Digestion is a multi-step process that starts as soon as food enters your mouth, and it doesn't end until you excrete the waste products through a bowel movement.
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The six steps of digestion each serve a different purpose and have their own benefits.
The Major Stages of the Digestive System
The steps in the process of digestion in human beings involves six stages. These include:
In order to explain the first stage of digestion, we must figure that digestion starts in the mouth — as soon as food enters it. This is called ingestion.
Using your teeth to chew and grind the food breaks it down physically, but some chemical digestion also takes place in your mouth as a result of enzymes in your saliva.
"This first enzymatic digestion in the mouth is primarily acting to break down starches," says Hailey Crean, RD, a Boston-based registered dietitian and certified diabetes educator.
The next step, propulsion, includes the swallowing process. "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 this connecting 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 from the mouth and keep stomach acid from rising back up into the esophagus.
3. Physical Digestion
Although it's the third step in digestion, physical digestion actually begins in the mouth, with the food being chewed and ground up by your teeth.
Physical digestion also continues as food moves through the esophagus and into your stomach. Muscles lining the esophagus and stomach help churn and mix it — eventually combining what you've eaten with stomach acid. The resulting mixture of chewed food and digestive acid is 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, according to Crean. But that's just the start. Chemical digestion continues all the way through the digestive tract. The acid in your stomach, besides physically mixing with food, also helps to further break it down into its chemical components.
Chemical digestion refers to the work performed by enzymes throughout your digestive tract, which break the bonds that hold molecules together so that proteins, carbohydrates and fats are split into single molecules.
Only these smaller molecules can pass through the lining of your small intestine and be absorbed into your body.
While we think of the chemical digestion of protein as occurring in the stomach, the majority of chemical digestion occurs in the small intestine, per the National Library of Medicine.
This is where several enzymes — including bile, which helps digest fats and some vitamins — are secreted and act on stomach contents (called chyme, says the National Institute of Diabetes and Digestive and Kidney Diseases) to further break it down into its individual chemical components.
Absorption is the step that involves the transport of material that your body can use from your digestive system to your bloodstream. This includes fat, protein, carbohydrates, vitamins, minerals and antioxidants. Most absorption occurs in the small intestine, according to the Mayo Clinic.
But Crean points out that some absorption — primarily absorption of fluid and electrolytes — takes place in the large intestine, the final organ in your digestive tract.
Even though the large intestine does not produce enzymes, bacteria that thrive there continue the digestive process by fermenting carbohydrates that weren't digested in the small intestine. This process produces a small amount of energy, as well as vitamin K, 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 NIDDK. Digestion is important so that your body can break down these nutrients and use them.
According to the Centers for Disease Control and Prevention, some conditions that affect the digestive tract, like Crohn's disease or ulcerative colitis, can limit your body's ability to absorb nutrients.
6. Waste Elimination
The last step in digestion is waste elimination. Once digested food reaches the large intestine and any water or electrolytes remaining have been reabsorbed into the bloodstream, what's left is stool, explains the Mayo Clinic.
The large intestine is where the stool is stored until it passes through the anus in the form of a bowel movement — the end stage of digestion.
Types of Digestive Enzymes
As we've discussed, several organs and accessory organs release enzymes that aid in the digestive process.
When you eat a meal rich in all of the macronutrients -- carbohydrates, proteins and fats – the digestive enzymes work together to break down the nutrients into particles that are small enough for your body to absorb.
There are three main types of digestive enzymes, which include amylases, lipases and proteases.
Amylase is a digestive enzyme predominantly secreted by the pancreas and salivary glands and found in other tissues in very small levels, per StatPearls Publishing.
Its primary function is to break down complex carbohydrates to simple sugars, like glucose, that your body can absorb.
The salivary glands release amylase, along with mucus, electrolytes and water, in the form of saliva when food enters your mouth. The amylase in saliva starts the breakdown of starches, so this is where the digestion of carbohydrates first begins.
When the digested material reaches your duodenum — the first section of your small intestine — the pancreas releases amylase to finish the breakdown of carbohydrates so that your small intestine can absorb the sugars.
Lipase is an enzyme that breaks down triglycerides into fatty acids and glycerol, per StatPearls Publishing.
Lipase breaks down the fat you eat down into smaller molecules that can pass through your small intestine and into your blood.
While your mouth and stomach produce some lipase, the largest volume is produced by the pancreas. Most people produce enough lipase to adequately break down the fats they eat, but those with celiac disease, cystic fibrosis or Crohn's disease may be lacking in the enzyme.
Proteases play an important role in the digestion of protein, as explained in Comprehensive Biotechnology.
Proteases break down protein into small peptides and amino acids, which are then absorbed by your small intestine.
While protein digestion begins in the stomach, the bulk of digestion occurs in the small intestine where the pancreas releases proteases.
Your body can use the amino acids to build new proteins that are needed for various physiological functions.
Is This an Emergency?
- Cleveland Clinic: “Laparoscopic Antireflux Surgery”
- Hailey Crean, RD, CDE, registered dietitian and certified diabetes educator, Boston
- Mayo Clinic: “Slide Show: See How Your Digestive System Works”
- Mayo Clinic: “Esophagus”
- National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK): “Your Digestive System and How It Works”
- National Library of Medicine/StatPearls: "Physiology, Digestion"
- Mayo Clinic: "Slide Show: See How Your Digestive System Works"
- Colorado State University: "Microbial Fermentation"
- StatPearls Publishing: "Amylase"
- StatPearls Publishing: "Lipase"
- Comprehensive Biotechnology: "Proteases"
- Methods in Enzymology: "Nuclease"