4 Digestion and Nutrient Absorption
Learn how food moves through the digestive tract, how nutrients are broken down and absorbed, and which factors can affect these processes.
and
mechanically and chemically breaks food into molecules that can cross the digestive tract’s lining. moves those molecules, along with water and minerals, into blood or lymph for distribution through the body. Most and nutrient take place in the small intestine.
The route food takes
Food moves through the in order. At each stage, physical movement and digestive processes prepare food for the next part of the tract.
Mouth: Teeth break food into smaller pieces, and saliva moistens it. Salivary amylase begins starch . Swallowing moves the resulting bolus into the esophagus.
Esophagus: Muscular waves called move the bolus to the stomach.
Stomach: Muscular mixing turns food into a fluid mixture called . Stomach acid helps unfold proteins and activates pepsin, an enzyme that begins protein . The stomach releases gradually into the small intestine.
Small intestine: The duodenum, jejunum, and ileum receive and digestive secretions. Enzymes complete much of , and the intestinal lining absorbs most nutrients.
Large intestine: The colon absorbs remaining water and electrolytes. Gut microbes ferment some undigested carbohydrates, including certain fibers. The remaining material forms stool, which passes through the rectum and exits through the anus.
Organs that aid
The pancreas, liver, and gallbladder are : food does not pass through them, but they aid . The pancreas releases digestive enzymes and bicarbonate into the duodenum; bicarbonate helps neutralize acidic .
The liver produces bile, and the gallbladder stores and releases it. Bile disperses dietary fat into small droplets, making it easier for lipase to act.
The small intestine’s absorbing surface
Folds, , and microscopic projections called give the small intestine a large inner surface area for . Cells lining the intestine, called , take up nutrients from intestinal contents and pass them into blood or lymph.
Nutrient and routes
Different nutrients are digested into different products and use different routes into the body.
Carbohydrates: Amylase breaks starch into smaller sugars, and enzymes on the intestinal surface complete the process. Glucose, galactose, and fructose cross into intestinal cells and then enter the blood. For example, lactase splits milk sugar (lactose) into glucose and galactose.
Proteins: Stomach pepsin and pancreatic and intestinal enzymes break proteins into amino acids and small peptides. These products cross intestinal cells and enter the blood, which carries them to the liver and other tissues.
Fats: Bile disperses fat droplets, and pancreatic lipase breaks triglycerides into smaller lipid products. Many long-chain fats are reassembled inside intestinal cells and packaged into particles called . These enter lymphatic vessels in the before reaching the bloodstream.
Vitamins, minerals, and water: These generally do not need to be digested into smaller building blocks, though some depend on specific digestive conditions or binding partners. They use different routes and locations. For example, vitamin B₁₂ depends on made in the stomach and takes place in the ileum. Water and electrolytes are absorbed in both the small and large intestines.
Some dietary components resist human digestive enzymes. Much of the fiber in plant foods passes into the colon, where microbes may ferment part of it; the remainder contributes to stool.
Factors that affect and
and depend on digestive secretions, the condition of the intestinal lining, movement through the tract, food composition, and nutrient-specific requirements.
Enzymes and digestive secretions: Effective depends on the right enzymes and conditions. Insufficient lactase leaves lactose partly undigested. Reduced pancreatic enzyme output can interfere with the of fats, proteins, and carbohydrates.
Bile and fat handling: Inadequate bile delivery can make fat and less effective, including of the fat-soluble vitamins A, D, E, and K.
Intestinal surface and health: Damage or inflammation to the small-intestinal lining can reduce its capacity to absorb nutrients. The large surface area of healthy and supports efficient uptake.
Transit and mixing: and intestinal mixing bring food into contact with digestive juices and the intestinal lining. Changes in movement can affect how long and have to occur.
Food composition and microbes: Food structure affects how readily enzymes can reach nutrients. Some fibers are fermented by colon microbes, while others pass through largely intact.
Specific nutrient requirements: Some nutrients rely on particular transport mechanisms or partners. Vitamin B₁₂, for example, requires and an intact ileum for normal .