What Does It Mean If Your Red Blood Cells Are Low?

Low red blood cells, a condition broadly called anemia, means your blood is carrying less oxygen than your body needs. Red blood cells are produced in your bone marrow and contain hemoglobin, the protein that picks up oxygen in the lungs and delivers it throughout the body. When their numbers drop or they carry less hemoglobin than normal, virtually every organ feels the shortfall. The causes range from something as fixable as an iron-poor diet to serious conditions like bone marrow disease, and the path to treatment depends entirely on which cause is driving the problem.

How Your Body Keeps Red Blood Cell Counts Steady

Your kidneys constantly monitor the oxygen level in your blood. When oxygen dips, they ramp up production of a hormone called erythropoietin (EPO), which travels to the bone marrow and signals it to churn out more red blood cells. This oxygen-sensing feedback loop keeps red blood cell levels in a tight range and can respond quickly to stresses like blood loss or high altitude.¹ When any part of this chain breaks down, whether the kidneys fail to produce enough EPO, the bone marrow can’t respond, or red blood cells are lost or destroyed faster than they’re replaced, the count drops.

A low red blood cell count shows up on a standard blood test called a complete blood count, or CBC. The key numbers are hemoglobin concentration and hematocrit (the percentage of blood volume made up of red cells). Doctors generally diagnose anemia when hemoglobin falls below about 13 g/dL in men or 12 g/dL in non-pregnant women, though exact cutoffs vary slightly by lab and population.

What Symptoms Actually Show Up

The textbook list of anemia symptoms includes fatigue, weakness, shortness of breath, pale skin, dizziness, cold hands and feet, and a fast or irregular heartbeat. But the connection between anemia and symptoms is looser than most people expect. A large population-based study found that after accounting for other health factors, anemia was linked to a feeling of low energy but did not show a clear independent association with fatigue, poor concentration, or shortness of breath.² That doesn’t mean anemia never causes those symptoms. It means that in a general population, many people with mild anemia feel fine, and many people without anemia feel tired for other reasons.

How noticeable the symptoms are usually depends on two things: how low the count drops and how fast it gets there. If anemia develops slowly over weeks or months, your body compensates by increasing heart rate and adjusting blood flow, so you might not notice anything until levels are quite low. A sudden drop from acute bleeding, by contrast, can cause dramatic lightheadedness and rapid heart rate at levels that would barely register in chronic anemia.

Iron Deficiency and Other Nutritional Causes

Iron deficiency is by far the most common cause of anemia worldwide. Iron is an essential building block of hemoglobin. When iron stores run low, the bone marrow produces red blood cells that are smaller than normal and carry less hemoglobin, a pattern called microcytic anemia.³ Women with heavy menstrual periods, people with poor dietary iron intake, and anyone with chronic gastrointestinal bleeding (from ulcers, polyps, or inflammatory bowel disease) are particularly at risk.

Vitamin B12 and folate deficiencies cause a different pattern. These vitamins are needed for DNA synthesis during cell division, and without them, developing red blood cells in the bone marrow grow abnormally large. The result is megaloblastic anemia, characterized by oversized red cells (macrocytes) and a mismatch between the cell’s nuclear and cytoplasmic development.⁴ B12 deficiency can sneak up on people because the body stores years’ worth of the vitamin; strict vegans, people with absorption disorders, and older adults with reduced stomach acid are at highest risk. Folate deficiency tends to develop faster and is especially concerning during pregnancy, when folate needs spike.

Blood Loss You Know About and Blood Loss You Don’t

Acute blood loss from surgery, trauma, or a major bleed is usually obvious and treated immediately. Chronic blood loss is trickier. Slow, steady bleeding from the GI tract can drain iron stores over months without visible symptoms. A man or postmenopausal woman presenting with iron-deficiency anemia with no obvious cause will typically get checked for GI bleeding, because that scenario raises concern about ulcers, colon polyps, or colorectal cancer. In premenopausal women, heavy periods are the most common source, though GI causes still need to be considered if the anemia doesn’t respond to iron supplementation.

Whether the problem is reduced production or increased loss of red blood cells matters for treatment. A review in the Journal of Clinical Medicine framed the diagnostic challenge clearly: anemia stems either from decreased production of hemoglobin and red blood cells or from increased consumption and blood loss, and addressing the cause is the central goal of managing it.⁵

Chronic Diseases That Quietly Lower Your Count

Anemia of chronic disease (sometimes called anemia of inflammation) is the second most common form of anemia after iron deficiency, and it’s easily confused with it. Conditions like rheumatoid arthritis, chronic infections, cancer, and inflammatory bowel disease trigger ongoing inflammation that interferes with iron use. The body has iron in its stores, but inflammatory signals from a protein called hepcidin effectively lock iron away from the bone marrow, preventing it from being used to make hemoglobin.⁶ The anemia tends to be mild to moderate and often improves when the underlying disease is brought under control.

