The intestines are the workhorse of digestion and absorption — roughly 6-7 meters of small intestine coiled through the abdomen plus 1.5 meters of large intestine (colon) framing them. The small intestine absorbs almost everything you actually use from food. The colon reclaims water and hosts the microbial ecosystem that ferments what you did not absorb. Together they hold more immune tissue than any other organ.
At a glance
What it does
The small intestine — duodenum, jejunum, ileum — does the heavy absorption. Pancreatic enzymes and bile enter at the duodenum and finish breaking down carbohydrates, fats, and proteins. Monosaccharides, amino acids, fatty acids, vitamins, and minerals are absorbed across the mucosa, which is folded into plicae circulares, then villi, then microvilli — amplifying surface area to roughly 30-40 square meters. Specific transporters pick up specific molecules: SGLT1 for glucose, fatty acids passively via micelle uptake, iron via DMT1, B12 with intrinsic factor in the ileum, bile acids recycled in the terminal ileum.
The colon absorbs water and electrolytes, ferments indigestible carbohydrates into short-chain fatty acids (acetate, propionate, butyrate), and forms and stores stool until defecation. Butyrate is the primary fuel for colonocytes, which is one reason fiber intake matters.
Both segments house the gut-associated lymphoid tissue — Peyer's patches in the ileum, diffuse lymphocytes throughout. Roughly 70% of the body's immune cells reside in or around the gut, which makes sense because this is the largest exposed surface the body has to the outside world.
How it works
Peristalsis moves contents distally. Segmentation mixes them. The enteric nervous system — the "second brain," with about 500 million neurons — coordinates local reflexes independently of CNS input, modulated by vagal and sympathetic signals.
The mucosal barrier is a single layer of epithelial cells sealed with tight junctions, covered by a mucus layer and an antimicrobial peptide shield. Goblet cells produce mucus; Paneth cells at the crypt bases produce defensins. Microfold (M) cells sample luminal antigens and pass them to immune cells. The epithelium turns over every 3-5 days — one of the fastest tissues in the body — driven by stem cells in the crypts of Lieberkühn.
The microbiome is the trillions of microorganisms in the colon (and fewer in the small intestine). A healthy adult has on the order of 38 trillion bacteria, dominated by Bacteroidetes and Firmicutes at the phylum level, with substantial interpersonal variation. The microbiome digests fiber, synthesizes vitamin K and some B vitamins, modulates immunity, and communicates with the brain via vagal and metabolic signals.
When it goes wrong
Irritable bowel syndrome is the common functional disorder — cramping, bloating, altered bowel habits — without structural disease on standard testing. It is real, not imagined, and current understanding emphasizes gut-brain axis dysregulation, visceral hypersensitivity, and sometimes post-infectious changes. Treatment is individualized: FODMAP diets, soluble fiber, tricyclic antidepressants, rifaximin for diarrhea-predominant, laxatives or secretagogues for constipation-predominant.
Inflammatory bowel disease — Crohn's and ulcerative colitis — is a different beast. Genuine immune-mediated inflammation of the intestinal wall producing bloody diarrhea, weight loss, fistulae, strictures, and extra-intestinal manifestations. Diagnosed by endoscopy and biopsy, treated aggressively with biologics (anti-TNF, anti-integrin, anti-IL-23) and immunomodulators. Not the same as IBS and not a "leaky gut" story.
Celiac disease is autoimmune reaction to gluten in HLA-DQ2/DQ8 genetic carriers, causing small intestinal villous atrophy, malabsorption, and varied systemic symptoms. Lifelong strict gluten avoidance is the only real treatment. Non-celiac gluten sensitivity is a separate, less well-characterized entity that probably exists but is overdiagnosed.
Colorectal cancer is one of the most common cancers worldwide, largely arising from adenomatous polyps over 10+ years. Screening colonoscopy starting at 45 saves lives; the data are strong. Processed meat, heavy alcohol, obesity, and inactivity raise risk; fiber, exercise, and aspirin (in specific contexts) lower it.
"Leaky gut syndrome" is a popular framing that overstates real biology. Increased intestinal permeability exists and is associated with several diseases, but "leaky gut" as a standalone diagnosis driving every vague symptom is a functional medicine invention. There is no validated test, no targeted treatment, and most interventions marketed for it have no trials.
Interactions
Antibiotics disrupt the microbiome, sometimes for months. C. difficile infection is the most dangerous consequence and has made fecal microbiota transplantation a real therapy for recurrent C. diff. Probiotic products for general health have weaker data than marketing suggests; specific strains for specific conditions (certain pediatric diarrhea, pouchitis) have better evidence.
GLP-1, GIP, PYY, and cholecystokinin are intestinal hormones that regulate satiety, gastric emptying, and pancreatic secretion. GLP-1 agonists like semaglutide exploit this system pharmacologically. Incretin physiology explains why injected glucose produces less insulin release than oral glucose of the same dose.
Fiber is the single most under-consumed nutrient in most industrial diets. Soluble fiber feeds the microbiome, produces short-chain fatty acids, improves lipids, and increases satiety. Target is roughly 25-35 g/day; most adults hit 10-15 g.
Honest take
The microbiome is real and important; the microbiome industry is roughly 80% marketing. Most direct-to-consumer microbiome tests have poor reproducibility and the "personalized recommendations" they generate are not validated against clinical outcomes. The interventions with actual evidence are boring: eat more fiber from diverse plant sources, limit ultra-processed foods, avoid unnecessary antibiotics, and get enough sleep. Fecal microbiota transplant for recurrent C. diff is the one microbiome intervention with strong RCT data. "Leaky gut" is a red flag when used as a standalone diagnosis — it almost always signals a provider who will sell you a long panel of supplements without the diagnostic workup your actual symptoms deserve.
Sources
- Rao et al., American Journal of Gastroenterology — ACG guideline on IBS.
- Rubin et al., American Journal of Gastroenterology — ACG guideline on ulcerative colitis.
- Lichtenstein et al., American Journal of Gastroenterology — ACG guideline on Crohn's disease.
- Human Microbiome Project Consortium, Nature — foundational human microbiome data.