The Body's Process of Taking In and Using Food Explained

October 29, 2025 •

4 min read

Every living organism, from single-celled bacteria to humans, requires energy to survive. This energy comes from the food we consume, which must undergo a complex and coordinated series of steps to be converted into a usable form. The process of taking in and using food involves two major biological systems: digestion and metabolism.

Quick Summary

This article explains the journey of food through the body, detailing the steps of digestion, nutrient absorption, and the metabolic processes that convert food into cellular energy (ATP).

Key Points

Ingestion to Digestion: The journey of food starts in the mouth, where it is chewed and mixed with saliva, before passing through the esophagus to the stomach for further breakdown by acids and enzymes.
Accessory Organ Contributions: The liver, gallbladder, and pancreas are crucial accessory organs that produce bile and key enzymes to complete the chemical digestion of carbohydrates, proteins, and fats in the small intestine.
Absorption of Nutrients: The small intestine, with its large surface area created by villi and microvilli, is where most nutrients are absorbed into the bloodstream and lymphatic system.
Cellular Respiration: Once absorbed, nutrients like glucose are used by cells in a metabolic process called cellular respiration to create ATP, the body's primary energy source.
Waste Elimination: The large intestine reabsorbs water and electrolytes from undigested food, with the help of gut bacteria, before the final waste products are eliminated from the body.
The Central Role of ATP: The ultimate goal of the entire process is the production of ATP, which provides the necessary energy for all cellular functions, including growth, repair, and movement.

The biological journey of food is a fascinating and intricate process. It begins the moment a morsel of food enters the mouth and continues until the waste products are expelled from the body. This entire process can be broadly divided into digestion, where food is broken down, and metabolism, where the absorbed nutrients are utilized for energy and growth.

The Journey Begins: From Mouth to Stomach

The initial stage of the process starts in the mouth with ingestion and mechanical digestion. As you chew, your teeth physically break down food into smaller pieces, increasing its surface area. Simultaneously, salivary glands release saliva, which contains the enzyme amylase to begin the chemical digestion of carbohydrates. The tongue helps mix the food with saliva, forming a softened mass called a bolus, which is then swallowed.

From the mouth, the bolus travels down the esophagus via peristalsis—rhythmic muscle contractions that push food along the digestive tract. It passes through the lower esophageal sphincter and enters the stomach. The stomach is a muscular, J-shaped organ that acts as a temporary reservoir, storing food and releasing it slowly into the small intestine. Here, strong stomach muscles churn the food, mixing it with gastric juices, including hydrochloric acid and pepsin, an enzyme that starts protein digestion. This acidic environment helps kill bacteria and denatures proteins. The result is a semi-liquid mixture known as chyme.

Nutrient Absorption in the Small Intestine

The chyme is gradually released into the small intestine, where the majority of chemical digestion and nutrient absorption occurs. This long, coiled tube is divided into three sections: the duodenum, jejunum, and ileum. As chyme enters the duodenum, it is met with digestive juices from the pancreas and bile from the liver and gallbladder.

Pancreatic Juice: Contains bicarbonate to neutralize stomach acid, along with enzymes like amylase (for carbohydrates), lipase (for fats), and proteases (for proteins).
Bile: Produced by the liver and stored in the gallbladder, bile emulsifies fats, breaking them into smaller droplets to aid lipase in digestion.

The intestinal walls are lined with millions of microscopic, finger-like projections called villi, which are themselves covered in microvilli. This creates an enormous surface area for efficient absorption of nutrients into the bloodstream. Absorbed simple sugars (from carbohydrates) and amino acids (from proteins) travel to the liver, while fatty acids and glycerol (from fats) are absorbed into the lymphatic system.

The Role of Metabolism and Cellular Respiration

Metabolism encompasses all the chemical reactions that occur in the body's cells, including converting food into energy. Once nutrients are absorbed, they are transported to cells throughout the body where they are used for energy, growth, and repair. The primary process for energy production is cellular respiration, which converts glucose and other nutrients into adenosine triphosphate (ATP), the cell's main energy currency.

