Iron deficiency anemia gets treated like simple math almost everywhere: not enough iron in, so take a supplement and move on. If you've swallowed iron pills for months and still feel exhausted, the math isn't the issue. For a meaningful share of people, low iron keeps returning because their gut can't absorb it well, regardless of how much they eat or take.
How your gut absorbs iron
Most of your iron gets absorbed in the duodenum, the short stretch of small intestine right after your stomach. Three things need to work together for that to happen: a healthy intestinal lining, enough stomach acid, and functioning transport proteins that carry iron across the gut wall.
Stomach acid converts iron into the form your gut can use. Long-term use of acid-reducing medications, including proton pump inhibitors, has been shown to lower iron absorption, according to the NIH Office of Dietary Supplements[1]. When any part of this chain breaks down, whether from inflammation, tissue damage, or low acid production, iron levels can drop even on an iron-rich diet.
Gut conditions that cause iron deficiency
Several digestive conditions interfere with iron absorption in different ways. Some damage the gut lining directly. Others cause blood loss, lower stomach acid, or remove the tissue where absorption happens. The table below covers four of the most common.
| Condition | How it causes iron deficiency |
|---|---|
| Celiac disease | Gluten triggers an immune attack on the small intestine lining, damaging the exact tissue that absorbs iron |
| Crohn's disease or ulcerative colitis | Chronic inflammation blocks absorption directly, and active disease often causes ongoing blood loss |
| H. pylori infection | Lowers stomach acid, competes with the body for iron, and can cause slow blood loss from gastritis or ulcers |
| Bariatric or gastric surgery | Removes or bypasses the stomach and duodenal tissue where iron absorption normally happens |
Celiac disease is one of the best documented gut causes, and anemia can show up before any digestive symptoms do, according to Mayo Clinic[2]. Atrophic gastritis, a thinning of the stomach lining that lowers acid production, and small intestinal bacterial overgrowth are two more conditions your doctor may want to rule out.
Why iron pills alone sometimes don't work
When your gut can't absorb iron well, swallowing more of it only goes so far. Some people get stomach irritation from high-dose iron pills without their blood levels moving at all. Doctors call this refractory iron deficiency anemia, and it's one of the clearest signals that something beyond diet is driving the problem. The American Society of Hematology[3] notes that when oral iron fails, doctors often turn to intravenous iron, which delivers iron straight into the bloodstream and skips gut absorption entirely.
Signs your anemia may start in your gut
Anemia paired with ongoing diarrhea, bloating, unexplained weight loss, or stomach pain deserves a closer look at your gut. A family history of celiac disease or inflammatory bowel disease raises your odds too, even without any digestive symptoms. If you've taken oral iron consistently for several months and your levels still haven't moved, bring that up with your doctor directly.
Getting the right testing
Ask your doctor for ferritin and iron saturation tests, not just a basic hemoglobin check. If a gut cause seems likely, your doctor may also order celiac antibody tests, a stool test for hidden blood, or refer you for an endoscopy or colonoscopy.
Bring these to your doctor:
- Anemia that hasn't improved after several months of consistent oral iron
- Digestive symptoms alongside low iron: diarrhea, bloating, weight loss, or stomach pain
- A family history of celiac disease or inflammatory bowel disease
- Long-term use of acid-reducing medication
- Ferritin and iron saturation results, not just a hemoglobin number
Iron deficiency and gut absorption problems can overlap with other conditions covered here. Gut health after ostomy surgery and POTS and your gut both touch on absorption and nutrient issues from a different angle, and what to bring to your first GI appointment can help you get the most out of a workup like this one.
What to do with this
Start today by writing down every digestive symptom you've had in the past month, even the ones that seem unrelated to your energy levels. Bring that list with you the next time you talk to your doctor about your iron levels.
