🟡 Preliminary Evidence
Red blood cells depend on three essential nutrients to form and function: iron, folate, and vitamin B12. According to Thieme’s Color Atlas of Pathophysiology, deficiency in any one of these micronutrients disrupts oxygen transport and triggers a cascade of clinical symptoms including fatigue, pale skin, brain fog, and numbness. Understanding how these three work together offers clinicians and patients a framework for diagnosing and preventing anaemia.
Key takeaways
- Iron absorption ranges from 3–15% of dietary intake, with heme iron from meat absorbed more efficiently than non-heme iron from plants
- Folate and B12 drive DNA synthesis in bone marrow; deficiency causes red blood cells to enlarge but fail to divide (megaloblastic anaemia)
- The body recycles approximately 25 mg of iron daily from old red cells, reducing dietary dependence
- Inflammation impairs iron absorption by raising hepcidin, a hormone that traps iron in storage cells
Iron absorption and recycling: the body’s efficient system
Daily iron intake, absorption rates, and recycling in healthy adults
Source: Color Atlas of Pathophysiology, Thieme | Georgian Medical Journal News
Iron: oxygen delivery and the absorption paradox
Iron is the core of haemoglobin, the protein that binds and transports oxygen throughout the body. According to Thieme’s Color Atlas of Pathophysiology, dietary iron intake typically ranges from 10–20 mg per day, yet the body absorbs only 3–15% of what is consumed. This absorption efficiency depends critically on iron source: heme iron from animal products (meat, poultry, fish) crosses the intestinal barrier readily, whilst non-heme iron from plant sources encounters significant barriers.
Vitamin C enhances non-heme iron absorption by reducing iron to a more readily absorbable form, a mechanism documented in nutritional biochemistry. Conversely, inflammation impairs absorption through a different mechanism—inflammatory signals trigger hepcidin, a hormone that sequesters iron inside storage cells (enterocytes and macrophages), rendering it unavailable for haemoglobin synthesis. This hepcidin-mediated trapping explains why patients with chronic infection or inflammatory disease often develop anaemia despite adequate dietary iron.
Fortunately, the body recycles iron efficiently. According to Thieme’s Color Atlas of Pathophysiology, approximately 25 mg of iron is recovered daily from senescent red blood cells, reducing absolute dietary dependence. In the absence of bleeding or haemolysis, this recycling loop can sustain haemoglobin production for months even on a marginal diet—until storage reserves deplete.
Folate and B12: the DNA synthesis gatekeepers
Red blood cell production in bone marrow is fundamentally a process of DNA synthesis: precursor cells (erythroblasts) must replicate their DNA before division to generate mature red cells. According to Thieme’s Color Atlas of Pathophysiology, folate and vitamin B12 are non-interchangeable cofactors in this process. Folate (also called vitamin B9) donates one-carbon units required for nucleotide synthesis; B12 (cobalamin) reactivates folate after it has released its carbon unit.
Folate stores in the liver deplete rapidly—within weeks to months on a deficient diet. B12, by contrast, is stored in larger quantities and can sustain function for 2–3 years after dietary intake ceases. However, B12 absorption is uniquely dependent on intrinsic factor, a glycoprotein secreted by gastric parietal cells. Without intrinsic factor (as occurs in pernicious anaemia, gastrectomy, or autoimmune gastritis), even adequate dietary B12 cannot be absorbed.
When either folate or B12 is deficient, DNA synthesis stalls. Erythroblasts enlarge but cannot divide, producing fewer but oversized red cells—a condition called megaloblastic anaemia. Clinically, patients experience fatigue, shortness of breath, and if B12 is deficient, neurological symptoms including paraesthesia, ataxia, and cognitive impairment from demyelination of spinal cord and peripheral nerves.
The integrated triad: how one deficiency cascades
Iron, folate, and B12 are not isolated nutritional factors but nodes in a single physiological network. Iron transports oxygen; folate and B12 enable the cell division required to produce red cells. Break any one link and the entire system falters. According to Thieme’s Color Atlas of Pathophysiology, this explains why anaemic patients often present with overlapping symptoms: fatigue reflects both reduced oxygen carrying capacity and impaired energy metabolism in tissues; pale skin signals low haemoglobin; brain fog and numbness can arise from either oxygen deprivation or B12-induced neurodegeneration.
Diagnosis therefore requires measurement of all three: serum iron, ferritin, and transferrin saturation; serum folate and red cell folate; and serum cobalamin with methylmalonic acid or homocysteine (more sensitive B12 markers). In clinical practice, a single micronutrient panel risks missing the true cause of anaemia, particularly in older adults, vegans, or patients with malabsorption.
The body recycles approximately 25 mg of iron daily from old red blood cells, a volume that exceeds typical dietary absorption by a factor of five—making efficient recycling more critical than dietary intake for sustaining haemoglobin production.
— Color Atlas of Pathophysiology, Thieme
What this means
Frequently asked questions
Why is iron from meat absorbed better than iron from plants?
Heme iron in animal products exists in a form that bypasses many absorption barriers, entering enterocytes through a dedicated transporter. Non-heme iron from plants must be reduced to the ferrous form (iron 2+) by stomach acid and vitamin C, then compete with calcium, phytates, and polyphenols for absorption. This is why a spinach salad (despite high non-heme iron) delivers less absorbable iron than a small steak.
Can inflammation cause anaemia even if iron intake is adequate?
Yes. Chronic inflammation (infection, autoimmune disease, cancer) triggers hepcidin, which sequesters iron in macrophages and enterocytes, preventing absorption and recycling. This anaemia of inflammation or anaemia of chronic disease is one of the most common causes of anaemia worldwide, occurring even in iron-replete individuals. Treatment requires addressing the underlying inflammation, not iron supplementation alone.
How long can the body survive on stored B12 without dietary intake?
The liver stores 2,000–5,000 mcg of B12, and the body uses only 1–2 mcg daily. On average, stored B12 lasts 2–3 years after dietary intake ceases. However, those with pernicious anaemia or malabsorption cannot access stored B12 efficiently and become deficient within months if not supplemented. This is why vegans and older adults (who have impaired B12 absorption) require regular supplementation or monitoring.
As our understanding of micronutrient interdependency deepens, anaemia prevention increasingly centres on integrated screening and supplementation rather than single-nutrient approaches. Global health initiatives must recognise that iron, folate, and B12 deficiencies often coexist in low-income populations, and addressing all three simultaneously is more cost-effective than sequential treatment. Clinicians who understand this triad can diagnose anaemia more precisely and guide patients toward sustainable dietary and supplemental strategies.
Source: Color Atlas of Pathophysiology, Thieme
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Medically reviewed by Prof. Giorgi Pkhakadze, MD, MPH, PhD. Spotted an error? Contact the editorial team.
