Iron Deficiency Anemia (IDA) occurs when the body lacks sufficient iron to produce adequate hemoglobin, the protein in red blood cells responsible for carrying oxygen. This shortage results in fewer healthy red blood cells, leading to symptoms such as fatigue and weakness. Colorectal Cancer (CRC) is a malignant growth originating in the lining of the colon or rectum. The answer to whether CRC can cause IDA is a definitive yes. Colorectal cancer is a common underlying cause of otherwise unexplained iron deficiency anemia, particularly in older adults.
The Link Between Colon Cancer and Anemia
The presence of IDA often serves as a clinical clue pointing toward an underlying gastrointestinal problem. In adults, iron loss is most frequently attributed to chronic blood loss from the digestive tract rather than inadequate dietary intake. Anemia is a common finding in patients diagnosed with colorectal cancer, with prevalence reported between 30% and 75% of cases overall. Up to 50% of patients with colon cancer may present with anemia before surgical intervention.
Because of this association, unexplained IDA is treated as a “red flag” symptom requiring immediate investigation. Clinical guidelines recognize that iron deficiency in men and post-menopausal women must be evaluated for a gastrointestinal source until proven otherwise. This is due to the high likelihood that anemia is a presenting symptom of a silent malignancy or a pre-cancerous lesion. For many patients, the gradual onset of iron-deficiency symptoms is the only signal that prompts the medical workup leading to a cancer diagnosis.
The Mechanisms of Iron Depletion
The development of IDA in CRC is driven by two distinct physiological processes that often occur simultaneously.
Chronic Occult Blood Loss
The primary mechanism is chronic occult blood loss, which is the slow, continuous seepage of blood from the tumor’s surface. As the tumor grows, its delicate tissue can become ulcerated or eroded, causing intermittent bleeding into the digestive tract. This blood loss is typically small enough that it does not cause visible blood in the stool, making it “occult” or hidden. Over time, this persistent leakage results in a net negative iron balance, as iron lost exceeds the amount absorbed from the diet. The body’s iron stores become severely depleted, leading to absolute iron deficiency due to the physical loss of iron-containing red blood cells.
Anemia of Chronic Inflammation (ACD)
The second mechanism is the body’s systemic response to the malignancy, resulting in Anemia of Chronic Inflammation (ACD), or functional iron deficiency. Cancer triggers an inflammatory state, causing the release of immune signaling proteins called cytokines. These cytokines stimulate the liver to produce high levels of hepcidin, a master regulator of iron metabolism. Hepcidin limits iron availability by trapping it within storage sites, such as macrophages and liver cells, and reducing absorption from the small intestine. Consequently, even with adequate iron stores, the iron cannot be released to the bone marrow to produce new red blood cells, making it functionally unavailable.
Anemia as an Indicator for Screening
The subtle nature of IDA means its symptoms are often dismissed as simple aging or tiredness, delaying medical investigation. Patients may experience fatigue, paleness, shortness of breath upon exertion, and dizziness due to reduced oxygen-carrying capacity. Recognizing these symptoms and linking them to potential iron deficiency is the first step toward diagnosis.
A physician confirms iron deficiency through blood tests measuring hemoglobin levels and specific iron indicators like ferritin. Low ferritin levels are a direct indicator of depleted iron stores, characteristic of absolute iron deficiency. For instance, in men with unexplained anemia, a serum ferritin level of 100 nanograms per milliliter or less is considered an indicator for prompt endoscopic evaluation.
Once IDA is confirmed and no other obvious source of blood loss is apparent, the diagnostic pathway involves a thorough search for the bleeding site, including endoscopic investigation. This typically requires a colonoscopy to examine the large intestine and often an upper endoscopy. Treating the anemia itself, often through iron supplements, is secondary to locating and addressing the underlying cause, which may be colorectal cancer. Prompt investigation is necessary because anemia may be the earliest sign of a treatable malignancy.