Jejunal Absorption – Nutrient Uptake in the Small Intestine

What is Jejunal Absorption?

Jejunal absorption is the process by which nutrients are taken up in the jejunum, the middle portion of the small intestine. Located between the duodenum and the ileum, the jejunum is the primary site for the absorption of macronutrients and many essential micronutrients. It plays a central role in converting digested food into bioavailable energy and building blocks for the body.

Anatomy and Structure of the Jejunum

The jejunum is approximately 1.5 to 2.5 metres in length and is characterized by a highly folded mucosal lining. These circular folds, known as plicae circulares, are covered with intestinal villi – finger-like projections that further increase the absorptive surface area. Each villus is in turn covered with microvilli, forming the brush border. Together, these structures expand the effective surface area of the jejunum to approximately 200 square metres, enabling efficient nutrient uptake.

Which Nutrients Are Absorbed in the Jejunum?

Carbohydrates

Dietary carbohydrates are broken down by amylase and brush border enzymes (such as maltase, sucrase, and lactase) into simple sugars (glucose, fructose, and galactose). Glucose and galactose are actively transported into enterocytes via the SGLT1 transporter, while fructose enters via GLUT5. All monosaccharides then pass into the portal bloodstream via GLUT2.

Proteins

Proteins are digested in the stomach and small intestine by enzymes such as pepsin, trypsin, and chymotrypsin, producing amino acids and small peptides. These are absorbed through specific transporters on the brush border and enter the portal circulation for distribution throughout the body.

Fats

Dietary fats are emulsified by bile salts and broken down by pancreatic lipase into fatty acids and monoglycerides. These diffuse into enterocytes, where they are reassembled and packaged into chylomicrons. Chylomicrons are secreted into the lymphatic system and eventually enter the bloodstream via the thoracic duct.

Vitamins and Minerals

The jejunum is also responsible for absorbing most fat-soluble vitamins (A, D, E, and K) and several water-soluble vitamins (including vitamin C, B1, B2, B6, and folate). Essential minerals such as calcium, magnesium, iron, and zinc are primarily absorbed in the jejunum through active and facilitated transport mechanisms.

Mechanisms of Jejunal Absorption

Nutrient uptake in the jejunum occurs via several distinct transport mechanisms:

• Active transport: Energy-dependent uptake against a concentration gradient, e.g. glucose and amino acids.
• Passive diffusion: Movement along a concentration gradient without energy expenditure, e.g. short-chain fatty acids and water.
• Facilitated diffusion: Carrier-mediated transport without direct energy use, e.g. fructose via GLUT5.
• Transcytosis: Vesicle-mediated uptake of larger molecules, relevant for certain proteins and immunoglobulins.

Clinical Significance of Jejunal Absorption

Impaired jejunal absorption can lead to malabsorption syndromes, in which the body fails to absorb sufficient nutrients. Key conditions affecting jejunal absorption include:

• Coeliac disease: An autoimmune disorder in which gluten damages the intestinal villi, drastically reducing the absorptive surface.
• Crohn's disease: A chronic inflammatory bowel disease that can affect any part of the gastrointestinal tract, including the jejunum.
• Short bowel syndrome: A condition following surgical removal of large portions of the intestine, resulting in a permanently reduced absorptive capacity.
• Lactase deficiency: The absence or insufficiency of lactase leads to incomplete breakdown of lactose and associated digestive symptoms.

Diagnosis of Impaired Jejunal Absorption

Several diagnostic tools are used to evaluate the absorptive function of the jejunum:

• D-xylose test: Assesses small intestinal absorption capacity by measuring urinary or blood levels of D-xylose after oral ingestion.
• Breath tests: Including the hydrogen breath test and lactose breath test, used to identify enzyme deficiencies and bacterial overgrowth.
• Small intestinal biopsy: Tissue samples are taken for histological examination of mucosal architecture, particularly in suspected coeliac disease.
• Blood tests: Measurement of nutrient levels (e.g. iron, folate, vitamin B12) as indirect indicators of malabsorption.

References

1. Guyton A.C., Hall J.E. - Textbook of Medical Physiology, 13th Edition, Elsevier (2016).
2. Sleisenger M.H., Fordtran J.S. - Gastrointestinal and Liver Disease, 11th Edition, Elsevier (2020).
3. World Gastroenterology Organisation (WGO) - Global Guidelines: Celiac Disease (2016). Available at: https://www.worldgastroenterology.org