Anemia and Colon Cancer

Low red blood cells or hemoglobin can be an early sign of a tumor in the colon

Anemia can be an early warning sign of colon cancer. It is caused when the tumor starts to bleed, reducing the levels of red blood cells (RBCs) and an iron-rich protein called hemoglobin needed to deliver oxygen and nutrients to tissues.

This can lead to symptoms such as fatigue, weakness, dizziness, and skipped or rapid heartbeats.

Doctor talking to patient in doctor's office
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This article explains what anemia is, how colon cancer causes anemia, and what signs and symptoms suggest you may have colon cancer. While not always present, especially in the beginning, recognizing any that do occur can help you catch the disease in its early stages when it is most treatable.

Symptoms and Types of Anemia

Anemia can often go unnoticed, though it may be picked up on blood tests done for routine reasons. It is only when the loss of RBCs or iron is severe enough that symptoms may develop and get your attention, most commonly:

There are different types of anemia, each of which is differentiated by the underlying cause. These can be broadly categorized as:

How Colon Cancer Causes Anemia

While anemia with colon cancer is mainly due to blood loss, there are other mechanisms that can complicate the condition.

When a malignant (cancerous) tumor develops, it releases chemicals that trigger the rapid formation of new blood vessels (known as angiogenesis). These new blood vessels provide the tumor with the nutrients it needs to grow but are also very fragile and burst quite easily as the tumor increases in size.

As the tumor gets larger, the bleeding and blood loss worsen. This not only leads to decreased RBCs but a decrease in the amount of iron in the blood, causing iron deficiency anemia.

On top of this, chronic inflammation caused by the tumor alters how the body uses iron. Rather than releasing iron into the blood to form hemoglobin, the iron is stored in body tissues, further aggravating iron-deficiency anemia.

Chemotherapy and radiation can make iron-deficiency anemia worse by suppressing bone marrow, the spongy part of bones where red blood cells are made.

Studies suggest that more than 30% of people with cancer will experience anemia at some point, with almost half developing iron-deficiency anemia. In those who get chemotherapy or radiation, around 67% can expect to have anemia.

Recognizing the Signs of Colon Cancer

On its own, iron-deficiency anemia is not something one would automatically connect with colon cancer—particularly if it occurs on its own. More likely culprits include heavy menstrual bleeding, an iron-deficient diet, or digestive disorders that impair the absorption of dietary iron (like celiac disedase).

With that said, iron-deficiency anemia can also be caused by slow, chronic internal bleeding. If no other cause can be found, healthcare providers will typically check for gastrointestinal bleeding caused by things like a peptic ulcer, hiatal hernia, or colon cancer.

With colon cancer, the healthcare provider will look for other signs of the disease, including:

  • A change in bowel habits, such as altering diarrhea and constipation
  • Narrow, pencil-like stools
  • A feeling that your bowel is not emptied even after defecating
  • Rectal bleeding with bright red blood
  • Dark, tarry stools
  • Cramping or abdominal pain
  • Fatigue and weakness
  • Unintended weight loss

Severe anemia rarely occurs on its own with colon cancer and it is more commonly seen with some or all of these accompanying symptoms.

If any of these symptoms persist for more than two weeks, call your healthcare provider and request the necessary tests to better pinpoint the cause.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Wilson MJ, Dekker JWT, Harlaar JJ, Jeekel J, Schipperus M, Zwaginga JJ. The role of preoperative iron deficiency in colorectal cancer patients: prevalence and treatment. Int J Colorectal Dis. 2017;32(11):1617-24. doi:10.1038/s41598-018-19572-y

  2. American Society of Hematology. Anemia.

  3. Väyrynen JP, Tuomisto A, Väyrynen SA, et al. Preoperative anemia in colorectal cancer: relationships with tumor characteristics, systemic inflammation, and survival. Sci Rep. 2018;8(1):1126. doi:10.1038/s41598-018-19572-y

  4. Cross AJ, Wooldrage K, Robbins EC, et al. Whole-colon investigation vs. flexible sigmoidoscopy for suspected colorectal cancer based on presenting symptoms and signs: a multicentre cohort study. Br J Cancer. 2019;120(2):154-64. doi: 10.1038/s41416-018-0335-z

Additional Reading

By Suzanne Dixon, MPH, RD
Suzanne Dixon, MPH, MS, RDN, is an award-winning registered dietitian and epidemiologist, as well as an expert in cancer prevention and management.