How Digestion and Absorption Work

Digestion is the process of breaking food down into nutrients; absorption is the passing of those nutrients through the gut lining into the blood so the body can use them. It starts in the mouth, where chewing and saliva begin to break food down. In the stomach, acid and enzymes (proteins that speed up chemical reactions) turn food into a thick liquid. In the first part of the small intestine, bile from the liver and gallbladder helps to digest fat, and enzymes from the pancreas break down protein, fat and starch. Most nutrients are then absorbed along the small intestine: iron mainly in its first part, and vitamin B12 and bile salts in its last part (the ileum). What is left reaches the colon (large bowel), which absorbs water and salts, and where gut bacteria ferment fibre. The leftovers leave the body as stool. Muscles in the gut wall move food along in waves, and a network of nerves and hormones coordinates the whole process.

Everyone digests food, and digestive symptoms are very common. In a large survey of people in 33 countries (the Rome Foundation Global Study, published in 2021), about four in ten adults met the criteria for at least one functional gut disorder, meaning a disorder of how the gut works rather than one with visible damage. Problems with actual absorption (malabsorption) are much less common but are important to recognise.

Digestion can be disturbed at any step. Examples include a lack of an enzyme (for instance lactase, which digests milk sugar, so lactose intolerance causes gas and loose stools after dairy), too little pancreatic enzymes or bile, damage to the lining of the small intestine (as in coeliac disease, where gluten triggers the immune system to injure the gut), infections, medicines, surgery that removes or bypasses part of the gut, and changes in how fast food moves. Eating very quickly, large fatty meals, and stress can also trigger symptoms without any disease. Many symptoms come from the gut being over-sensitive rather than damaged.

Your doctor starts with your story: what you eat, when symptoms occur, stool pattern, weight, medicines and family history. Blood tests may look for anaemia, low iron, vitamin B12 or vitamin D, and coeliac disease antibodies. Stool tests can check for infection, inflammation (such as faecal calprotectin) or pancreatic enzyme output. Breath tests are sometimes used for lactose intolerance, though their results need careful interpretation. Endoscopy (a camera test of the upper gut with small tissue samples called biopsies) and colonoscopy look directly at the lining. A food and symptom diary kept for two weeks is often very helpful.

For most people with occasional digestive symptoms, simple changes in how and what they eat bring clear improvement. If a specific cause such as coeliac disease, lactose intolerance or enzyme deficiency is found, targeted treatment usually restores comfortable digestion and good nutrition. Early assessment of warning signs allows serious conditions to be found at a more treatable stage.