Which foods are digested by the small intestine?

January 12, 2026 •

4 min read

The small intestine, despite its name, is the longest part of the gastrointestinal tract, responsible for over 90% of nutrient absorption. It is the primary site where the final breakdown and absorption of carbohydrates, proteins, and fats take place. This critical process ensures your body gets the energy and building blocks it needs from the foods you eat.

Quick Summary

The small intestine is the major site of chemical digestion and nutrient absorption. Specialized enzymes from the pancreas and intestinal walls break down carbohydrates, proteins, and fats into smaller molecules. Bile aids in fat digestion, and the vast surface area of the intestinal lining absorbs these nutrients into the bloodstream and lymphatic system.

Key Points

Carbohydrate Digestion: Complex carbs like starches are broken down into simple sugars (glucose, fructose) primarily in the small intestine by pancreatic and brush border enzymes.
Protein Digestion: Proteins are broken down into amino acids, dipeptides, and tripeptides by pancreatic enzymes (trypsin, chymotrypsin) and intestinal peptidases.
Fat Digestion: Fats are emulsified by bile from the liver and then digested by pancreatic lipase into fatty acids and monoglycerides.
Absorption: Digested nutrients are absorbed through the vast surface area of the small intestine's villi and microvilli into the bloodstream and lymphatic system.
Accessory Organs: The pancreas and liver play vital roles by providing crucial enzymes and bile to aid digestion in the small intestine.
Nutrient Delivery: Absorbed carbohydrates and proteins enter the bloodstream, while fats are transported via the lymphatic system before reaching general circulation.

The Small Intestine: A Digestion Powerhouse

At approximately 22 feet long in adults, the small intestine is a tightly coiled, muscular tube that is the most important organ for nutrient absorption. It works in concert with the pancreas, liver, and gallbladder to complete the digestive process that begins in the mouth and stomach. The walls of the small intestine, lined with millions of finger-like projections called villi and microvilli, create a massive surface area to maximize the absorption of digested food molecules.

How Food is Prepared for the Small Intestine

Before reaching the small intestine, food undergoes preliminary digestion. In the mouth, chewing and salivary amylase begin breaking down starches. In the stomach, churning and gastric juices containing hydrochloric acid and pepsin initiate protein digestion. The resulting semi-liquid mixture, known as chyme, is then slowly released into the duodenum, the first section of the small intestine.

The Breakdown of Macronutrients

Once in the small intestine, the chyme is bathed in digestive juices from the pancreas and bile from the liver. This neutralizes the stomach acid and provides the enzymes necessary for the final stages of chemical digestion.

Carbohydrates

Most carbohydrate digestion occurs here, with starches and complex sugars broken down into simple sugars like glucose, fructose, and galactose.

Pancreatic Amylase: Secreted by the pancreas, this enzyme continues the breakdown of starch into disaccharides (e.g., maltose) and shorter carbohydrate chains.
Brush Border Enzymes: Embedded in the lining of the small intestine, enzymes such as maltase, sucrase, and lactase break down disaccharides into single sugar molecules (monosaccharides) that can be absorbed directly into the bloodstream.

Proteins

The small intestine completes the digestion of proteins that started in the stomach. Pancreatic proteases and intestinal enzymes work together to break down polypeptides into their most basic form: amino acids.

Pancreatic Proteases: The pancreas secretes inactive enzymes like trypsinogen and chymotrypsinogen, which are activated in the small intestine to become trypsin and chymotrypsin. These break down large proteins into smaller peptides.
Brush Border Peptidases: Enzymes like aminopeptidase and dipeptidase, located on the intestinal wall, further break down peptides into individual amino acids, which are then absorbed.

Fats (Lipids)

Fats, or lipids, pose a unique challenge because they are not water-soluble. Their digestion largely depends on a two-step process in the small intestine.

Emulsification by Bile: Bile, produced by the liver and stored in the gallbladder, contains bile salts that break down large fat globules into smaller fat droplets. This increases the surface area for enzymes to act.
Pancreatic Lipase: This enzyme then breaks down the smaller fat droplets into monoglycerides and free fatty acids, which are small enough to be absorbed by intestinal cells.

