Does oral iron cause constipation by promoting growth of intestinal bacteria?

Why do oral iron supplements cause constipation? The present theory is somewhat convoluted and involves oral iron ions causing oversecretion of fluids into the upper GI tract at the expense of the lower GI tract. The decreased fluid in the lower GI tract (large intestine, colon) creates constipation. Could there be another explanation?

Let’s consider the following observations.

1. We all normally have bacteria in the colon. These bacteria are beneficial to health and can break down toxins, suppress the growth of harmful bacteria, and synthesize vitamins and neurotransmitters. Some types of bacteria in the colon will proliferate when taking oral iron supplements as iron is a rate-limiting cofactor in their division and replication.(1)

2. The overgrowth of certain bacteria in the colon can induce constipation.(2,3)

A prime candidate for a bacteria that normally resides in the colon and both requires iron for its growth and induces constipation when it proliferates is a bacterial species known as Bacteroides.(1) The proliferation of Bacteroides is fueled by iron, and medications that bind iron can inhibit the growth of Bacteroides.(4) Patients with constipation have higher levels of Bacteroides in the colon.(2,3)

Is this the real reason oral iron causes constipation?  Sounds plausible?  If ultimately proven correct, you read about it here first.

References

1. Dostal, Alexandra, et al. Iron supplementation promotes gut microbiota metabolic activity but not colitis markers in human gut microbiota-associated rats. Br J Nutr. 2014 Jun 28;111(12):2135-45. 

2. Ohkusa, Toshifumi, et al. Gut Microbiota and Chronic Constipation: A Review and Update. Front Med (Lausanne). 2019; 6: 19. 

3. Parthasarathy, Gopoanandan, et al. Relationship Between Microbiota of the Colonic Mucosa vs Feces and Symptoms, Colonic Transit, and Methane Production in Female Patients With Chronic Constipation. Gastroenterology. 2016 Feb;150(2):367-79.e1 

4. Rocha E R, et al. Effect of ferric and ferrous iron chelators on the growth of Bacteroides fragilis under anaerobic conditionsFEMS Microbiol Lett. 1991 Nov 1;68(1):45-50.