This process occurs in three main stages:

Glycolysis: A molecule of glucose is broken down into two molecules of pyruvate in the cell's cytoplasm. A small amount of ATP is produced during this stage.
The Krebs Cycle (or Citric Acid Cycle): In the mitochondria, pyruvate is further broken down, producing more ATP and high-energy electron carriers (NADH and FADH2).
The Electron Transport Chain: The electron carriers from the Krebs cycle donate their electrons, powering a series of reactions that generate a large amount of ATP.

Waste Management in the Large Intestine and Elimination

After nutrient absorption in the small intestine is complete, the remaining undigested material, including fiber, moves into the large intestine. The large intestine's main function is to absorb the remaining water and electrolytes, solidifying the waste material into feces. The gut microbiome—trillions of bacteria residing in the large intestine—play a crucial role here, further breaking down some remaining nutrients and producing beneficial vitamins, such as vitamin K. The feces are stored in the rectum until elimination through the anus.

Digestion vs. Metabolism: A Comparative Look


Feature	Digestion	Metabolism
Primary Function	Mechanical and chemical breakdown of food into absorbable nutrients.	All chemical reactions in the body; converting nutrients to energy and other molecules.
Location	Primarily the gastrointestinal (GI) tract: mouth, stomach, small and large intestines.	Occurs inside every cell of the body.
Scope	A preparatory, macro-level process that precedes cellular use.	A cellular-level process that uses the products of digestion.
End Products	Simple sugars, amino acids, fatty acids, and glycerol, ready for absorption.	Energy (ATP), heat, and cellular building blocks.
Control	Regulated by nerves and hormones responding to the presence of food.	Regulated by hormones like insulin and glucagon, responding to nutrient availability.

Conclusion

The intricate process of how our body takes in and uses food is a marvel of biological engineering. Starting with the simple act of chewing, the digestive system systematically breaks down complex food particles into their most basic components. These components are then absorbed and enter the metabolic pathways of trillions of cells, where they are converted into the essential energy currency (ATP) that powers every single cellular function, from a single muscle contraction to complex thought. Proper functioning of both digestion and metabolism is fundamental to overall health, providing the fuel needed for growth, repair, and daily activities.

For additional details on metabolic pathways, explore this resource: Khan Academy: Cellular Respiration

Frequently Asked Questions

Digestion is the process of breaking down food into smaller, absorbable molecules in the gastrointestinal tract. Metabolism, on the other hand, is the sum of all chemical reactions within the body's cells that use these absorbed nutrients for energy, growth, and repair.

Fats are emulsified by bile in the small intestine, then broken down by lipase into fatty acids and glycerol. These components form small spheres called micelles that are absorbed by intestinal cells and repackaged into chylomicrons, which enter the lymphatic system.

ATP, or adenosine triphosphate, is the primary energy currency of the cell. It is produced during cellular respiration by breaking down nutrients like glucose. When a phosphate bond in ATP is broken, it releases energy to power various cellular activities.

Humans cannot digest cellulose because they lack the specific enzymes and the specialized digestive structures, like the multi-chambered stomach found in ruminants, that are necessary to break it down. Ruminants possess certain bacteria in their gut that can process cellulose.

The gut microbiome, or gut flora, is a community of microorganisms in the large intestine that helps break down remaining undigested material, like fiber. These bacteria also produce some beneficial vitamins, such as vitamin K.

The digestive process is controlled by a combination of nerves and hormones. The enteric nervous system, a network of nerves in the GI tract walls, regulates local movements and secretions. Hormones like gastrin and secretin are released in response to food and signal different parts of the digestive system.

The body can store excess nutrients for later use. For example, glucose can be stored in the liver and muscles as glycogen, and excess calories can be converted and stored as fat in adipose tissue.