Kidney disease deserves special mention because it hits both sides of the production equation. Damaged kidneys produce less erythropoietin, which means the bone marrow gets a weaker signal to make red blood cells. On top of that, chronic kidney disease exacerbates iron deficiency by limiting iron availability.⁷ Anemia is so common in advanced kidney disease that it’s often one of the first problems nephrologists address.

Genetic Conditions That Affect Red Blood Cells

Some people are born with genes that produce abnormal hemoglobin or too little of it. Sickle cell disease, caused by a mutation in the gene for the beta chain of hemoglobin, leads to rigid, crescent-shaped red blood cells that break down easily and clog small blood vessels. The beta-thalassemias are another major group, characterized by reduced production of beta-globin chains. These conditions are caused by a wide variety of molecular defects, mostly involving the beta-globin gene directly, and are generally inherited in a recessive pattern, meaning you need defective copies from both parents to develop severe disease.⁸

People who carry only one copy of these mutations (carriers, or those with “trait”) usually have mild or no anemia and may never know about it unless it shows up on a routine blood test. Carrier status is important to know about for family planning, since two carriers can have a child with severe disease. Genetic hemoglobin disorders are most common in people of African, Mediterranean, Middle Eastern, and South Asian descent, populations where these traits historically offered some protection against malaria.

When the Immune System Destroys Red Blood Cells

In hemolytic anemias, red blood cells are destroyed faster than the bone marrow can replace them. This can happen for mechanical reasons (artificial heart valves can physically shear red cells), because of infections like malaria, or because the immune system mistakenly targets its own red blood cells. Autoimmune hemolytic anemia (AIHA) involves antibodies that attach to red blood cells and trigger their destruction, often with the help of the complement system, a set of immune proteins that can punch holes in cell membranes.⁹

Signs that hemolysis is happening include jaundice (yellowing of the skin and eyes) from the breakdown products of hemoglobin, dark urine, and an enlarged spleen. Lab tests can pick up the hallmarks: elevated bilirubin, low haptoglobin (a protein that mops up free hemoglobin), and a high reticulocyte count showing the marrow is working overtime to compensate.

Bone Marrow Problems and Chemotherapy

If the bone marrow itself is damaged or diseased, red blood cell production drops at the source. Aplastic anemia, myelodysplastic syndromes, leukemias, and cancers that have spread to the bone marrow can all crowd out or suppress normal blood cell production. Chemotherapy is another major cause. Chemotherapy drugs target rapidly dividing cells, and blood-forming cells in the bone marrow divide fast, making them collateral damage. The resulting drop in blood counts is called myelosuppression, and it affects red blood cells, white blood cells, and platelets simultaneously.¹⁰ This is why cancer patients undergoing chemotherapy often need transfusions or EPO-stimulating drugs during treatment.

How Doctors Figure Out the Cause

Finding out your red blood cells are low is the easy part. Finding out why takes some detective work. The initial CBC provides clues based on the size of the red blood cells. Small cells (microcytic) point toward iron deficiency or thalassemia. Normal-sized cells (normocytic) suggest chronic disease, kidney problems, or early blood loss. Large cells (macrocytic) steer the investigation toward B12 or folate deficiency, liver disease, or certain bone marrow disorders.

One of the most informative early tests is the reticulocyte count, which measures young, newly released red blood cells. A reticulocyte production index below about 2% in someone who’s anemic suggests the bone marrow isn’t keeping up, pointing toward a production problem. A count well above that threshold signals the marrow is responding appropriately, meaning red blood cells are being lost or destroyed rather than underproduced.¹¹ From there, additional blood tests like iron studies, B12 and folate levels, kidney function, inflammatory markers, and sometimes a bone marrow biopsy help narrow down the specific cause.

Medications That Can Lower Your Count

Several common drugs contribute to anemia, either by suppressing the bone marrow directly or by interfering with nutrient absorption. Nonsteroidal anti-inflammatory drugs (NSAIDs) like ibuprofen can cause chronic GI bleeding over time. Certain antibiotics and anticonvulsants can suppress blood cell production. And some widely used drug combinations create nutrient deficiencies that lead to anemia indirectly. A study of over 11,000 people with type 2 diabetes found that those taking metformin together with a proton pump inhibitor (a common acid-reducing drug) had a roughly 18% higher risk of developing B12 deficiency compared to those on metformin alone.¹² Since B12 deficiency leads to macrocytic anemia, people on long-term metformin are often advised to have their B12 levels checked periodically.

