Where are fat-soluble vitamins (FSVs) absorbed in the body?

Fat-Soluble Vitamin Absorption Location

Fat-soluble vitamins (A, D, E, and K) are absorbed primarily in the upper half of the small intestine, specifically in the duodenum and jejunum. 1

Anatomical Site of Absorption

• The small intestine, particularly the proximal portions (duodenum and upper jejunum), serves as the primary absorption site for fat-soluble vitamins. 1
• Fat-soluble vitamins follow the same absorption pathway as dietary lipids throughout the gastrointestinal tract. 1
• The jejunum and ileum are specifically noted as the main absorption sites for vitamins E and K. 1

Mechanism of Absorption

• Fat-soluble vitamins require bile acids and normal fat digestion for adequate absorption, as they are incorporated into mixed micelles formed during lipid digestion. 1, 2
• At physiological doses, vitamin A (retinol) uptake occurs via a saturable carrier-mediated process at the intestinal mucosal level, while passive diffusion predominates at pharmacological doses. 1
• Multiple specific transport proteins are involved in the absorption process at the intestinal mucosal level, contrary to earlier beliefs that absorption occurred solely via passive diffusion. 1
• Bile salts are essential for the digestion and absorption of fats and fat-soluble vitamins in the small intestine. 2

Clinical Implications

• Patients with malabsorptive procedures affecting the proximal small intestine (such as bariatric surgery with duodenal bypass, short bowel syndrome, or biliopancreatic diversion) are at particularly high risk for fat-soluble vitamin deficiencies. 1
• Conditions causing fat malabsorption (pancreatic insufficiency, bile acid deficiency, bacterial overgrowth) significantly impair fat-soluble vitamin absorption. 1
• Patients on bile acid sequestrants require monitoring for fat-soluble vitamin deficiencies, as these medications can interfere with normal fat absorption and reduce absorption of vitamins A, D, E, and K. 1, 3

Absorption Efficiency

• Retinoids (preformed vitamin A) demonstrate excellent absorption rates of 75-100%, whereas carotenoid absorption varies greatly depending on the food matrix and carotenoid type. 1
• The presence of adequate bile flow and pancreatic enzymes is essential for optimal absorption of all fat-soluble vitamins. 1, 4