The Absorption Process

After digestion, the resulting small nutrient molecules are ready for absorption. The small intestine's enormous surface area, created by folds, villi, and microvilli, is key to this process.

Monosaccharides and Amino Acids: Simple sugars and amino acids are absorbed by the epithelial cells of the small intestine and transported into the bloodstream, where they travel to the liver for further processing and distribution.
Fatty Acids and Monoglycerides: These products of fat digestion are absorbed into the intestinal cells, where they are reassembled into triglycerides. They are then packaged into lipoproteins called chylomicrons and transported via the lymphatic system before entering the bloodstream.

Nutrient Absorption in Different Small Intestine Sections

While absorption occurs throughout the small intestine, different sections have specific roles.

Comparison of Small Intestine Sections


Feature	Duodenum	Jejunum	Ileum
Function	Receives chyme from stomach; mixes with pancreatic enzymes and bile; primary site for iron absorption.	Primary site for nutrient absorption, including carbohydrates and amino acids.	Absorbs nutrients not absorbed earlier, especially vitamin B12 and bile salts.
Length	Shortest segment, about 25 cm (10 inches).	Middle segment, about 2.5 m (8 feet) long.	Longest segment, about 3 m (9.8 feet) long.
Role in Digestion	Initial and most intensive chemical digestion with accessory organ secretions.	Continues chemical digestion and absorbs most macronutrients.	Primarily focused on absorption; finishes any remaining digestion.

The Importance of Digestive Health

Efficient digestion and absorption in the small intestine are crucial for overall health. A diet rich in whole foods, fiber, and adequate hydration supports the intricate process carried out by the small intestine and its accessory organs. Proper intestinal function ensures the body receives essential nutrients for energy, growth, and repair. For more detailed physiological information, you can explore the NIH StatPearls guide to digestion.

Conclusion

In summary, the small intestine is the central hub for digesting and absorbing carbohydrates, proteins, and fats. Through a coordinated effort involving specialized enzymes from the pancreas, bile from the liver, and the massive surface area provided by villi and microvilli, the small intestine breaks down these foods into their basic components. It is this efficient process that allows the body to harness the energy and building blocks from your diet, fueling all cellular functions and sustaining life.

Frequently Asked Questions

Digestion begins in the stomach with hydrochloric acid and pepsin primarily breaking down proteins. The stomach’s acidic environment is unsuitable for carbohydrate and fat digestion. The small intestine, however, is where the majority of all three macronutrients are broken down and absorbed with the help of pancreatic enzymes and bile, in a more neutral environment.

Because fats are not water-soluble, they require special handling. Bile salts first emulsify large fat globules into smaller droplets. These smaller droplets are then digested by lipase into fatty acids and monoglycerides. The products are packaged into micelles for transport to the intestinal wall, where they are re-formed into triglycerides and absorbed into the lymphatic system via lacteals.

The pancreas is an essential accessory organ that secretes digestive juices containing enzymes into the small intestine. These enzymes, including pancreatic amylase, lipase, and proteases (trypsin, chymotrypsin), are crucial for the complete breakdown of carbohydrates, fats, and proteins.

No, the small intestine does not digest fiber. Humans lack the necessary enzymes to break down fiber. Instead, fiber passes into the large intestine, where some types may be fermented by gut bacteria.

Villi and microvilli are tiny, finger-like projections that line the walls of the small intestine. They drastically increase the surface area available for nutrient absorption. This expanded surface ensures that the maximum amount of nutrients from digested food is absorbed into the body.

Bile, produced by the liver, acts as an emulsifier. When fats enter the small intestine, bile breaks the large fat globules into smaller droplets, a process that dramatically increases the surface area for pancreatic lipase to act upon, making fat digestion more efficient.

No, different nutrients are absorbed in different segments. For example, most carbohydrates, proteins, and fats are absorbed in the jejunum. The duodenum is a primary site for iron absorption, while the ileum is responsible for absorbing vitamin B12 and bile salts.