Comorbidities
If you have Crohn's disease and feel exhausted no matter how much you sleep, you are not alone, and you are not imagining it. Fatigue is one of the most common complaints among Crohn's patients, and in many cases, anemia is a direct and treatable cause. Studies show that roughly one in three people with Crohn's disease has anemia, with some research finding that 42% develop it within a year of diagnosis [1]. Yet anemia often goes underdiagnosed because its symptoms, such as fatigue, brain fog, and shortness of breath, overlap so heavily with the general experience of living with inflammatory bowel disease. Understanding why Crohn's causes anemia is the first step toward getting the right bloodwork and the right treatment.
How Crohn's Disease Causes Iron Deficiency Anemia
Iron deficiency is the single most common cause of anemia in Crohn's disease, and it develops through several overlapping pathways [2]. The most straightforward is chronic blood loss. When Crohn's-related ulcers and fissures penetrate beneath the mucosal lining of the intestine, they can rupture small blood vessels, leading to ongoing bleeding that may or may not be visible in your stool [3]. Over weeks and months, even small amounts of daily blood loss deplete your iron stores.
Malabsorption is the second major driver. Crohn's frequently affects the duodenum and proximal small intestine, which is exactly where dietary iron is absorbed. Chronic inflammation in these areas damages the intestinal lining and reduces its ability to take up iron from food. Research has also identified a genetic component: a loss-of-function mutation in the gene PTPN2, found in 19-20% of the IBD population compared to 14-16% of the general population, disrupts blood proteins that regulate iron levels [4].
The third mechanism is inflammation itself. Pro-inflammatory cytokines like interleukin-6, which are elevated during active Crohn's, trigger the liver to produce a hormone called hepcidin. Hepcidin blocks iron absorption from the gut and locks existing iron inside cells, making it unavailable for red blood cell production [2]. This means that even if you are eating iron-rich foods, your body may be actively preventing itself from using that iron.
B12 Deficiency: The Other Anemia in Crohn's
Iron deficiency gets the most attention, but vitamin B12 deficiency is another significant cause of anemia in Crohn's disease, particularly for patients with ileal involvement or a history of ileal resection. B12 is absorbed almost exclusively in the terminal ileum, so when that section of the bowel is inflamed or has been surgically removed, absorption drops significantly [5].
The numbers are striking. Approximately 33-36% of Crohn's patients have B12 deficiency, compared to 16% of those with ulcerative colitis [5][6]. Ileal inflammation alone carries nearly a four-fold increase in risk, and the odds climb further after surgery. Patients who have had more than 20 cm of terminal ileum resected are roughly seven times more likely to be B12 deficient than those without resection [5]. Unlike iron deficiency anemia, which produces smaller-than-normal red blood cells, B12 deficiency causes megaloblastic anemia, where red blood cells are abnormally large and cannot function properly. It can also cause neurological symptoms, including numbness, tingling, and difficulty with memory and concentration.
If you have ileal Crohn's or have had ileal surgery, ask your gastroenterologist about regular B12 monitoring. Treatment is straightforward: B12 injections bypass the damaged gut entirely and restore levels reliably.
IV Iron vs. Oral Iron: Why the Route Matters
Once iron deficiency anemia is confirmed, the question becomes how to replace iron. For many Crohn's patients, oral iron supplements are a poor first choice. Oral iron is notorious for causing nausea, constipation, and abdominal pain, and in one study, 38% of IBD patients on oral iron experienced gastrointestinal side effects, compared to just 6% reporting mild reactions with IV iron [7]. Beyond tolerability, oral iron is simply less effective when the gut is inflamed. If your intestine cannot absorb iron from food, it will struggle to absorb it from a supplement.
Intravenous iron bypasses the gut entirely. In clinical trials, IV iron increased hemoglobin by 2.6 g/dL at eight weeks, compared to 1.3 g/dL with oral iron. By week 12, 82% of IV iron patients had normalized their hemoglobin versus 54% in the oral group [7]. European Crohn's and Colitis Organisation (ECCO) guidelines now recommend IV iron as the first-line treatment for patients with active IBD, hemoglobin below 10 g/dL, or prior intolerance to oral iron [2]. Oral iron remains reasonable for patients with mild anemia whose disease is in remission and who tolerate it well.
The practical takeaway: if you have been prescribed oral iron and it is making your GI symptoms worse, or if your levels are not improving after several weeks, talk to your doctor about switching to IV iron. It is not a last resort. For many Crohn's patients, it is the more appropriate starting point.
What You Can Do Now
Fatigue in Crohn's disease deserves investigation, not dismissal. If you have not had your iron panel (ferritin, transferrin saturation, serum iron) and B12 levels checked recently, request them at your next appointment. These are simple blood tests that can reveal a highly treatable problem.
In the meantime, track your energy levels and fatigue alongside your other symptoms in Aidy. Showing your GI a fatigue trend over weeks is more actionable than saying "I'm always tired." A clear pattern on a chart can be the difference between a vague conversation and a concrete plan, whether that means bloodwork, an iron infusion, or a B12 injection.
References:
Abomhya, A. et al. "Iron Deficiency Anemia: An Overlooked Complication of Crohn's Disease." Journal of Hematology, 2022. PMC9076139
Gasche, C. et al. "Iron Deficiency Anemia in Inflammatory Bowel Diseases—A Narrative Review." Nutrients, 2021. PMC8624004
Mount Sinai. "Crohn's And Iron Deficiency Anemia: What's The Link?" 2018. mountsinai.org
UC Riverside. "Genetic link found between iron deficiency & Crohn's disease." 2025. news.ucr.edu
Battat, R. et al. "Prevalence and Risk Factors for Functional Vitamin B12 Deficiency in Patients with Crohn's Disease." Inflammatory Bowel Diseases, 2015. PubMed 26296064
Li, T. et al. "An assessment of serum vitamin B12 and folate in patients with Crohn's disease." Medicine, 2022. PMC9771213
Aksan, A. et al. "Iron Therapy in Inflammatory Bowel Disease." Nutrients, 2020. PMC7697745