Pregnancy, Exercise, and Other Situations That Skew the Numbers

Pregnancy creates a kind of false alarm. Blood volume increases dramatically to support the growing fetus, but plasma (the liquid part of blood) expands more than the red blood cell mass does. A systematic review found that plasma volume rises by about 48% by weeks 35 to 38 of pregnancy compared to the non-pregnant state.¹³ Because the red cells are diluted in a larger pool of plasma, hemoglobin concentration drops even though the total number of red blood cells hasn’t fallen by the same amount. This dilutional anemia is considered physiologically normal, though pregnant women do also need more iron and folate, so genuine deficiency can layer on top of the dilution effect.

Endurance athletes, especially runners, sometimes develop what’s called sports anemia. Part of it is the same dilution phenomenon: regular training expands plasma volume. But runners also experience a small amount of red blood cell destruction from the repeated impact of feet hitting the ground. A study comparing running and cycling at the same intensity found that markers of hemolysis were about four times higher after running, implicating footstrike as the primary mechanical cause.¹⁴ That said, even after an ultramarathon, the actual amount of red blood cell destruction from footstrike appears to be clinically negligible. A study of runners after a 60-kilometer race found that although markers of hemolysis increased, the red blood cell count itself didn’t drop significantly.¹⁵ Iron loss through sweat, GI bleeding during intense exercise, and chronic inflammation from training likely contribute more to anemia in serious athletes than footstrike alone.

Why Anemia in Older Adults Is Often Harder to Pin Down

Anemia becomes more common with age, but the cause is frequently elusive. Nutritional deficiencies, chronic diseases, and kidney decline all become more prevalent. Yet in roughly a third of older adults with anemia, standard workups don’t identify a clear cause.¹⁶ Some of this unexplained anemia may stem from age-related changes in the bone marrow’s ability to respond to EPO or from low-level chronic inflammation associated with aging itself. A portion of these cases eventually turn out to be early myelodysplastic syndromes, a group of bone marrow disorders that are most common in people over 65 and can evolve into leukemia over time. This is one reason persistent, unexplained anemia in an older adult warrants closer follow-up rather than a shrug.

Treatment Depends Entirely on the Cause

There’s no one-size-fits-all treatment for low red blood cells. Iron-deficiency anemia is treated with iron supplementation, either oral or intravenous. Oral iron is cheap and widely available, but it doesn’t work well for everyone. People with inflammatory bowel disease, celiac disease, or autoimmune gastritis often absorb oral iron poorly, making intravenous iron the better option.¹⁷ B12 deficiency is corrected with B12 injections or high-dose oral supplements, and folate deficiency responds to oral folic acid.

For anemia caused by chronic kidney disease, synthetic erythropoietin (EPO) injections can stimulate the bone marrow when the kidneys no longer produce enough of the hormone on their own. In surgical patients, both intravenous iron and EPO-stimulating agents are used to reduce the need for blood transfusions.¹⁸

Red blood cell transfusions are reserved for severe anemia or situations where symptoms are dangerous. Clinical trials have shown that a threshold of 7 to 8 g/dL for triggering a transfusion is generally as safe as transfusing at the higher traditional threshold of 9 to 10 g/dL in most settings.¹⁹ This “restrictive” strategy reduces unnecessary transfusions, which carry their own risks including iron overload, transfusion reactions, and infection. The critical point in every case is that transfusions treat the symptom, not the disease. Identifying and correcting the root cause is what actually resolves the anemia long-term.

When Low Red Blood Cells Start Affecting the Heart

One consequence of long-standing anemia that doesn’t get enough attention is its effect on the heart. When hemoglobin is low, the heart compensates by pumping harder and faster to deliver the same amount of oxygen. Over time, this extra workload can cause the heart muscle to thicken, a condition called left ventricular hypertrophy, which can lead to heart cell death and worsen or trigger congestive heart failure.²⁰ This is especially relevant for people who already have heart disease or kidney disease, where anemia, heart failure, and kidney decline can feed each other in a worsening cycle. Treating the anemia early in this context isn’t just about relieving fatigue; it’s about protecting the heart from structural damage that may not be fully reversible.