What Nutrients Does the Small Intestine Absorb?

November 27, 2025 •

3 min read

Over 90% of the nutrients and water your body receives from food comes from the digestive process within the small intestine. As the central hub of nutrient assimilation, the small intestine is a remarkable organ responsible for absorbing everything from carbs and fats to vitamins and minerals.

Quick Summary

The small intestine absorbs macronutrients (carbohydrates, fats, proteins) and micronutrients (vitamins, minerals) through specialized structures like villi and microvilli. Different sections of the small intestine are responsible for absorbing specific nutrients, which then enter the bloodstream or lymphatic system.

Key Points

Macronutrient Absorption: The small intestine absorbs carbohydrates as simple sugars, proteins as amino acids, and fats as fatty acids and monoglycerides.
Micronutrient Absorption: Essential vitamins (A, D, E, K, C, B-complex) and minerals (iron, calcium) are absorbed, with specific vitamins like B12 having dedicated absorption sites.
Regional Specialization: Different sections of the small intestine—duodenum, jejunum, and ileum—specialize in absorbing different nutrients.
Absorption Pathways: Water-soluble nutrients enter the bloodstream via capillaries, while fats are absorbed into the lymphatic system via lacteals.
Villi and Microvilli: The extensive surface area for absorption is created by millions of finger-like projections called villi and microvilli lining the intestinal wall.
Water Absorption: The small intestine absorbs a significant volume of water each day via osmosis, which is crucial for hydration.

The small intestine is a vital organ in the digestive system, a long, coiled tube approximately 20 feet long, where the final stages of digestion and the vast majority of nutrient absorption take place. Its unique structure, featuring circular folds, villi, and microvilli, increases the surface area for absorption significantly, creating an extensive "brush border". Here, digested food, or chyme, is efficiently absorbed into the bloodstream or lymphatic system.

Macronutrient Absorption

Macronutrients—carbohydrates, proteins, and fats—are broken down into simpler forms for absorption.

Carbohydrates: Absorbed as simple sugars (monosaccharides) like glucose, fructose, and galactose. Glucose and galactose use active transport, often with sodium, while fructose uses facilitated diffusion, before entering the bloodstream.
Proteins: Broken down into amino acids, dipeptides, and tripeptides, absorbed via active transport into blood capillaries.
Fats (Lipids): Emulsified by bile salts, then broken into fatty acids and monoglycerides. These form micelles, diffuse into cells, reassemble into triglycerides, and are packaged into chylomicrons for absorption into the lymphatic system's lacteals.

Micronutrient and Water Absorption

Essential vitamins, minerals, and water are also absorbed.

Vitamins: Water-soluble vitamins are absorbed actively. Fat-soluble vitamins are absorbed with fats in micelles and chylomicrons. Vitamin B12 requires intrinsic factor and is absorbed in the ileum.
Minerals: Absorbed through various processes. Iron and calcium are primarily absorbed in the duodenum, with calcium absorption regulated by Vitamin D. Other minerals like magnesium, potassium, and zinc are absorbed throughout.
Water: Approximately 7 to 8 liters are absorbed daily via osmosis.

Section-Specific Absorption

Absorption is specialized along the small intestine's length:


Section	Primary Absorption Functions
Duodenum	Iron, calcium, magnesium, water-soluble vitamins, some fats and carbohydrates
Jejunum	Most carbohydrates, amino acids, fatty acids, fat-soluble vitamins (A, D, E, K), and water
Ileum	Vitamin B12, bile salts, and any remaining nutrients not absorbed earlier

The Role of Villi and Microvilli

The inner wall of the small intestine is covered with villi and microvilli, dramatically increasing the surface area for efficient nutrient transfer. Villi contain capillaries and lacteals. Water-soluble nutrients enter the capillaries and go to the liver, while fats and fat-soluble vitamins enter the lacteals and the lymphatic system.

Conclusion

The small intestine efficiently absorbs macronutrients, micronutrients, and water, crucial for health. Its structure, with villi and microvilli, maximizes this process. Understanding what nutrients the small intestine absorbs is vital, as malfunctions can lead to deficiencies. For further information, the NCBI offers an authoritative resource on the small intestine's anatomy and physiology: NCBI - Anatomy, Abdomen and Pelvis, Small Intestine.

Lists of Absorbed Nutrients

Macronutrients

Carbohydrates: Absorbed as simple sugars like glucose, fructose, and galactose.
Proteins: Absorbed as amino acids, dipeptides, and tripeptides.
Fats: Absorbed as fatty acids and monoglycerides.

Micronutrients

Vitamins: Includes water-soluble (B vitamins, C) and fat-soluble (A, D, E, K) vitamins.
Minerals: Includes iron, calcium, magnesium, zinc, and potassium.

Other

Water: Large amounts absorbed through osmosis.
Bile Salts: Recycled and reabsorbed in the terminal ileum.

Key Factors Influencing Absorption

Transit Time: Affects the duration of nutrient contact for absorption.
Surface Area: Villi and microvilli increase surface area for efficiency.
Enzymes and Bile: Essential for breaking down nutrients.
Transport Mechanisms: Various methods move nutrients across the intestinal wall.

How Absorption Pathways Differ

Bloodstream: Water-soluble nutrients go to the liver via capillaries.
Lymphatic System: Fats and fat-soluble vitamins enter via lacteals.

Regional Specialization

Duodenum: Key for iron and calcium.
Jejunum: Absorbs most macronutrients.
Ileum: Absorbs Vitamin B12 and recycles bile salts.

Frequently Asked Questions

If the small intestine fails to absorb nutrients properly, a condition called malabsorption occurs. This can lead to nutritional deficiencies, weight loss, and other health complications, which can be caused by diseases like Celiac disease or Crohn's disease.

The majority of nutrient absorption takes place in the jejunum, the middle section of the small intestine. However, the duodenum and ileum are also vital for absorbing specific nutrients like iron and Vitamin B12, respectively.

Fats are first emulsified by bile salts, then broken down by enzymes. The resulting fatty acids and monoglycerides form micelles, which diffuse into intestinal cells. Inside the cells, they are reassembled into triglycerides, packaged into chylomicrons, and absorbed into the lacteals of the lymphatic system.

The villi and microvilli are specialized structures that dramatically increase the surface area of the small intestine's lining. This extensive surface area maximizes contact with digested food, allowing for highly efficient nutrient absorption.

Vitamin B12 absorption is a unique process that occurs in the terminal ileum. It requires a specific carrier protein called intrinsic factor, which is produced in the stomach, to bind to the vitamin for its absorption.

Water-soluble nutrients (carbohydrates, proteins, B vitamins, C) are absorbed directly into the blood capillaries within the villi. Fat-soluble nutrients (fats, vitamins A, D, E, K) are absorbed into the lacteals, which are lymphatic vessels.

While the small intestine is the primary site for nutrient absorption, the large intestine also plays a role in absorbing water, electrolytes, and some vitamins produced by its resident bacteria, but on a much smaller scale.