already know not to take an iron supplement when you have an acute infection like a cold, because iron feeds bacteria and can make the infection worse. Do you see where this is going?

Pathogenic bacteria thrive on iron

If the iron from your supplement isn't absorbed in the small intestine, it travels to the large intestine and feeds the “bad bacteria”.

This condition of imbalanced bacteria is called dysbiosis.

With dysbiosis, we suffer from symptoms of:

  • gas
  • bloating
  • constipation.

An additional theory is that free iron causes oxidative damage to the wall of the colon, contributing to intestinal hyperpermeability or “leaky gut”.

While the theory is sound – iron does cause oxidative damage, and oxidative damage does compromise the integrity of the intestinal barrier – there are no studies to date proving the two are directly linked.

Can you boost your iron levels and have a healthy gut?

Yes! It starts with selecting an iron supplement that will be quickly absorbed in the small intestine, so there is little left to feed the harmful bacteria in the colon.

There are factors to look for in a supplement to maximise its absorption:

1. Ionic Charge 

Ferric Iron versus Ferrous Iron. 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.

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 co factors; 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 contribute to dysbiosis and constipation. 

2. Form

Supplemental iron comes in many forms. Getting the most absorbable form is the key to preventing dysbiosis and constipation. 

Most Common and Most Constipating

Ferrous Sulphate & 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 down the bowels.

We know for sure that supplementation with ferrous sulphate will more often than not cause gastrointestinal symptoms including constipation.

Other symptoms include: 

  • black stools
  • gas
  • bloating 
  • heartburn

A small study demonstrated that ferrous sulphate supplementation caused significantly more gastrointestinal side effects than a placebo.

Another showed that the higher the dose, the more severe the gastrointestinal complaints.

In a 2007 study, most participants were unaware that there are different forms of iron available in supplements, and relied on their physicians to make appropriate recommendations. Despite the well-documented association between constipation and these formulations, most physicians recommended preparations containing ferrous fumarate and ferrous sulphate. These preparations are often the cheapest and most readily available in pharmacies, and are so frequently prescribed because they often have the biggest dose of iron per tablet.

The good news is that there are alternatives

Likely Most Absorbable:

  • Amino Acid Chelate
  • Ferrous Diglycinate
  • Ferrous Bisglycinate

‚Äč Iron diglycinate is an iron atom bound to two (di or bis) glycine molecules (glycinate). Iron chelated amino acids are readily absorbed by the body – about 400% more readily absorbed than iron sulphate

With increased absorption, there will be less iron travelling to the colon, so we can assume there will be fewer bowel-related side effects. The literature backs this up – a double-blind, randomised trial in 2013 showed that iron amino acid chelates gave far fewer adverse side effects in perimenopausal women, as compared to ferrous sulphate supplementation. And to top it off, amino acid chelates may have a longer lasting effect on iron levels. A study on school-age children found that the glycine-bound iron was associated with higher storage levels of iron, when measured 6 month after supplementation was stopped.

3. Dosage 

The body will only absorb so much of any mineral at a time. Even with highly absorbable forms of iron such as ferrous diglycinate, a large dose will cause some iron to remain in the gut and contribute to constipation. 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 nutritionist or naturopath about the best way to split your dose across the day to maximise absorption and reduce side effects.

Support Your Body

  • Drink plenty of water and eat fibre-rich foodsvitamin c
  • Eat vitamin C rich foods or take a vitamin C supplement alongside with your iron to increase its absorption.
  • Take iron away from food and other supplements to prevent others nutrients inhibiting its absorption.
  • Take a probiotic to boost beneficial bacteria in the colon and prevent dysbiosis.
  • Consider these habits to improve iron absorption and avoid the common mistakes that lead to iron deficiency.

By taking a high quality, bioavailable form of iron in split doses, you can reduce the risk dysbiosis, leaky gut, and constipation. Ferrous diglycinate appears to be a highly absorbable form of iron supplementation with the fewest gastrointestinal side effects. Speak to a nutritionist or naturopath if you continue to experience constipation while taking an iron supplement.  Australia’s best online discount chemist


Pereira, D. I., Couto Irving, S. S., Lomer, M. C. & Powell, J. J. (2014) A rapid, simple questionnaire to assess gastrointestinal symptoms after oral ferrous sulphate supplementation. BMC Gastroenterology, 4, 103.

Souza, A. et al. (2009) Adherence and side effects of three ferrous sulfate treatment regimens on anemic pregnant women in clinical trials. Cad. Saúde Pública, 25:6, 1225 – 1233.

Melamed, N., Ben-Haroush, A., Kaplan, B. & Yogev, Y. (2007) Iron supplementation in pregnancy – does the preparation matter? Arch Gynecol Obstet., 276:6, 601 – 604.

Abdel-Lah, M., Rasheed, S. l., Hassan, I. I. & El-sayed, A. (2006) Iron chelated amino acid therapy versus oral iron therapy for the treatment of iron deficiency anemia with pregnancy. The Journal of the Egyptian Society of Obstetrics and Gynecology, 32:4, 419-428.

Fouad, G. T., Evans, M., Sharma, P., et al., (2013) A randomized, double-blind clinical study on the safety and tolerability of an iron multi-amino acid chelate preparation in premenopausal women. J Diet Suppl., 10:1, 17 – 28.

Duque, X., Martinez, H., Vilchis-Gil, J., Mendoza, E., et al. (2014) Effect of supplementation with ferrous sulfate or iron bis-glycinate chelate on ferritin concentration in Mexican schoolchildren: a randomized controlled trial. Nutrition Journal, 13, 71.

Nappi, C., et al. (2009) Efficacy and tolerability of oral bovine lactoferrin compared to ferrous sulfate in pregnant women with iron deficiency anemia: A prospective controlled randomized study. Acta Obstetricia et Gynecologica, 88, 1301 – 1035.

 Santiago, P. (2012) Ferrous versus Ferric Oral Iron Formulations for the Treatment of Iron Deficiency: A Clinical Overview. ScentificWorldJournal, 2012.   

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