Gut basics
What is digestion? How food moves through your body
Digestion is the process of breaking food down into molecules small enough to be absorbed into the blood. It begins in the mouth, continues in the stomach and is completed in the small intestine, where almost all nutrients are absorbed. What remains passes to the large intestine. The whole journey usually takes one to three days.
In short
- Digestion has two parts: mechanical (chewing and churning) and chemical (acid, bile and enzymes).
- Most digestion and nearly all absorption happen in the small intestine.
- The large intestine absorbs water and lets bacteria ferment what is left.
- How you eat matters as much as what you eat: pace, timing and chewing all affect digestion.
What happens at each stage?
Mouth: seconds to a minute
Teeth grind food into small pieces. Saliva moistens it and starts to break down starch. Thorough chewing reduces the work the stomach has to do.
Food pipe: a few seconds
A wave of muscle contraction carries each swallow to the stomach. A valve at the lower end opens to let it in and closes to keep it there.
Stomach: two to four hours
The stomach stores the meal, mixes it with acid and enzymes that begin to digest protein, and releases it in small amounts. Fatty and very large meals leave more slowly.
Small intestine: three to six hours
Bile from the liver breaks fat into droplets. Enzymes from the pancreas digest fat, protein and carbohydrate. The lining, folded into millions of tiny projections, absorbs the nutrients into the blood.
Large intestine: one to two days
Water and salts are absorbed. Bacteria ferment fibre, producing substances that nourish the lining of the colon. The remainder is formed into stool.
What controls digestion?
Digestion is coordinated by the gut’s own nervous system and by hormones released from the gut wall. The sight and smell of food begin the flow of saliva and stomach juices. Food arriving in the stomach signals the colon to move, which is why many people feel the urge to pass stool after a meal.
Stress shifts the body away from “rest and digest”. Blood flow to the gut falls, and its movements change, which is why hurried or anxious meals are followed by discomfort.
What helps digestion?
- Sit down to eat, and take about twenty minutes over a meal
- Chew each mouthful well
- Eat at regular times, with the last meal two to three hours before bed
- Include fibre from vegetables, fruit, pulses and whole grains
- Drink enough water through the day
- Walk for ten to fifteen minutes after meals
What disturbs it?
Meal habits
Very large, late or hurried meals, and long gaps followed by overeating.
Medicines
Painkillers, some diabetes medicines, iron tablets and antibiotics can all upset digestion.
Conditions of the gut
Reflux, gastritis, gallstones, coeliac disease, bacterial overgrowth and disorders of gut movement.
Lifestyle
Poor sleep, long hours of sitting, alcohol and tobacco.
When does indigestion need attention?
Occasional fullness or gas after a heavy meal is normal. See a doctor if discomfort after meals lasts for more than a few weeks, or if you have difficulty swallowing, vomiting, weight loss, black stools or pain that wakes you at night. These are assessed with an examination and, where needed, an endoscopy.
Questions patients ask me
Is it harmful to drink water with meals?
No. A glass of water with a meal does not dilute digestive juices in any way that matters. Drink as you feel comfortable.
Why do I feel sleepy after lunch?
A large meal, particularly one rich in refined carbohydrate, draws blood flow to the gut and changes blood sugar and hormone levels. A smaller lunch with more protein and vegetables, followed by a short walk, usually helps.
Do I need digestive enzyme supplements?
Most people make all the enzymes they need. Enzyme supplements have a clear role in a few conditions, such as disease of the pancreas and lactose intolerance. They are not needed for routine indigestion.
Why does food seem to pass through me quickly after eating?
What you pass after a meal is from an earlier one. Eating triggers a reflex that makes the colon contract. In people with a sensitive gut this reflex is stronger.
Sources
This page is written for patients and is based on the guidelines and studies below. It is for education and does not replace a consultation.
- Review of the enteric nervous system. Furness JB. Nat Rev Gastroenterol Hepatol. 2012. PubMed 22392290
- Systematic reviews of carbohydrate quality and human health. Reynolds A et al. Lancet. 2019. PubMed 30638909
- American College of Gastroenterology guideline on gastroparesis. Camilleri M et al. Am J Gastroenterol. 2022. PubMed 35926490