Absorption, Assimilation, and Metabolism: What is the taking in of food nutrients by the body called?

December 23, 2025 •

3 min read

Approximately 90% of nutrient absorption occurs in the small intestine, primarily in the jejunum and ileum. This process, along with subsequent assimilation and metabolism, addresses the fundamental question: what is the taking in of food nutrients by the body called and how does it sustain life.

Quick Summary

Nutrient uptake involves the digestive system breaking down food into simpler molecules, which are then absorbed into the bloodstream. These absorbed nutrients are subsequently assimilated by the body's cells and used to power metabolic activities, including energy production and the construction of new tissues.

Key Points

Absorption is Not the Whole Story: While absorption is the initial intake of nutrients into the body, assimilation is the cellular utilization of those nutrients, and metabolism is the overall set of chemical reactions involved.
The Small Intestine's Critical Role: The majority of nutrient absorption happens in the small intestine, specifically aided by millions of finger-like villi that increase the surface area for nutrient uptake.
Two Transport Pathways: Water-soluble nutrients (sugars, amino acids) are absorbed into the blood capillaries, while fat-soluble nutrients (fatty acids, glycerol) are absorbed into the lymphatic system via lacteals.
Metabolism is Fuel and Construction: Metabolism is split into catabolism, which breaks down molecules for energy (ATP), and anabolism, which uses that energy to build complex molecules and new tissues.
Liver's Processing Plant Function: After absorption, water-soluble nutrients are transported to the liver, which stores, processes, and distributes them to the rest of the body as needed.
Digestion Precedes Intake: The entire process is preceded by digestion, the mechanical and chemical breakdown of food into simpler molecules that are small enough to be absorbed.

The process of using food nutrients begins with the journey of digestion, a complex cascade of mechanical and chemical breakdowns. This prepares the food for its eventual utilization, which is a multi-step process known most broadly as metabolism. While digestion breaks food down, the actual 'taking in' is split into two critical phases: absorption and assimilation.

Digestion: The Preliminary Breakdown

Before the body can take in nutrients, it must first break down large food molecules into smaller, absorbable components. This occurs throughout the digestive tract with the help of various enzymes.

Mechanical Digestion: The physical process begins in the mouth with chewing and continues in the stomach with churning and mixing. This increases the surface area for enzymes to act upon.
Chemical Digestion: Enzymes in saliva, stomach acid, and pancreatic juices systematically break down complex carbohydrates into simple sugars, proteins into amino acids, and fats into fatty acids and glycerol.

Absorption: Entering the Body's Interior

Absorption is the specific process where digested food molecules pass through the lining of the small intestine into the bloodstream or lymphatic system. The small intestine is uniquely designed for this task, with millions of finger-like projections called villi, which are themselves covered in microvilli to maximize surface area.

The Fate of Absorbed Nutrients

Different nutrients take different routes after absorption:

Water-soluble nutrients (simple sugars, amino acids, some vitamins, and minerals) cross into capillaries within the villi and are transported by the bloodstream. This nutrient-rich blood travels directly to the liver via the hepatic portal vein for initial processing before being distributed throughout the body.
Fat-soluble nutrients (fatty acids, glycerol, and fat-soluble vitamins) are absorbed into specialized lymphatic vessels within the villi called lacteals. The lymphatic system eventually empties these nutrients into the bloodstream, bypassing the liver's initial processing stage.

Assimilation: The Cellular Work

Assimilation is the final step, referring to the movement of absorbed nutrients from the blood into the cells of the body's tissues and organs. Once inside the cells, these molecules are put to use for energy, growth, and repair.

Assimilation is where the true power of nutrition is unleashed. A glucose molecule absorbed into the blood is assimilated by a muscle cell and used as fuel for contraction. An amino acid is assimilated by a cell to build a new protein for repair or function. This process is managed by the body's metabolism.

The Role of Metabolism

Metabolism encompasses all the life-sustaining chemical reactions that occur within cells, including both the building and breaking down of molecules.

Catabolism: The destructive phase where larger molecules (like glucose) are broken down to release energy in the form of ATP. This energy powers all cellular activities, from muscle movement to nerve impulses.
Anabolism: The constructive phase where energy from catabolism is used to build and store complex molecules from simpler ones. This is crucial for cell growth, tissue repair, and storing energy for future use.


---	Absorption	Assimilation
Primary Location	Small intestine	Body cells and tissues
Process	Nutrients move from the digestive tract into the bloodstream or lymph	Nutrients move from the bloodstream into the body's cells
Result	Nutrients are made available for transport	Nutrients are utilized by cells for energy, growth, and repair
Analogy	Delivery of raw materials	Processing and using materials at the factory

Conclusion

While the term 'absorption' most directly describes the initial taking in of food nutrients, a complete answer involves the subsequent processes of assimilation and metabolism. Absorption moves nutrients from the digestive tract into the circulatory system, assimilation incorporates them into body cells, and metabolism manages their overall utilization for energy, repair, and growth. Together, these processes transform the food we eat into the fundamental fuel that powers all physiological functions, a testament to the intricate workings of the human body.

For more information on how the body uses nutrients, consult resources like the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).

Frequently Asked Questions

Absorption is the process of moving digested nutrients from the digestive tract into the bloodstream or lymphatic system. Assimilation is the subsequent process where the body's cells take up and use these absorbed nutrients for energy, growth, and repair.

Most nutrient absorption occurs in the small intestine. Its inner walls are lined with villi and microvilli, which create a large surface area optimized for absorbing molecules into the body.

After absorption, water-soluble nutrients are transported via the bloodstream to the liver. The liver processes and stores these nutrients before releasing them to the rest of the body's cells for utilization.

The two main parts of metabolism are catabolism and anabolism. Catabolism is the breakdown of large molecules to release energy, while anabolism is the building of complex molecules, which requires energy.

The inner wall of the small intestine is covered in millions of microscopic, finger-like projections called villi. These villi, in turn, have even smaller projections called microvilli, which significantly increase the surface area available for nutrient absorption.

No, nutrients follow different paths. Water-soluble nutrients like simple sugars and amino acids enter the blood capillaries, while fat-soluble nutrients such as fatty acids and fat-soluble vitamins are absorbed into the lymphatic system through vessels called lacteals.

No, for the body to use food for energy, growth, or repair, the food must first be broken down into small, simple molecules through digestion. These simple molecules must then be absorbed into the bloodstream, where they can be transported to the body's cells.