Small Intestine

Small intestine anatomy and function

The small intestine is the primary organ of nutrient absorption and a critical component of immune defense and endocrine function. Measuring approximately 20-25 feet (6-7.5 meters) in length and divided into three segments (duodenum, jejunum, ileum), the small intestine presents an enormous surface area for absorption through specialized structures called villi and microvilli ^[1]. These microscopic projections increase the absorptive surface area approximately 30-fold compared to a smooth tube, enabling efficient extraction of amino acids, fatty acids, vitamins, minerals, and glucose from digested food. Research shows that the small intestine absorbs 90% of ingested nutrients within the first 100-150 cm of the intestinal tract ^[2].

Beyond absorption, the small intestine synthesizes hormones including GLP-1 and GIP that regulate blood glucose, appetite, and motility. It houses approximately 70% of the body's immune tissue in the form of gut-associated lymphoid tissue (GALT), making it central to both nutritional status and immune function ^[3]. The small intestine also maintains a controlled barrier that prevents pathogenic bacterial translocation while permitting nutrient passage.

Intestinal barrier function and permeability

The intestinal barrier—formed by a single layer of epithelial cells connected by tight junction proteins—selectively permits nutrient absorption while preventing harmful bacteria and large molecules from entering the bloodstream. This selective permeability is maintained through tight junction proteins including claudins, occludin, and zonula occludens proteins. Studies demonstrate that barrier integrity depends on continuous epithelial cell renewal, with the entire epithelial lining replaced every 3-5 days ^[4].

Increased intestinal permeability (often termed "leaky gut") can result from prolonged inflammation, dysbiosis (microbial imbalance), infection, or irritating substances such as alcohol or NSAIDs. When barrier function is compromised, bacterial lipopolysaccharides and food antigens may cross into the bloodstream, triggering systemic inflammation and immune activation. Research indicates that intestinal permeability contributes to conditions including autoimmune disease, food intolerances, and metabolic disorders ^[5].

Small intestinal bacterial overgrowth (SIBO)

SIBO occurs when bacteria that normally reside in the colon migrate into the small intestine and multiply excessively. This overgrowth disrupts normal digestive processes, interferes with nutrient absorption, and produces gas and bloating. Studies estimate SIBO prevalence at 6-15% in the general population, rising to 30-85% in individuals with irritable bowel syndrome ^[6]. Risk factors include decreased gastric acid production, impaired intestinal motility, structural abnormalities, and certain medications including proton pump inhibitors.

Common symptoms include bloating, abdominal pain, diarrhea, and nutrient deficiencies—particularly fat-soluble vitamins (A, D, E, K) and vitamin B12. Diagnosis typically involves breath testing or jejunal aspirate culture. Treatment usually combines antibiotics (such as rifaximin) with dietary modifications and prokinetic agents to restore normal motility patterns ^[7].

Nutrient absorption in the small intestine

Different segments of the small intestine specialize in absorbing specific nutrients. The duodenum receives partially digested food from the stomach and neutralizes acidic chyme while beginning absorption of iron, calcium, and water-soluble vitamins. The jejunum handles most carbohydrate and protein absorption through specialized transporters. The ileum completes nutrient absorption and is the exclusive site of vitamin B12 absorption through intrinsic factor-mediated uptake ^[8].

Malabsorption disorders affecting the small intestine include celiac disease, Crohn's disease, tropical sprue, and short bowel syndrome. These conditions can lead to serious nutritional deficiencies, weight loss, and systemic complications if left untreated.