Roles of the Large and Small Intestines in the Digestive System
The small intestine is the primary site for digestion and absorption of nutrients, while the large intestine functions to absorb water, harbor microbiota, and facilitate waste elimination. 1, 2
Small Intestine Functions
Primary Digestive and Absorptive Role
The small intestine is key in the digestion and absorption of macro and micronutrients, receiving approximately 1.5 liters of pancreatico-biliary secretions daily containing bicarbonate to neutralize gastric acid and digestive enzymes for carbohydrate, fat, and protein breakdown. 3, 2
Bile salts from the liver enter the duodenum and are essential for fat emulsification and absorption, while pancreatic enzymes complete the breakdown of proteins, carbohydrates, and fats. 3
The jejunal mucosa produces passive secretions to render the lumen isotonic during nutrient passage, with brush border enzymes produced by enterocytes performing final carbohydrate and protein digestion. 3
Anatomical Specialization
The small intestine comprises 56% of total gut volume in humans, reflecting its adaptation for nutrient extraction, while the large intestine comprises only about 20%—these proportions are reversed in apes. 1
The proximal small intestine (duodenum and jejunum) is not only dedicated to nutrient digestion and absorption but is also a highly regulated immune site exposed to environmental factors, maintaining host-protective responses against pathogens while tolerating food antigens. 4
Clinical Significance
Gastric emptying rate is of fundamental importance to postprandial glycemia, accounting for at least a third of the variance in blood glucose response, as even minor changes in carbohydrate delivery to the small intestine can substantially affect glycemia. 1
In short bowel syndrome, patients absorbing less than one-third of oral energy intake require parenteral nutrition, highlighting the small intestine's critical absorptive capacity. 1
Large Intestine Functions
Water and Electrolyte Absorption
The colon has a large capacity to absorb sodium and water, which is why patients with a short bowel and preserved colon are rarely in negative water and sodium balance and rarely need supplements. 1
The large intestine is essential for the absorption of water to allow adequate defecation, serving as the final processing station before waste elimination. 2
Bacterial Fermentation and Energy Salvage
The large intestine harbors intestinal microbiota, for which their nutritional role is as important as it is unknown, with bacterial fermentation of plant polysaccharides producing short-chain fatty acids (SCFAs). 2, 5
Colonic fermentation in humans provides 5-10% of energy requirements from digestion of starch, nonstarch polysaccharides, and protein not absorbed in the small bowel, though this becomes more important in patients with short bowel syndrome. 5
Bacterial fermentation of carbohydrate salvages some energy in patients with jejunum-colon anatomy, though D-lactic acidosis can occur if the diet is high in mono and oligosaccharides. 1
Gut Proportions and Adaptation
Gut proportions in humans are adapted to meat eating, with the small intestine specialized for nutrient extraction and the large intestine reduced in size compared to herbivorous primates. 1
In patients with preserved colon after small bowel resection, unabsorbed long chain fatty acids reduce transit time and water/sodium absorption, making diarrhea worse, while also being toxic to bacteria and reducing carbohydrate fermentation. 1
Coordinated Function
Integration of Motility and Secretion
Gastrointestinal motility and secretions are tightly coordinated to optimize digestion and absorption, with disruption of this coordination leading to symptoms such as abdominal pain after eating. 3
The stomach stores masticated food and delivers acidic chyme into the duodenum at a rate commensurate with the rate at which it can be digested and absorbed—too fast overwhelms the small intestine causing malabsorption and diarrhea; too slow inhibits consumption of another meal. 6
Sequential Processing
The gut works like the reverse of an assembly line with different regions specialized sequentially for storage, acid digestion, alkaline digestion, absorption, fermentation, and disposal of waste products. 6
Intestinal health plays a crucial role in the digestion and absorption of nutrients and elimination of waste from the body, with maintaining a healthy intestinal tract important for overall health and well-being of both small and large intestines. 1
Clinical Implications
Short Bowel Syndrome Classification
SBS patients can be classified into three anatomical groups that predict outcomes: end-jejunostomy (most severe), jejunocolonic anastomosis, and jejuno-ileo-colic anastomosis (most favorable), with the presence or absence of colon dramatically affecting management. 7
Patients with end-jejunostomy typically do not demonstrate functional adaptation over time, while those with preserved colon may show gradual reduction in nutritional requirements due to intestinal adaptation. 7
Dietary Considerations
For SBS patients with preserved colon, a diet high in complex carbohydrates (polysaccharides) and normal in fat content is recommended, while keeping it low in oxalate to reduce renal stone risk. 8
A nutritious and balanced diet is essential for good intestinal health as it promotes regular bowel movement, with probiotics being key to maintaining intestinal health. 1