Anemia happens when your body doesn’t have enough healthy red blood cells to carry oxygen to your tissues. The most common cause is iron deficiency, but anemia can also result from vitamin shortages, chronic diseases, blood loss, genetic conditions, or bone marrow problems. Understanding which type you’re dealing with matters because each has different causes and different solutions.
How Anemia Develops in Your Body
Your bone marrow constantly produces new red blood cells, and each one needs hemoglobin, an iron-rich protein that grabs oxygen in your lungs and delivers it throughout your body. Anything that disrupts this process can make you anemic: too little iron, not enough of the right vitamins, damage to the bone marrow itself, or red blood cells being destroyed or lost faster than your body can replace them.
The process often unfolds in stages. With iron deficiency, for example, your body first burns through its stored iron (mostly kept in the liver). Then circulating iron drops. Eventually, your bone marrow can’t build enough hemoglobin, so it produces red blood cells that are smaller and paler than normal. By this point, you’re feeling the effects.
Iron Deficiency: The Most Common Cause
Iron deficiency accounts for more cases of anemia worldwide than any other single cause. Your body needs iron to make hemoglobin, and when intake is too low, absorption is impaired, or losses are too high, your stores run dry. Women between 19 and 50 need about 18 mg of iron daily, while men the same age need only 8 mg. During pregnancy, the requirement jumps to 27 mg per day.
Three main pathways lead to iron deficiency. The first is simply not getting enough iron from food, which is common among vegetarians and vegans since plant-based iron is harder for the body to absorb. The second is impaired absorption: conditions like celiac disease damage the gut lining, and weight-loss surgery bypasses the part of the intestine where iron is best absorbed. The third, and often most serious, is blood loss. Every milliliter of blood contains iron, so heavy menstrual periods, stomach ulcers, colon polyps, or regular use of anti-inflammatory painkillers that irritate the stomach lining can all drain iron stores over time.
This is why doctors investigating unexplained iron deficiency in men or postmenopausal women will often look for hidden bleeding in the digestive tract. The anemia itself may be the first clue to something else going on.
Vitamin B12 and Folate Deficiency
Iron isn’t the only nutrient your bone marrow needs. Developing red blood cells rely on vitamin B12 and folate to copy their DNA and divide properly. Without these vitamins, the cells can’t multiply fast enough. Many of the immature cells die before they ever leave the bone marrow, a process called ineffective red blood cell production.
The red blood cells that do survive tend to be abnormally large. On a standard blood test, their average size (measured as mean corpuscular volume, or MCV) rises above the normal range of 80 to 100 femtoliters. This is the opposite of iron deficiency, where cells come out too small.
B12 deficiency often stems from absorption problems rather than diet. Your stomach produces a protein needed to absorb B12 in the small intestine, and some people stop making it, a condition called pernicious anemia. Older adults, people who’ve had stomach surgery, and those with certain autoimmune conditions are at higher risk. Folate deficiency, on the other hand, is more often dietary, though alcohol use and certain medications can deplete folate levels too. B12 deficiency can also cause neurological symptoms like numbness, tingling, and difficulty with balance, which folate deficiency does not.
Chronic Diseases and Inflammation
If you have a long-term inflammatory condition like rheumatoid arthritis, inflammatory bowel disease, chronic kidney disease, or certain infections, your body may effectively lock iron away where it can’t be used. This is sometimes called anemia of chronic disease, and it works through a surprisingly specific mechanism.
When your immune system is activated, inflammatory signals trigger your liver to produce more of a hormone called hepcidin. Hepcidin blocks the main doorway that iron uses to enter your bloodstream, both from food in your intestine and from storage cells that recycle iron from old red blood cells. The result: iron gets trapped, even though your total iron stores may be normal or even high. Your bone marrow is essentially starved of iron in the middle of plenty.
What makes this tricky is that standard iron supplements often don’t help much, because the same hepcidin blockade prevents supplemental iron from being absorbed. Treating the underlying inflammation is usually the more effective path.
Blood Loss: Sudden and Gradual
Losing blood means losing red blood cells and the iron they contain. The experience differs dramatically depending on whether the loss is sudden or slow.
Acute blood loss from trauma, surgery, or a ruptured blood vessel can cause an emergency. When a large volume of blood is lost quickly, the body can’t compensate fast enough. Heart rate climbs, blood pressure drops, skin turns pale, and in severe cases, confusion or loss of consciousness follows. This is hemorrhagic shock, and it requires immediate medical treatment.
