How are iron and vitamin B12 processed in the adult human body?

Iron and Vitamin B12 Processing in the Adult Human Body

Iron Metabolism

Iron Absorption

Iron absorption occurs primarily in the duodenum and proximal jejunum, with bioavailability heavily dependent on the form of iron consumed. 1

• Heme iron (from meat, poultry, and fish) has superior bioavailability compared to non-heme iron from plant sources 1
• Vitamin C enhances iron absorption, while polyphenols (certain vegetables), tannins (tea), phytates (bran), and calcium (dairy products) inhibit absorption 1
• In adults, approximately only 5% of iron required for red blood cell production comes from dietary sources, with 95% recycled from breakdown of senescent red blood cells 1
• Systemic inflammation significantly inhibits iron absorption, making oral iron supplementation ineffective in active inflammatory disease 1

Iron Transport and Storage

• Absorbed iron binds to transferrin in the bloodstream for transport to tissues 2
• Transferrin saturation (calculated as serum iron/total iron binding capacity × 100) reflects the amount of iron available for erythropoiesis 2
• Iron is stored primarily as ferritin (70% in men, 80% in women) and hemosiderin in the liver, bone marrow, spleen, and skeletal muscles 1
• Men store approximately 1.0-1.4 g of body iron, while women store only 0.2-0.4 g 1

Iron Loss

• Adults lose approximately 1 mg of iron daily through feces and desquamated mucosal and skin cells 1
• Women of childbearing age require an additional 0.3-0.5 mg daily to compensate for menstrual blood loss 1
• Pregnancy demands an average of 3 mg daily over 280 days' gestation for tissue growth and blood loss at delivery 1

Iron Deficiency Spectrum

Iron deficiency progresses through three stages: iron depletion (reduced stores), iron-deficient erythropoiesis (reduced transport iron), and iron-deficiency anemia (reduced functional iron). 1

• Absolute iron deficiency is defined as transferrin saturation <20% and ferritin <30 ng/mL 1
• Functional iron deficiency occurs when transferrin saturation is 20-50% or ferritin is 30-800 ng/mL, indicating inadequate iron mobilization despite adequate stores 1
• Ferritin <15 μg/L is highly specific for absent iron stores in the absence of inflammation 2

Vitamin B12 Metabolism

Vitamin B12 Absorption

Vitamin B12 absorption is a complex, multi-step process requiring intrinsic factor produced by gastric parietal cells. 1

• Meat and animal-derived products are the only natural dietary sources of vitamin B12 3
• Vitamin B12 binds to intrinsic factor in the stomach, and this complex is absorbed in the terminal ileum 1
• Malabsorption can occur with gastric pathology (loss of intrinsic factor), terminal ileal disease or resection, or bacterial overgrowth 1

Vitamin B12 Storage and Turnover

• The body stores vitamin B12 primarily in the liver, with reserves typically lasting several years before deficiency develops after absorption ceases 1
• Unlike iron, vitamin B12 has minimal daily losses, making deficiency primarily an absorption problem rather than an intake or loss issue 1

Clinical Implications of Combined Deficiencies

Iron deficiency and vitamin B12 deficiency often coexist, particularly in elderly patients, where more than 50% of anemia cases involve iron deficiency, sometimes with concurrent B12 and/or folate deficiency. 1

• Both deficiencies should be screened for and corrected before initiating erythropoiesis-stimulating agents, as many insurers require normal studies for reimbursement 1
• Vitamin B12 levels may appear falsely low in iron deficiency and can increase during iron replacement therapy, even without B12 supplementation 4
• In inflammatory bowel disease, both iron and B12 deficiency are common and should be assessed together, with ferritin levels up to 100 μg/L potentially reflecting iron deficiency despite inflammation 1

Key Diagnostic Pitfall

Ferritin is an acute phase reactant and can be falsely elevated in inflammatory conditions, masking true iron deficiency. 1 In the presence of inflammation, transferrin saturation and other iron indices should be measured alongside ferritin for accurate assessment 1, 2.