Anemia happens when your blood doesn’t carry enough healthy red blood cells or hemoglobin to deliver oxygen throughout your body. It’s diagnosed when hemoglobin drops below 13.5 g/dL in men or below 12.0 g/dL in women. The causes range from something as simple as not getting enough iron in your diet to serious conditions like bone marrow failure or chronic disease.
Iron Deficiency: The Most Common Cause
Iron deficiency accounts for more cases of anemia worldwide than any other cause. Your body needs iron to build hemoglobin, the protein inside red blood cells that carries oxygen. When iron stores run low, your body can’t produce enough functional red blood cells, and oxygen delivery to your tissues drops.
Three main pathways lead to iron deficiency:
- Blood loss. Every time you lose blood, you lose iron. Heavy menstrual periods are one of the most common triggers in women of reproductive age. Slower, hidden blood loss is also a major factor: peptic ulcers, colon polyps, colorectal cancer, and hiatal hernias can all cause steady internal bleeding that drains iron reserves over months. Regular use of over-the-counter pain relievers like aspirin, ibuprofen, and naproxen can irritate the stomach lining enough to cause this kind of slow bleed. Frequent blood donation is another overlooked cause.
- Not enough iron in your diet. Vegetarians and vegans face a higher risk if they don’t consistently eat iron-rich plant foods like lentils, beans, fortified cereals, and dark leafy greens. The iron found in plant foods (called nonheme iron) is harder for the body to absorb than the iron in meat, which makes dietary planning especially important.
- Poor absorption. Even if you eat plenty of iron, your body may not absorb it properly. Celiac disease, Crohn’s disease, ulcerative colitis, and other digestive conditions can damage the lining of the intestine where iron is taken up. Gastric bypass and other stomach surgeries physically remove or bypass sections of the gut responsible for iron absorption.
Vitamin B12 and Folate Deficiency
Your body also needs vitamin B12 and folate to build red blood cells. When either vitamin is in short supply, the bone marrow produces red blood cells that are abnormally large and underdeveloped. These oversized cells don’t function properly and break down faster than normal ones, a condition called megaloblastic anemia.
B12 deficiency often stems from absorption problems rather than diet alone. A condition called pernicious anemia, in which the stomach stops producing a protein needed to absorb B12, is a classic example. Older adults, people who’ve had stomach surgery, and those with digestive disorders are at higher risk. Strict vegans can also become deficient because B12 occurs naturally only in animal products. Folate deficiency tends to be more dietary, though pregnancy dramatically increases folate needs, making supplementation critical during that time.
Chronic Diseases and Inflammation
Long-term illnesses like rheumatoid arthritis, kidney disease, cancer, and inflammatory bowel disease can cause anemia even when your iron intake is perfectly fine. The mechanism is counterintuitive: your body has plenty of iron stored away, but inflammation prevents it from reaching the places where red blood cells are made.
Here’s what happens. When your body is fighting chronic inflammation, the liver ramps up production of a hormone that controls iron flow. This hormone locks iron inside storage cells and blocks its release into the bloodstream. The bone marrow, starved of the iron it needs, can’t produce red blood cells at a normal rate. This is why treating the anemia itself with iron supplements often doesn’t work in these cases. The underlying disease has to be managed first.
Red Blood Cells That Break Down Too Fast
Hemolytic anemia occurs when red blood cells are destroyed faster than the bone marrow can replace them. Normal red blood cells live about 120 days. In hemolytic anemia, that lifespan is cut short, sometimes dramatically.
Some forms are inherited. Sickle cell anemia causes red blood cells to become rigid and crescent-shaped, making them fragile and prone to breaking apart. Thalassemia reduces the body’s ability to produce normal hemoglobin, leading to smaller, shorter-lived red blood cells. Both conditions are passed from parents to children through specific genes, and they disproportionately affect people of African, Mediterranean, Middle Eastern, and Southeast Asian descent.
Other forms are acquired later in life. Autoimmune disorders like lupus and rheumatoid arthritis can cause the immune system to mistakenly attack healthy red blood cells. Certain medications, including some antibiotics and antimalarial drugs, can trigger destruction. Blood cancers, severe infections (viral or bacterial), an overactive spleen, and even mechanical heart valves that physically damage red blood cells as they pass through can all shorten red blood cell survival.
Bone Marrow Failure
In aplastic anemia, the bone marrow itself stops working properly. Instead of producing enough red blood cells, white blood cells, and platelets, the marrow slows down or shuts down production. This is rare but serious.
Known triggers include exposure to toxic substances like benzene, pesticides, and arsenic. Radiation therapy and chemotherapy for cancer can damage the marrow directly. Certain viral infections, including hepatitis, Epstein-Barr virus, and HIV, can also trigger marrow failure. In many cases, however, the cause is never identified. The immune system appears to attack the marrow’s stem cells, but the reason it starts doing so remains unclear.
Who Is Most at Risk
Anemia is staggeringly common. Globally, about 40% of children under age 5, 37% of pregnant women, and 30% of women of reproductive age are affected. The burden falls heaviest on low-income countries, rural populations, and communities with limited access to nutrition and healthcare.
Several groups face elevated risk for specific biological reasons. Pregnant women need far more iron and folate to support a growing fetus and expanded blood volume. Menstruating women lose iron every month and may not fully replenish it between cycles. Infants and toddlers are vulnerable because rapid growth demands large amounts of iron relative to their body size. Older adults face increasing absorption problems and are more likely to have chronic diseases that interfere with red blood cell production.
How Diet Affects Iron Absorption
What you eat alongside iron-rich foods matters as much as the iron itself. Vitamin C is the strongest enhancer of plant-based iron absorption. Eating foods like orange juice, bell peppers, tomatoes, strawberries, or broccoli in the same meal as beans, lentils, or fortified grains can significantly boost how much iron your body actually takes in. Eating plant-based iron sources alongside meat also improves absorption.
On the other hand, several common foods and drinks interfere with iron uptake. Tannins in tea and coffee, phytates in whole grains and legumes, and oxalates in spinach all reduce nonheme iron absorption. Calcium is a particularly strong inhibitor, blocking the absorption of both plant-based and meat-based iron when consumed in the same meal. If you’re trying to improve your iron status, spacing your dairy intake away from your iron-rich meals is a practical step that can make a real difference.