Jejunal Absorption – Nutrient Uptake in the Small Intestine

What Is Jejunal Absorption?

Jejunal absorption describes the process by which nutrients are taken up in the jejunum, the middle and longest segment of the small intestine. The jejunum follows the duodenum and precedes the ileum. It is the primary site of nutrient absorption in the human body and plays a central role in nutrition and metabolism.

Anatomy and Structure of the Jejunum

The jejunum is approximately 1.5 to 2.5 meters long and features a greatly enlarged mucosal surface designed to maximize nutrient uptake. This surface expansion is achieved through three anatomical structures:

• Plicae circulares (Kerckring folds): Circular mucosal folds that enlarge the intestinal lumen.
• Villi: Finger-like projections of the mucosa that further multiply the absorptive surface area.
• Microvilli (brush border): Tiny projections on the enterocytes (intestinal epithelial cells) that dramatically increase the surface area.

Together, these structures give the jejunum a total surface area of up to 200 square meters.

Which Nutrients Are Absorbed in the Jejunum?

Carbohydrates

Carbohydrates are first broken down by enzymes such as amylase and disaccharidases into simple sugars including glucose, fructose, and galactose. These are then transported -- actively or passively -- through the enterocytes into the blood capillaries and carried via the portal vein to the liver.

Proteins and Amino Acids

Proteins are degraded by proteases (e.g., trypsin, chymotrypsin) into amino acids and small peptides. These are absorbed through specific transporters in the intestinal epithelial cells and also enter the circulation via the portal vein.

Fats and Fat-Soluble Vitamins

Fats are emulsified by bile acids and cleaved by pancreatic lipase into fatty acids and monoglycerides. Together with bile salts, these form micelles that are taken up through the cell membranes of enterocytes. Inside the cells, triglycerides are reassembled, packaged into chylomicrons, and released into the lymphatic system (lacteals) before entering the bloodstream. Fat-soluble vitamins (A, D, E, K) are absorbed via the same pathway.

Water-Soluble Vitamins and Minerals

Water-soluble vitamins such as vitamin C and B vitamins (except vitamin B12, which is absorbed in the ileum), as well as minerals including calcium, iron, magnesium, and zinc, are also absorbed in the jejunum via specific transporters or passive diffusion.

Water and Electrolytes

Several liters of water are absorbed in the jejunum each day along with electrolytes such as sodium, potassium, and chloride. This is essential for maintaining the body's fluid balance.

Mechanisms of Absorption

Nutrient uptake in the jejunum occurs via several transport mechanisms:

• Active transport: Nutrients are moved against a concentration gradient using energy (ATP), e.g., glucose and amino acids.
• Passive diffusion: Substances move along their concentration gradient without energy expenditure.
• Facilitated diffusion: Transport through specific channel proteins without energy input.
• Endocytosis: Uptake of larger molecules or particles by enclosing them in vesicles.

Clinical Relevance of Jejunal Absorption

Disruptions in jejunal absorption can lead to malabsorption syndromes, in which nutrients are not adequately taken up. Common conditions that impair jejunal absorption include:

• Celiac disease: An autoimmune reaction to gluten that damages the intestinal villi.
• Crohn's disease: A chronic inflammatory bowel disease that can affect all segments of the intestine.
• Short bowel syndrome: Reduced absorptive surface following surgical removal of segments of the small intestine.
• Tropical sprue: Infection-related malabsorption occurring in tropical regions.

Symptoms of impaired jejunal absorption include diarrhea, weight loss, nutrient deficiencies, and fatigue.

Diagnosis of Absorption Disorders

Several diagnostic methods are available to evaluate absorption disorders:

• Blood tests to measure nutrient levels (e.g., iron, vitamins, albumin)
• Stool analysis for fat content (steatorrhea)
• Endoscopy with biopsy of the small intestinal mucosa
• Imaging studies such as MRI enterography or capsule endoscopy
• Breath tests (e.g., hydrogen breath test for lactose intolerance)

References

1. Kasper, D. L. et al. - Harrison's Principles of Internal Medicine, 21st Edition, McGraw-Hill Education, 2022.
2. Guyton, A. C. and Hall, J. E. - Textbook of Medical Physiology, 14th Edition, Elsevier, 2020.
3. World Gastroenterology Organisation (WGO) - Global Guidelines: Celiac Disease (2016). Available at: https://www.worldgastroenterology.org