Anemia develops when your body either can’t produce enough healthy red blood cells or loses them faster than it can replace them. The result is the same regardless of the cause: too little hemoglobin circulating in your blood to deliver adequate oxygen to your tissues. In adult men, anemia is diagnosed when hemoglobin drops below 13.6 g/dL; in adult women, the threshold is below 12 g/dL. Globally, about 31% of women of reproductive age and nearly 40% of young children are affected.
Iron Deficiency: The Most Common Path
Your body needs iron to build hemoglobin, the protein inside red blood cells that carries oxygen. When iron stores run low, your bone marrow simply can’t manufacture enough functional red blood cells. This is the single most frequent cause of anemia worldwide, and it happens through two basic routes: not absorbing enough iron, or losing too much of it through bleeding.
Blood loss is the leading driver, and it doesn’t have to be dramatic. Heavy menstrual periods are one of the most common causes in premenopausal women. Slow, invisible bleeding inside the digestive tract is another major source. In one study of patients with persistent iron deficiency who had no obvious gut symptoms, researchers found bleeding-related conditions in 37% of them, including colon cancer, stomach cancer, peptic ulcers, and colon polyps. This kind of gradual blood loss can drain iron stores for months or years before anemia shows up on a blood test.
On the intake side, your diet matters. Data from the UK Biobank found that vegetarians and vegans generally had lower hemoglobin levels than regular meat eaters. Among white premenopausal women, 12.8% of vegetarians were anemic compared to 8.7% of regular meat eaters. Plant-based diets can provide iron, but the form found in vegetables and grains is harder for the body to absorb than the iron in meat. Without deliberate planning, deficiency is more likely.
Pregnancy and Increased Demand
Pregnancy creates an enormous spike in iron demand. Each pregnancy requires roughly 1,000 milligrams of total iron to support the growth of the placenta and fetus, the expansion of the mother’s red blood cell supply, and the blood loss that comes with delivery. At the same time, blood plasma volume increases by 40% to 50%, while actual red blood cell production only rises by 15% to 25%. This mismatch creates a dilutional effect that lowers hemoglobin concentration even in women who started pregnancy with adequate iron stores. About 35.5% of pregnant women worldwide are anemic.
Vitamin B12 and Folate Deficiency
Iron isn’t the only nutrient your body needs to make red blood cells. Vitamin B12 and folate are both essential for red blood cells to mature properly. When either is missing, the bone marrow produces oversized, underdeveloped red blood cells that don’t function well.
B12 deficiency has several possible origins. The most well-known is pernicious anemia, an autoimmune condition where the immune system destroys cells in the stomach lining that produce a protein needed to absorb B12. Stomach surgeries, inflammatory bowel conditions like Crohn’s disease, and certain medications (including common acid reflux drugs and the diabetes drug metformin) can also block B12 absorption. Because B12 is found almost exclusively in animal products, people on strict vegan diets who don’t supplement are at particular risk.
Folate deficiency follows a similar pattern. Celiac disease and other conditions affecting the gut lining can impair folate absorption. Some medications used for epilepsy, rheumatoid arthritis, and inflammatory bowel disease also interfere with folate levels.
Chronic Inflammation and Disease
Anemia doesn’t always come from a nutritional gap. Chronic infections, autoimmune diseases, cancer, and kidney disease can all trigger a form of anemia driven by inflammation rather than deficiency. The mechanism is indirect but powerful: when the immune system stays activated for a long time, it releases signaling molecules that raise levels of a liver protein called hepcidin. Hepcidin acts like a gatekeeper for iron. It blocks iron from entering the bloodstream, both from food in the gut and from the body’s own storage sites in immune cells and the liver.
This creates an unusual situation where the body may have adequate iron stores but can’t access them. It’s essentially a lockdown. Evolutionarily, this response may have developed as a defense against infection, since bacteria and other pathogens also need iron to grow. But in people with chronic inflammatory conditions, the lockdown persists and starves the bone marrow of the iron it needs. At the same time, inflammation suppresses the hormone that signals the kidneys to stimulate red blood cell production, and it shortens the lifespan of existing red blood cells.
Red Blood Cells Destroyed Too Fast
Normally, a red blood cell lives about 120 days before being recycled. In hemolytic anemia, red blood cells are destroyed well before that, and the bone marrow can’t keep up with replacements. The causes fall into two broad categories.
Inherited conditions involve defects built into the red blood cell itself. Sickle cell disease warps cells into rigid, crescent shapes that get trapped and destroyed in small blood vessels. Hereditary spherocytosis produces fragile, sphere-shaped cells. Thalassemia disrupts hemoglobin production at the genetic level.
Acquired causes come from outside the red blood cell. The immune system can mistakenly target its own red blood cells for destruction, a condition called autoimmune hemolytic anemia. Certain infections directly invade and rupture red blood cells (malaria is the classic example). Mechanical heart valves can physically shear cells apart. Toxins from lead exposure, copper poisoning, or venomous snake bites can damage cell membranes. Even some common medications, including certain antibiotics, can trigger immune-mediated destruction.
Bone Marrow Failure
In aplastic anemia, the problem sits at the source. The stem cells in your bone marrow that are supposed to develop into blood cells become damaged or depleted, so the marrow produces fewer red blood cells, white blood cells, and platelets all at once. The most common cause is autoimmune: the body’s own immune system attacks the stem cells.
Other triggers include radiation and chemotherapy (which can damage stem cells as a side effect of targeting cancer), exposure to toxic chemicals like benzene (found in gasoline) and certain pesticides, viral infections, and some medications used to treat rheumatoid arthritis and bacterial infections. In rare cases, pregnancy itself can provoke an immune response against bone marrow stem cells. Aplastic anemia is far less common than iron deficiency, but it tends to be more serious because it affects the production of all blood cell types, not just red blood cells.
Who Faces the Highest Risk
Certain groups are more vulnerable to developing anemia simply because of their biology or circumstances. Premenopausal women lose iron with every menstrual cycle, and those with heavy periods are at especially high risk. Pregnant women face a near-doubling of iron requirements. Young children between 6 months and 5 years are vulnerable because of rapid growth combined with diets that may not meet their iron needs.
People with chronic conditions like inflammatory bowel disease, celiac disease, kidney disease, or rheumatoid arthritis face ongoing risk through both inflammation and impaired nutrient absorption. Frequent blood donors can gradually deplete iron stores. And anyone who has had stomach or intestinal surgery may permanently lose some capacity to absorb iron, B12, or both.
The takeaway is that anemia isn’t one condition with one cause. It’s a common endpoint reached through dozens of different pathways, from something as simple as a diet low in iron to something as complex as an autoimmune attack on bone marrow. Understanding which pathway applies to you is what determines the right response.