Anaemia is a condition where your blood can’t carry enough oxygen to meet your body’s needs, either because you have too few red blood cells or because those cells contain too little hemoglobin, the protein that actually binds oxygen. The World Health Organization defines it using hemoglobin thresholds: below 130 g/L for men, below 120 g/L for non-pregnant women, and below 110 g/L during pregnancy. It affects roughly 1.92 billion people worldwide, making it one of the most common health conditions on the planet.
How Red Blood Cells Carry Oxygen
Each hemoglobin molecule contains four iron atoms, and each iron atom can grab one molecule of oxygen. That means a single hemoglobin protein carries up to four oxygen molecules from your lungs to every tissue in your body. About 98% of all oxygen in your blood travels this way; only 2% dissolves directly in plasma. So when hemoglobin drops, your oxygen delivery drops with it.
Your body constantly produces new red blood cells in the bone marrow through a process driven by a hormone called erythropoietin, which your kidneys release when they detect low oxygen levels. This feedback loop normally keeps your red blood cell count stable. Anaemia develops when something disrupts the loop: not enough raw materials to build hemoglobin, destruction of red blood cells faster than they can be replaced, or problems with the bone marrow itself.
Why It Makes You Feel Exhausted
The hallmark symptoms of anaemia are tiredness, weakness, and shortness of breath, all of which trace back to the same problem: your tissues aren’t getting the oxygen they need to produce energy. Your heart tries to compensate by pumping harder and faster, which is why a rapid or irregular heartbeat is common. Over time, this extra workload can strain the heart.
Other signs include pale skin, cold hands and feet, dizziness, and headaches. Mild anaemia often goes unnoticed because these symptoms creep in gradually. Many people assume they’re just run down or not sleeping well. The symptoms tend to become obvious only when hemoglobin drops low enough that the body’s compensatory mechanisms can’t keep up.
Iron Deficiency: The Most Common Cause
Iron deficiency accounts for the largest share of anaemia cases globally, and it doesn’t happen overnight. Your body first draws down its iron reserves, measured by a blood protein called ferritin. The WHO considers ferritin below 15 μg/L in adults to signal depleted stores, though in clinical practice, levels below 30 μg/L already indicate a real deficit. At this early stage, your hemoglobin may still look normal on a blood test. Anaemia is actually a late manifestation of iron deficiency, meaning your stores can be running on empty well before you’d be formally diagnosed.
Iron enters your body through food in two forms. Heme iron, found in meat, fish, and poultry, is absorbed at roughly 25%. Non-heme iron, found in beans, spinach, nuts, lentils, and fortified grains, is absorbed at 17% or less. Despite making up only 10% to 15% of total dietary iron in Western diets, heme iron contributes about 40% of what your body actually takes in because of its superior absorption rate.
You can improve non-heme iron absorption significantly. Eating vitamin C alongside plant-based iron sources converts the iron into a form your gut absorbs more easily. Adding meat, fish, or poultry to a meal increases non-heme iron absorption two- to threefold. On the other hand, tea, coffee, and dairy products contain compounds that block absorption, so spacing these away from iron-rich meals makes a practical difference.
Vitamin B12 and Folate Deficiency
Your bone marrow needs both vitamin B12 and folate to build DNA when producing new red blood cells. Without them, the genetic material inside developing cells can’t replicate properly, but the rest of the cell keeps maturing on schedule. This mismatch produces abnormally large, malformed red blood cells that die off prematurely. The result is called megaloblastic anaemia.
B12 and folate work together in a tightly linked chemical pathway. B12 acts as a helper molecule that recycles folate into its active form. When B12 is missing, folate gets trapped in an unusable state, so a B12 deficiency effectively creates a folate deficiency too. This is why the two conditions produce nearly identical blood abnormalities. B12 deficiency, however, also causes nerve damage, including numbness, tingling, and balance problems, which folate deficiency does not.
Anaemia From Chronic Illness
Long-term conditions like rheumatoid arthritis, kidney disease, inflammatory bowel disease, and certain infections can cause anaemia even when your diet is fine. The culprit is chronic inflammation. When your immune system stays activated for a long time, it produces signaling molecules that trigger the release of a hormone called hepcidin. High hepcidin levels block iron absorption in the gut and lock existing iron inside immune cells, preventing it from reaching the bone marrow where red blood cells are made.
At the same time, the same inflammatory signals blunt the production of erythropoietin and directly impair the bone marrow’s ability to respond. The net effect is iron that’s technically present in your body but unavailable for use. This is why standard iron supplements often don’t resolve anaemia of chronic disease. Treating the underlying condition is what allows iron metabolism to normalize.
Inherited Forms of Anaemia
Some types of anaemia are genetic. In sickle cell disease, a mutation changes the shape of hemoglobin molecules so that red blood cells deform into rigid, crescent-shaped cells. These sickled cells don’t flow smoothly through small blood vessels and break down far faster than normal cells, causing both anaemia and painful blockages.
Thalassemia involves mutations that reduce or eliminate production of one of the protein chains that make up hemoglobin. In beta-thalassemia, the body can’t produce enough of the beta globin chain, leaving an excess of alpha chains that damages developing red blood cells. The severity ranges widely. Some people carry a single gene variant and experience only mild anaemia, while others inherit mutations from both parents and need regular blood transfusions to survive.
How Anaemia Is Classified
A complete blood count is the starting point for diagnosis. Beyond hemoglobin, doctors look at the average size of your red blood cells, measured in femtoliters (fL). Normal red blood cells fall between 80 and 100 fL. This size measurement splits anaemia into three categories that help narrow down the cause:
- Microcytic (below 80 fL): Small red blood cells, most commonly caused by iron deficiency or thalassemia.
- Normocytic (80 to 100 fL): Normal-sized cells, typical of anaemia from chronic disease, sudden blood loss, or kidney disease.
- Macrocytic (above 100 fL): Oversized cells, pointing toward B12 or folate deficiency, or certain bone marrow disorders.
Additional tests like ferritin, B12 levels, and markers of inflammation help confirm the specific type. Because the treatment for each form is completely different, getting the classification right matters. Taking iron supplements for B12-deficiency anaemia, for instance, won’t help and delays the real fix.
Who Is Most at Risk
Globally, about 24.3% of people have some form of anaemia. The burden falls unevenly. Children under five and women of reproductive age are the most affected groups, driven largely by the iron demands of growth, menstruation, and pregnancy. Women have higher anaemia prevalence than men across all age groups. Geographically, sub-Saharan Africa and South Asia carry the heaviest burden, where dietary iron intake, infectious diseases like malaria, and limited healthcare access compound each other.
In higher-income countries, older adults face increasing risk due to chronic disease, medication side effects that cause subtle gastrointestinal bleeding, and reduced dietary intake. Vegetarians and vegans also need to pay closer attention, since eliminating heme iron sources means relying entirely on the less efficiently absorbed non-heme form.