Here is how digestion works, step by step: after you eat, food travels through a long, hollow tube that runs from your mouth to your anus. Muscles push and mix it, and digestive juices break it into nutrients small enough to pass into your blood. Your body uses those nutrients, and what is left leaves as stool.
This is general information, not medical advice. Talk to your doctor or pharmacist about your own situation.
Your digestive system has two kinds of parts. The first is the GI tract, a chain of hollow organs: the mouth, esophagus, stomach, small intestine, large intestine and anus. Cleveland Clinic says the whole tract would be about 30 feet (9 meters) long if you stretched it out. The second is a set of solid organs that make or store digestive juices: the liver, the pancreas and the gallbladder.
Digestion matters because your body cannot use a sandwich as it is. It needs the protein, fat, carbohydrates, vitamins, minerals and water inside it, broken into pieces small enough to absorb. NIDDK explains that your body uses those pieces for energy, growth and cell repair.
Before the first bite. Seeing or smelling food can make your mouth water. Your brain signals your salivary glands to get ready.
Mouth. Chewing breaks food into smaller pieces, and saliva moistens it. Saliva also contains an enzyme that starts breaking down starches. When you swallow, a small flap called the epiglottis folds over your windpipe so food goes the right way.
Esophagus. Food does not just fall to your stomach. Waves of muscle, called peristalsis, squeeze it along: the muscle behind the food tightens while the muscle ahead relaxes. At the bottom, a ring of muscle relaxes to let food into the stomach.
Stomach. The stomach's muscles mix food with acid and enzymes, which are especially good at breaking down protein. The result is a mixture called chyme, which the stomach slowly empties into the small intestine.
Small intestine. This is where most of the work gets finished. The pancreas sends in juice with enzymes that break down carbohydrates, fats and proteins. The liver makes bile, which helps digest fats and some vitamins. The gallbladder stores bile and releases it after meals. The walls of the small intestine then absorb water and nutrients into your bloodstream.
Large intestine. Whatever is left, including undigested food, fluid and old cells from the gut lining, moves on. The large intestine absorbs more water and turns the waste from liquid into stool.
Rectum and anus. The rectum stores stool until a bowel movement pushes it out through the anus.
Carbohydrates are broken into simple sugars, which pass into your blood.
Proteins are broken into amino acids, which also pass into your blood.
Fats are broken into fatty acids and glycerol. Glycerol goes into your blood, while the lymph system absorbs the fatty acids.
Vitamins take both routes: some go into your blood and others into the lymph system.
Water is absorbed in the small intestine and again in the large intestine.
Your blood carries sugars, amino acids, glycerol, some vitamins and salts to the liver. The liver stores, processes and sends out nutrients as the rest of your body needs them.
Take a slice of toast with peanut butter as an example. The starch in the bread starts breaking down in your mouth. The protein in the peanut butter gets a head start in your stomach. The fat waits for bile and pancreatic juice in the small intestine. By the time the meal reaches your large intestine, most of its nutrients have already been absorbed.
Mayo Clinic says food takes about six hours on average to move through the stomach and small intestine. Time in the large intestine can take up to 36 to 48 hours. Add those together and a meal can take around two days to make the whole trip. Timing varies from person to person and depends on what you ate and how much.
Hormones. Cells in your stomach and small intestine release hormones that switch on digestive juices and help signal hunger or fullness.
Nerves. Your gut has its own network of nerves in its walls. When food stretches the gut, these nerves adjust how fast things move and how much juice is released.
Gut bacteria. The bacteria living in your gut, often called the gut microbiome, help with digestion. In the small intestine they make some of the enzymes needed to digest carbohydrates. In the large intestine they break down leftover nutrients and make vitamin K.
Cleveland Clinic suggests a few simple habits:
Drink enough water for your size and activity level.
Get fiber from balanced meals with fruits, vegetables and lean protein.
Eat mindfully and chew your food well.
Avoid alcohol and smoking. Cleveland Clinic notes smoking almost doubles your risk of acid reflux.
Find ways to manage stress, which is linked to constipation, diarrhea and irritable bowel syndrome.
Cleveland Clinic advises seeing a provider about any digestive symptom that lasts more than a few days, gets worse, or goes away and comes back. The NHS says to get an urgent appointment if you have stomach pain or bloating that does not go away or keeps coming back, trouble swallowing food, weight loss without trying, bleeding from your bottom, or diarrhea that has not gone away after a few days.
Call your local emergency number right away for sudden or severe stomach pain, pain when you touch your stomach, vomiting blood or vomit that looks like coffee grounds, bloody or black and sticky poo, being unable to poo or pass gas, or chest pain.
Yes. Chewing breaks food down, and an enzyme in saliva starts breaking down starches. Your mouth even starts making saliva when you see or smell food.
In the small intestine. Juices from the pancreas, liver and the intestine itself finish breaking down food there, and its walls absorb most of the nutrients into your blood.
The liver makes bile, which helps digest fats and some vitamins. It also receives nutrient-rich blood from the gut, then stores and processes those nutrients for the rest of your body.
They help with digestion. They make some enzymes for digesting carbohydrates, break down nutrients your body did not absorb, and make vitamin K.
If you learn better by seeing it, Body Unpacked: Health Atlas is a free iPhone app about how food and the body connect. Its Physiology Lab steps through a teaching model of digestion, and Nutrient Journeys follow 14 nutrients through the body on an animated map, from absorption to excretion, each with a source and a short quiz.
It is built for learning, not diagnosis. There is no account and no ads, and the content works offline.
Get Body Unpacked free on the App Store