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. The six steps of digestion each serve a different purpose and have their own benefits.
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Step 1: Ingestion
Digestion begins as soon as food enters your mouth. 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, CDE, a Boston-based registered dietitian and certified diabetes educator.
Step 2: Propulsion
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.
According to the Mayo Clinic, two muscles, known as sphincters — one on either side of this connecting tube — contract and relax to permit your food and liquids to pass. They also help prevent unchewed food from entering the esophagus from the mouth and keep stomach acid from rising back up into the esophagus.
Step 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.
Step 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.
However, according to the National Library of Medicine, the majority of chemical digestion occurs in the small intestine. That's 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.
Step 5: Absorption
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.
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.
Step 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.
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Is This an Emergency?
- Centers for Disease Control and Prevention: “Inflammatory Bowel Disease (IBD): What Is It?”
- 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"