Chronic blood loss is more insidious. A slowly bleeding stomach ulcer, heavy periods month after month, or a small polyp in the colon can drain iron stores so gradually that your body partially adapts. You might not notice symptoms until the anemia is fairly advanced. Many people with chronic blood loss first show up at a doctor’s office complaining of tiredness or breathlessness during exercise, only to discover their hemoglobin has been dropping for months.
Genetic and Inherited Causes
Some forms of anemia are written into your DNA. The two most well-known are sickle cell disease and thalassemia, both of which affect hemoglobin itself.
Sickle cell disease results from a single mutation in the gene for hemoglobin’s beta chain, where one amino acid (glutamic acid) is swapped for another (valine). This tiny change causes hemoglobin molecules to clump together under low-oxygen conditions, warping red blood cells into a rigid crescent or “sickle” shape. These misshapen cells get stuck in small blood vessels, causing pain crises, and they break apart much faster than normal cells, leading to chronic anemia.
Thalassemia involves reduced production of one of hemoglobin’s protein chains. Depending on which chain is affected and how many gene copies are involved, thalassemia can range from a barely noticeable trait to a severe condition requiring regular blood transfusions. Both conditions are inherited, meaning you receive the relevant gene mutations from one or both parents.
Bone Marrow Problems
Since all blood cells are made in the bone marrow, anything that damages or crowds out healthy marrow tissue can cause anemia. In aplastic anemia, the marrow essentially shuts down and stops producing enough blood cells. This can be triggered by autoimmune disorders, viral infections like hepatitis, long-term exposure to chemicals like benzene, certain medications (including some antiseizure drugs), or inherited conditions like Fanconi anemia. In many cases, no clear trigger is found.
Cancers that originate in or spread to the bone marrow, including leukemia and lymphoma, can also suppress red blood cell production. Myelodysplastic syndromes, a group of conditions where the marrow produces defective blood cells, are another cause. Risk factors for these syndromes include prior chemotherapy or radiation treatment and long-term exposure to pesticides or tobacco.
How Anemia Feels
Mild anemia often causes no obvious symptoms. As it progresses, the most common complaints are fatigue, weakness, and a general sense of low energy that rest doesn’t fix. You might notice you get winded doing things that used to be easy, like climbing stairs or walking uphill.
More specific signs depend on the type and severity. Pale skin, especially noticeable inside the lower eyelids or in the nail beds, suggests hemoglobin has dropped below about 9 g/dL. Spoon-shaped nails (where the nail curves upward at the edges) and cracked, sore corners of the mouth point toward iron deficiency specifically. Some people with iron deficiency develop pica, an unusual craving for non-food items like ice, dirt, or starch.
Restless legs, dizziness when standing up, a rapid heartbeat, and cold hands and feet are all common. Severe anemia can cause chest pain, fainting, and shortness of breath even at rest. B12 deficiency adds its own neurological layer: numbness or tingling in the hands and feet, difficulty sensing where your limbs are in space, and trouble with balance.
How Doctors Figure Out the Cause
A complete blood count (CBC) is the starting point. This single blood draw tells your doctor your hemoglobin level, how many red blood cells you have, and their average size. That size measurement, the MCV, immediately narrows the field. Small red blood cells (MCV below 80) point toward iron deficiency or thalassemia. Large red blood cells (MCV above 100) suggest B12 or folate deficiency, liver disease, or bone marrow disorders. Normal-sized cells with low hemoglobin often indicate chronic disease, acute blood loss, or kidney-related anemia.
From there, additional tests refine the picture. Ferritin (a measure of stored iron) is the most sensitive marker for iron deficiency. B12 and folate levels can be measured directly. A reticulocyte count shows how actively your bone marrow is trying to produce new red blood cells, which helps distinguish between production problems and destruction or loss. In some cases, a bone marrow biopsy is needed to look for cancer, aplastic anemia, or myelodysplastic syndromes.
Who Is Most at Risk
Certain groups face a higher baseline risk. Menstruating women lose iron every month and need more than twice the daily iron intake of men the same age. Pregnant women need even more, since blood volume expands significantly and the developing fetus draws on maternal iron stores. Children under five, particularly infants and toddlers, are vulnerable because of rapid growth combined with limited dietary variety.
People with chronic digestive conditions, those who’ve had gastrointestinal surgery, heavy drinkers, older adults with poor nutrition, and anyone with a family history of inherited blood disorders all carry elevated risk. Vegetarians and vegans aren’t guaranteed to become anemic, but they do need to be more intentional about iron and B12 intake, since the most readily absorbed forms of both nutrients come from animal sources.