How the Small Intestine Absorbs Nutrients

The small intestine does not absorb every nutrient through one route. Folds, villi and microvilli create a large surface, while specialised cells finish digestion and move different substances into blood or lymph.

Three sections share the work The small intestine has three sections called the duodenum, jejunum and ileum. Their functions overlap rather than dividing digestion into three completely separate jobs. The duodenum receives chyme from the stomach, bile and pancreatic secretions. The jejunum absorbs much of the digested carbohydrate, protein, fat, water, vitamins and minerals. The ileum continues absorption and is especially important for vitamin B12 and recycled bile salts. The exact contribution of each section varies with the nutrient and the person's anatomy. Folds, villi and microvilli increase surface area The inner wall has circular folds covered by finger like villi. Each villus is lined by absorptive cells called enterocytes. Tiny projections called microvilli cover the exposed surface of each enterocyte. Together, folds, villi and microvilli create a large area for digestion and absorption. A villus contains blood capillaries and a small lymphatic vessel called a lacteal. This arrangement gives absorbed substances access to different transport routes. The lining is thin, but absorption is controlled. Nutrients cross through or between cells using processes suited to their size and chemical properties. Digestion continues at the intestinal surface Pancreatic enzymes break carbohydrates, proteins and fats into smaller parts within the intestinal contents. Bile disperses fat into small droplets and supports the formation of structures called micelles. Enzymes attached to the microvilli complete several final digestion steps. These surface enzymes are often called brush border enzymes. For example, lactase breaks lactose into smaller sugars before absorption. Reduced lactase activity can cause lactose intolerance, but it does not damage the intestine. Sugars and amino acids mainly enter portal blood Most digested carbohydrates are absorbed as simple sugars. Digested proteins are absorbed mainly as amino acids and very small peptide fragments. These products enter capillaries within the villi. Nutrient rich blood then travels through the hepatic portal vein to the liver. The liver processes, stores or redistributes many absorbed substances. This first route through the liver is called portal circulation. Not every water soluble substance follows an identical transporter or absorption site. The shared portal route is a useful overview, not one mechanism for every nutrient. Most long chain dietary fat enters lymph first Fat digestion produces fatty acids and other lipid components. Bile supported micelles help deliver many of these components to enterocytes. Inside the cells, many long chain fats are rebuilt and packaged with proteins into particles called chylomicrons. Chylomicrons enter lacteals because they are too large to pass readily into villus blood capillaries. Lymph carries them towards larger lymphatic vessels before they enter the bloodstream. This route does not mean that every type of fat enters lymph. Shorter fatty acids can enter portal blood more directly. Fat soluble vitamins often travel with absorbed dietary fat, while other vitamins and minerals use different routes. Water and electrolytes also move across the lining The small intestine absorbs large amounts of water and electrolytes from food, drinks and digestive secretions. Water movement follows dissolved particles and pressure differences across the lining. Absorption is substantial but not complete. Material normally continues into the large intestine, which absorbs more water and electrolytes. The intestine's length and movement support contact with the lining. They do not guarantee that every nutrient is fully extracted from every meal. Malabsorption has several possible mechanisms Malabsorption means that the digestive system does not absorb enough of one or more nutrients. It can result from poor digestion, lining damage, reduced absorptive area, impaired bile or pancreatic function, or altered movement. Different conditions therefore cause different patterns. Damage to villi can affect several nutrients, but one nutrient deficiency does not prove widespread villus damage. Symptoms such as diarrhoea, bloating, weight loss, tiredness or greasy stool can occur with malabsorption. They also have many other possible causes. Tests provide selected clues Blood tests can identify anaemia or low levels of selected nutrients. They cannot show where the problem began or prove malabsorption by themselves. Stool tests can assess infection, inflammation or selected features of fat or pancreatic digestion. The test chosen depends on the clinical question. Coeliac blood tests look for selected antibodies. The person needs to keep eating gluten during testing because reducing gluten can make results less reliable. Do not start a gluten free diet before assessment when coeliac disease is suspected. Follow professional advice because later blood tests and biopsy may otherwise be harder to interpret. Gastroscopy can reach the duodenum and allow a biopsy of the lining. A biopsy samples a small area, and its findings must be combined with the person's symptoms and background and other tests. Know when to seek help Arrange a GP appointment for ongoing diarrhoea, unexplained weight loss or symptoms suggesting a nutrient deficiency. These patterns deserve assessment but do not identify one diagnosis. Ask for an urgent GP appointment or contact NHS 111 for bloody diarrhoea, black or dark red stool, persistent vomiting or signs of significant dehydration. Call 999 or go to A&E for severe abdominal pain, collapse, confusion, severe breathing difficulty or vomiting blood. Do not drive yourself.

The small intestine uses folds, villi, microvilli, enzymes and specialised transport routes. Sugars and amino acids mainly enter portal blood, while most long chain dietary fats enter lymph as chylomicrons.