Where are vitamin B12, folate (vitamin B9), and vitamin D absorbed in the gastrointestinal tract?

Sites of Absorption for Vitamin B12, Folate, and Vitamin D

Vitamin B12 Absorption

Vitamin B12 is absorbed exclusively in the terminal ileum through a highly specific receptor-mediated process. 1

The absorption mechanism involves several sequential steps:

• Gastric phase: Vitamin B12 is released from dietary proteins by gastric acid and pepsin in the stomach, then binds to intrinsic factor (a glycoprotein secreted by gastric parietal cells) 1, 2
• Duodenal transfer: In the duodenum, pancreatic proteases degrade haptocorrin (R-protein), and pH changes trigger the transfer of B12 to intrinsic factor 3
• Ileal uptake: The intrinsic factor-B12 complex is absorbed through receptor-mediated endocytosis in the terminal ileum via cubilin and amnionless receptors 3
• Distribution of receptors: Intrinsic factor-B12 receptors are present in significant amounts throughout the entire distal three-fifths of the small intestine, though the terminal ileum remains the primary absorption site 4

Clinical Implications of Ileal Disease

• Resection of more than 20 cm of distal ileum mandates prophylactic vitamin B12 supplementation (1000 mcg intramuscularly monthly for life) 1
• Ileal Crohn's disease involving more than 30-60 cm of ileum puts patients at risk for B12 deficiency even without resection 1
• Vitamin B12 absorption is adversely affected by sleeve gastrectomy, Roux-en-Y gastric bypass, and biliopancreatic diversion/duodenal switch because these procedures require an acidic environment and presence of intrinsic factor produced by gastric parietal cells 5

Folate Absorption

Folate (folic acid) is absorbed primarily in the small bowel, particularly the proximal jejunum. 5

• Absorption may be affected by Roux-en-Y gastric bypass or biliopancreatic diversion/duodenal switch procedures 5
• Deficiency following sleeve gastrectomy is more likely due to low dietary intake or non-adherence with vitamin and mineral supplementation rather than malabsorption 5

Important Safety Consideration

• Never administer folic acid before correcting vitamin B12 deficiency, as folic acid can mask megaloblastic anemia while allowing irreversible neurological damage (subacute combined degeneration of the spinal cord) to progress 5, 6

Vitamin D Absorption

Vitamin D and other fat-soluble vitamins (A, E, K) are absorbed in the upper half of the small intestine, following lipids through the gastrointestinal tract. 5

• Fat-soluble micronutrients including vitamin D are assumed to follow lipids in the gastrointestinal tract, with absorption presumably occurring in the upper small intestine 5
• Absorption varies depending on the food matrix and requires adequate fat digestion 5

Monitoring Recommendations

• Serum 25-hydroxyvitamin D levels of 75 nmol/L or greater are considered sufficient 5
• Check serum 25-hydroxyvitamin D levels at regular intervals post-bariatric surgery: at 3,6, and 12 months in the first year, then at least annually thereafter 5