The six steps of digestion start as soon as food enters your mouth, and the process doesn't end until you excrete the waste products through a bowel movement.
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Here are the different stages of digestion and a breakdown of the digestive enzymes involved in the process.
The 6 Steps of Digestion
Digestion is a process that involves many different organs, tissues and substances. Here's the process of digestion in the order it occurs:
The first digestion stage starts as soon as food enters your mouth — this is called ingestion.
Chewing and grinding food with your teeth breaks it down physically, and enzymes in your saliva also contribute to some chemical digestion while the food is still in your mouth, says Hailey Crean, RD, a Boston-based registered dietitian and certified diabetes educator.
The next step of digestion is propulsion, which is a fancy term for swallowing. "The propulsion step begins when we swallow food and it passes through the esophagus down into the stomach," Crean says.
Two muscles known as sphincters — one on either side of your esophagus tube — contract and relax to permit your food and liquids to pass, according to the Mayo Clinic.
They also help prevent unchewed food from entering the esophagus and keep stomach acid from rising upwards.
3. Physical Digestion
Although it's the third step to digestion, physical digestion really begins when you first start chewing your food and break down a bite into smaller, more digestible pieces, Crean says.
Physical digestion continues as food moves through the esophagus and into your stomach. Muscles lining the esophagus and stomach combine what you've eaten with stomach acid, resulting in a mixture of chewed food and digestive acid called chyme, Crean says.
4. Chemical Digestion
Like physical digestion, chemical digestion also begins in the mouth with enzymes in saliva that break down starches, Crean says. But that's just the start.
Chemical digestion continues all the way through the digestive tract as acid and enzymes break your food down into protein, carbohydrate and fat molecules, according to a September 2021 StatPearls article.
This is an important step for your digestive system because your body is only able to absorb these smaller substances.
The majority of chemical digestion occurs in the small intestine, per the StatPearls article. This is where several enzymes — including bile, which helps digest fats and some vitamins — break down chyme into its individual chemical components.
Next of the six activities of digestion is absorption.
This step involves transferring nutrients — like fat, protein, carbohydrates, vitamins, minerals and antioxidants — from the digestive system to your bloodstream.
Most absorption occurs in the small intestine, according to the Mayo Clinic. Some absorption — like that of fluid and electrolytes — takes place in the large intestine, the final organ in your digestive tract, Crean says.
Even though the large intestine does not produce enzymes, bacteria that thrive there continue the digestive process by fermenting carbohydrates that weren't broken down in the small intestine, according to Colorado State University.
Absorption allows your body to access the nutrients it needs from food and drink to produce energy and build new cells, tissues, enzymes and hormones, according to National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK).
Certain conditions —like celiac disease, cystic fibrosis and chronic pancreatitis — can limit your body’s ability to absorb nutrients, per Johns Hopkins Medicine.
6. Waste Elimination
The final stage of digestion is waste elimination. Once digested food reaches the large intestine and any remaining water or electrolytes have been reabsorbed into the bloodstream, what's left is stool, according to the Mayo Clinic.
The large intestine stores stool until it passes through the anus in the form of a bowel movement — the last of the six processes of digestion.
Types of Digestive Enzymes
As we've discussed, different enzymes released by your organs are an important part of the six steps of digestion. They help break down macronutrients — carbohydrates, proteins and fats — into particles that are small enough for your body to absorb.
Your stomach, small intestine and pancreas — particularly the pancreas — produce digestive enzymes, according to Johns Hopkins Medicine. There are three main types: amylases, lipases and proteases. Lactase and sucrase are two other important enzymes.
Here's what to know about each:
Amylase is a digestive enzyme predominantly secreted by the pancreas and salivary glands and found in other tissues in very small levels, per a July 2021 StatPearls article.
Its primary function is to break down complex carbohydrates to simple sugars (like glucose) that your body can absorb, according to Johns Hopkins Medicine.
The salivary glands release amylase — along with mucus, electrolytes and water — in the form of saliva when food enters your mouth. Salivary amylase starts the breakdown of starches, so this is where the digestion of carbohydrates first begins, per the StatPearls article.
Most of the digestion of carbohydrates occurs in the duodenum — the first section of your small intestine — when the pancreas releases amylase to finish breaking down carbs so your small intestine can absorb the sugars.
The 3 Sections of the Small Intestine
The small intestine has three sections: the duodenum, the jejunum and the ileum, per the Cleveland Clinic.
The duodenum regulates digestion, and this is where where bile, liver, gallbladder and pancreatic enzymes enter the small intestine. Next is the jejunum, where food mixes with digestive juices and absorption occurs. The last section, the ileum, further absorbs nutrients and moves waste products towards the large intestine.
Lipase is an enzyme that breaks down triglycerides into fatty acids and glycerol, according to a July 2021 StatPearls article.
Lipase breaks down the fat you eat down into smaller molecules that can pass through your small intestine and into your blood. Per the StatPearls article, there are different types of lipase, including:
- Hepatic lipases: Enzymes produced by your liver
- Hormone-sensitive lipases: Enzymes in certain fat cells
- Lipoprotein lipase: Enzymes in the lining of your blood vessels
- Pancreatic lipase: Enzymes produced in the pancreas and released into your small intestine
When it comes to what enzymes the duodenum produces for digestion, though, pancreatic lipase is the most common.
The enzymes that digest proteins in the small intestine are called proteases, according to July 2019 research in Comprehensive Biotechnology.
Your pancreas releases protease and then your duodenum secretes the enzyme, which breaks down protein into small peptides and amino acids that are then absorbed by your small intestine.
Lactase is another duodenum enzyme. It breaks down lactose, the sugar found in milk and other dairy products, according to Johns Hopkins Medicine.
Without it, it's hard for your body to process dairy, which can lead to symptoms like gas, bloating and cramping — a condition called lactose intolerance.
The sucrase enzyme is released into your small intestine to break down sucrose, a type of sugar found naturally in certain fruits, vegetables and honey, according to Johns Hopkins Medicine.
Without it, you can have digestive problems. For instance, some people have a disorder called congenital sucrase-isomaltase deficiency, which is when you don't have enough of the duodenal enzyme to break down sucrose.
- Cleveland Clinic: “Nissen Fundoplication"
- Mayo Clinic: “Slide Show: See How Your Digestive System Works”
- Mayo Clinic: “Esophagus”
- National Institute of Diabetes and Digestive and Kidney Diseases: “Your Digestive System and How It Works”
- StatPearls: "Physiology, Digestion"
- Colorado State University: "Microbial Fermentation"
- StatPearls: "Amylase"
- StatPearls: "Lipase"
- Comprehensive Biotechnology: "Proteases"
- Cleveland Clinic: "Small Intestine"
- Johns Hopkins Medicine: "Digestive Enzymes and Digestive Enzyme Supplements"
- Johns Hopkins Medicine: "Malabsorption"
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