Digestion refers to the process where your body breaks down food into smaller nutrients that the body can absorb and use for nourishment and energy. 

Digestion is vitality important to your health, but many factors such as poor diet, stress, environmental toxins, and even some medications can prevent you from properly digesting food and absorbing nutrients.  

This guide gives a high-level overview of how your digestive system works and common gastrointestinal issues that occur when your digestive system is not working optimally.


How Your Digestive System Works

Digestion starts even before you eat something. Just the sight, smell, or thought of food triggers your brain to send signals to the gastrointestinal tract to prepare for digestion, such as stimulating the flow of saliva or the release of digestive enzymes in the stomach. This is the cephalic phase of digestion.    


The next step is ingesting food. When you start chewing the food, this is an act of mechanical digestion that breaks food down into smaller pieces to increase the surface area where chemical breakdown can occur. The salivary glands in the mouth release the enzymes amylase and lipase to start the breakdown of starches and fats. 


When you swallow food it moves down the esophagus to the stomach, where the gastric phase of digestion begins. In this phase, hydrochloric acid is released into the stomach to start the breakdown of protein as well as stimulate the release of the enzyme pepsin, which also breaks down protein. Some lipid digestion also occurs in the stomach by the enzyme gastric lipase. The highly acidic environment of the stomach kills pathogens on your food so they don’t travel to the intestines. The stomach also releases intrinsic factor, which is necessary for absorbing vitamin B12. Very little absorption of nutrients occurs in the stomach.

Small Intestine

As the food is broken down in the stomach it’s turned into a semi-fluid mass called chyme, which is released into the small intestine to begin the intestinal phase of digestion. Once chyme enters the small intestine, digestive enzymes from the pancreas and bile from the gallbladder are secreted to finish breaking down food into the monosaccharides, amino acids, lipids, vitamins, and minerals that can be absorbed into the bloodstream and lymphatic system. The majority of nutrient absorption happens in the small intestine. The wall of the small intestine is made of up folds covered in small finger-like projections (i.e., villi) to increase the surface area available for nutrient absorption. 

 Large Intestine

Undigested material moves to the large intestine, which is made up of the cecum, colon, rectum, and anal canal. The absorption of water and some electrolytes occurs in the large intestine. The large intestine is also home to trillions of microorganisms known as the gut microbiota. The bacteria in the large intestine feed on any undigested food, such as fiber. Beneficial gut bacteria will also produce some B vitamins and vitamin K as they feed on undigested fiber. Finally, the digestion process ends in the large intestine with the excretion of waste products that were not absorbed during digestion. 


digestive health


Gastrointestinal Disorders

Issues with your body’s digestive system are not uncommon. For the digestive system to work optimally, the body needs to produce sufficient digestive enzymes, stomach acid, and bile to break down foods into a form that can be absorbed by the body. Here are some of the more common gastrointestinal issues that people can experience if their digestive system is not functioning optimally. 


Constipation occurs when you have fewer than three bowel movements per week. You may experience painful bowel movements, bloating, and a feeling like you haven’t fully emptied your bowels. Increasing your fiber intake and drinking plenty of water may help you have more frequent bowel movements. 


Diarrhea is watery, loose stools that occur at least three times per day. It happens when food moves through the large intestine too quickly for sufficient water to be absorbed. There are a variety of causes such as stress, irritation of the intestine, and gluten or lactose intolerance. Ingesting foods with bacteria or viruses may cause diarrhea as your digestive tract tries to quickly remove the pathogens from your body. 

Abdominal Bloating

Bloating occurs when the gastrointestinal tract becomes filled with air or gas, making your abdomen feel full, tight, or swollen. eating slowly and adequately chewing your food can help prevent bloating. 


Heartburn creates a burning sensation in your throat or chest. It occurs when the sphincter between the esophagus and stomach inappropriately opens up and the acidic contents of the stomach move up into your esophagus. Low stomach acid can cause heartburn, since the release of sufficient stomach acid is needed to keep the lower esophageal sphincter closed.  

Lactose Intolerance

Lactose intolerance is the inability to digest lactose, a simple sugar found in dairy products because your small intestine doesn’t produce enough lactase, which is the enzyme needed to digest lactose. Symptoms can include diarrhea, gas, abdominal pain, and bloating after eating dairy. 

Gluten Sensitivity and Celiac Disease

Gluten is a group of proteins found in wheat, barley, and rye. If you’re sensitive to gluten you may experience bloating, diarrhea, or constipation after eating foods that contain gluten. If you have celiac disease your body will have an immune response to gluten and start attacking the small intestine if you consume gluten.

Leaky Gut

Leaky gut is a condition where holes or cracks develop in the junctions along the intestinal lining allowing toxins and bacteria to leak through the intestinal wall. It can lead to food sensitivities, fatigue, skin issues, and other digestive issues. Some factors that may contribute to the increase in intestinal permeability include excessive sugar intake, stress, inflammation, nutrient deficiencies, excessive alcohol consumption, using NSAIDs, 

Small Intestinal Bacterial Overgrowth (SIBO)

SIBO occurs when bacteria that normally reside in other parts of the gastrointestinal tract start growing in the small intestine. This condition can cause diarrhea, bloating, nausea, vomiting, and weight loss. SIBO can lead to malnutrition if the bacteria are feeding off of the nutrients that would normally be absorbed by the small intestine. 

Irritable Bowel Syndrome (IBS)

IBS affects the large intestine and is a chronic condition that can cause lower abdominal pain, cramping, constipation or diarrhea, gas, and bloating. 


More Basic Nutrition Articles

Introduction to Intuitive Eating

Nutrition 101: How to Maintain Blood Sugar Levels

Nutrition 101: What are Macronutrients?

Nutrition 101: What are Probiotics and Prebiotics




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How your digestive system works