Vitamin and Mineral Absorption

Vitamin B12 needs four handoffs

B12 reaches blood only after coordinated handoffs in stomach, duodenum, and terminal ileum.

Gastric acid and pepsin free dietary B12 from animal protein; salivary and gastric haptocorrin, or R protein, then binds and protects it in acid.

In the duodenum, pancreatic proteases digest R protein and release B12 to bind intrinsic factor from gastric parietal cells.

The intrinsic-factor-B12 complex reaches terminal ileum, binds cubilin-dependent receptors, enters enterocytes, and circulates mainly on transcobalamin II.

Absorption shifts from proximal to distal

Nutrient uptake moves from proximal minerals and folate toward distal bile salts and B12.

Iron and much folate are absorbed proximally, whereas active uptake of bile salts and intrinsic-factor-B12 occurs at the far distal small bowel.

Many nutrients have reserve or secondary uptake, but B12 has no equivalent active absorption site beyond terminal ileum.

Iron begins proximally; active B12 uptake waits at the terminal ileum.

The absorption map

A compact bowel map turns a deficiency pattern into an anatomic localization.

Iron is absorbed mainly in duodenum and proximal jejunum; nonheme iron is reduced, transported in ferrous form, and exported to plasma transferrin.

Folate uptake centers in proximal small intestine, especially proximal jejunum, while vitamin D enhances calcium absorption in duodenum and jejunum.

Terminal ileum absorbs vitamin B12 and bile salts; fat-soluble-vitamin uptake also requires adequate bile salts, pancreatic digestion, and functioning small-bowel mucosa.

Iron and folate lead proximally; B12 and bile salts finish in terminal ileum.

Peyer patches sample the ileal lumen

Distal ileum samples luminal antigen while neighboring enterocytes handle nutrient absorption.

Peyer patches are unencapsulated lymphoid aggregates in lamina propria and submucosa, concentrated on the antimesenteric side of distal ileum.

Specialized microfold, or M, cells overlying the follicles transport luminal antigen to immune cells instead of acting as ordinary absorptive enterocytes.

Activated B cells become IgA-producing plasma cells in lamina propria; dimeric IgA crosses epithelium with a secretory component that protects it in the lumen.

Deficiency patterns localize the lesion

Anemia morphology starts the localization, and neurologic findings narrow the failed pathway.

After gastric bypass or proximal-small-bowel disease, iron deficiency reflects reduced acid exposure and loss of access to duodenum and proximal jejunum.

Proximal mucosal disease, poor intake, increased demand, or drug interference can cause folate deficiency and megaloblastic anemia without the classic B12 neurologic syndrome.

Parietal-cell loss, pancreatic failure, bacterial competition, or terminal-ileal disease or resection can cause B12 deficiency with both megaloblastic anemia and neurologic injury.

Neurologic megaloblastic anemia points to B12, then sends the search through stomach, pancreas, and ileum.

Resection and surgery create predictable deficits

Resection length sets the burden, but the missing segment predicts the signature deficiencies.

Loss of absorptive surface produces short bowel syndrome with diarrhea, dehydration, electrolyte loss, weight loss, and multiple micronutrient deficiencies.

Terminal-ileal loss impairs B12 and bile-salt absorption; unabsorbed bile acids stimulate colonic secretion, while later depletion of the bile-salt pool worsens fat and fat-soluble-vitamin absorption.

Bariatric procedures may reduce acid, intrinsic-factor exposure, pancreatic mixing, or access to duodenum and jejunum, so iron, B12, folate, calcium, vitamin D, thiamine, and other nutrients require structured monitoring.