Macrocytic anemia is a type of anemia that causes unusually large red blood cells. Like other types of anemia, macrocytic anemia means that the red blood cells also have low hemoglobin.
Hemoglobin is an iron-containing protein that transports oxygen around the body. Deficiencies in vitamin B-12 or folate often cause macrocytic anemia, so it is sometimes called vitamin deficiency anemia.
In this article, we look at the causes and symptoms of macrocytic anemia, the treatment options, and how to prevent complications.
Macrocytic anemia occurs if the red blood cells are unusually large. A unit called femtoliters (fL) is used to measure the size of blood cells. Usually, red blood cells are between 80–100 fL.
Red blood cells larger than 100 fL are considered macrocytic. When the cells grow too large, there are fewer of them than there needs to be and they carry less hemoglobin. This means the blood is not as oxygen-rich as it should be. Low blood oxygen can cause a range of symptoms and health problems.
Macrocytic anemia is not a single disease, but a symptom of several medical conditions and nutritional problems.
One of the most common types of macrocytic anemia is megaloblastic macrocytic anemia. This happens when red blood cells produce DNA too slowly to divide.
Anemia develops gradually, so a person may not notice any symptoms until it is severe. When symptoms appear, they may include:
- redness or swelling of the tongue (glossitis)
- low appetite
Macrocytic anemia is just one type of anemia. The symptoms of anemia are similar across all types, so it is important to use blood tests to diagnose the specific anemia a person has.
When people develop macrocytic anemia due to B-12 deficiency, they may have other symptoms. These include:
- weakness, pain, numbness, or tingling in the hands or feet
- instability when walking
- memory loss
These symptoms may resolve after treatment, but they can become permanent if left untreated.
Macrocytic anemia is almost always due to a deficiency of folate or vitamin B-12. A person may have a deficiency of one of these if their body cannot absorb vitamins due to an underlying disease, or because they do not eat enough foods with these vitamins.
B-12 is abundant in animal products, so vegans and vegetarians are more at risk of a B-12 deficiency. In some cases, people may eat enough foods with B-12 but are unable to absorb the vitamin due to autoimmune disorders, cancer, alcohol addiction, or inflammatory bowel disease.
A folate deficiency, sometimes known as vitamin B-9 deficiency, can also cause macrocytic anemia. Pregnant and breast-feeding women use more folate and have a higher risk of becoming deficient.
Other causes of macrocytic anemia may include:
- medications, including HIV drugs, cancer drugs, and others that suppress the immune system
- liver disease
- rare metabolic disorders
Each of these factors can make it more difficult for the body to absorb and metabolize essential nutrients.
In very rare cases, macrocytic anemia can be caused by a bone marrow disorder that prevents the body from producing enough healthy blood cells.
Doctors classify macrocytosis into two broad categories:
- Megaloblastic macrocytosis is the most common form. It occurs when DNA cannot be produced because of a vitamin deficiency.
- Nonmegaloblastic macrocytosis, which can happen when there is a problem with the liver, spleen, or bone marrow.
When a person shows signs of macrocytic anemia, a doctor will take several blood tests to find the underlying cause. They may also ask questions about a person’s diet, lifestyle, and other symptoms.
In most cases, vitamin injections are the first line of treatment. Injecting vitamins ensures the body can absorb these even if an underlying condition, such as celiac disease, prevents vitamin absorption.
Eating more foods containing vitamin B-12 may improve symptoms if a person is deficient in this vitamin because of their diet.
Other treatment options include:
- changing medications when a drug interferes with vitamin absorption
- taking medication for certain autoimmune or liver diseases
- taking medication for thyroid disorders
- making lifestyle changes, such as avoiding alcohol
- having blood transfusions or bone marrow transplants for bone marrow disorders
When the blood does not have enough hemoglobin, it will not have enough oxygen. The body may try to fix this by increasing the heart rate or blood pressure. Left untreated, anemia can cause heart failure, an enlarged heart, and circulatory problems.
When macrocytic anemia is due to a problem with the bone marrow or an organ, this underlying cause can trigger further complications. For example, people with untreated bone marrow disorders may develop leukemia.
Most cases of macrocytic anemia are due to vitamin deficiencies, which can cause different complications depending on the vitamin:
Vitamin B-12 deficiencies
A vitamin B-12 deficiency can cause:
- neural tube defects in babies born to women with a B-12 deficiency
- stomach cancer
- mobility problems and tingling in the hands and feet
- memory loss
A folate deficiency can cause:
- neural tube defects in babies born to women with a folate deficiency
- an increased risk of some cancers, including colon cancer
- cardiovascular health problems
- an increased risk of premature labor in pregnant women
- an increased risk of a placenta rupture before or during childbirth
Most cases of macrocytic anemia can be managed with vitamin injections. A doctor will continue to monitor a person’s B-12 or folate levels and use blood tests to determine whether red blood cells have returned to their normal size.
While most people recover with prompt treatment, they may need periodic checks for anemia.
When macrocytic anemia is due to an underlying disease, the outlook depends on what disease is causing the deficiency, and how it is treated.
Macrocytic anemia often goes undiagnosed until it becomes severe. People who have symptoms of anemia, a family history of anemia, or who have or are at risk of a condition linked to macrocytic anemia should see a doctor for a blood test.
Regular blood tests can help detect all forms of anemia before they undermine a person’s health. Most cases of anemia can be easily diagnosed and treated.