How does digestion work?
Digestion begins as soon as solid or liquid food is taken into the mouth and ends with the excretion of indigestible remains of the food pulp (feces, stool). The average digestion time is 33 to 43 hours, depending on the type of food.
Digestion in the mouth
The first stage of digestion begins in the mouth. Here, food is mechanically crushed by the teeth and mixed with saliva from the three salivary glands (ear, sublingual and mandibular). The saliva, of which between 0.5 and 1.5 liters are produced per day, already contains the first digestive enzymes (for example, ptyalin), which digest the food pulp.
The tongue and cheeks form small portions from the crushed, digested food pulp that can be swallowed easily. In the esophagus, this mush is transported to the stomach by rhythmic contractions of the wall muscles.
Digestion in the stomach
Certain cells in the gastric mucosa (main cells) secrete pepsinogen – the inactive precursor of the digestive enzyme pepsin. It is activated by the acidic environment in the stomach, which is caused by hydrochloric acid. This is released into the interior of the stomach by the vestibular cells (parietal cells) of the gastric mucosa. In addition, these cells produce the “intrinsic factor” – a glycoprotein that is needed in the small intestine for the absorption of vitamin B12 (cobalamin) into the blood.
To prevent the aggressive stomach acid from digesting the stomach wall, it is covered by a protective layer of mucus. The mucus is produced by the so-called accessory cells, another cell type of the gastric mucosa.
Digestion in the intestine
The digestion of fat
Fat digestion begins in the mouth with the fat-splitting enzyme lipase, which is contained in saliva. It continues in the stomach, where fats are emulsified by the motor activity of the stomach wall and further broken down by lipase from the gastric juice.
However, the main part of fat digestion takes place in the small intestine: The small intestinal wall releases the hormone cholecystokinin. It stimulates the pancreas and gallbladder to secrete their secretions into the duodenum. The pancreatic juice contains lipases that break down fats. Among other things, bile consists of bile acids, which are necessary for fat digestion.
Carbohydrate digestion
Carbohydrate digestion also begins in the mouth, with the enzyme amylase. However, on a large scale, it takes place in the small intestine (no carbohydrates are digested at all in the stomach): In the duodenum, carbohydrates are broken down by the pancreatic enzymes amylase, glucosidase and galactosidase.
The digestion of protein
Protein digestion continues in the small intestine. The responsible enzymes come from the pancreas: trypsin, chymotrypsin, elastase and the carboxypeptidases A and B. They, too, are first secreted as precursors and only activated in the intestine.
How long does digestion take?
Absorbed food remains in the stomach for about one to three hours. In the small intestine, the average retention time is seven to nine hours, and in the large intestine 25 to 30 hours. However, it can sometimes take much longer for the indigestible remains to be excreted as stool: The retention time in the rectum is 30 to 120 hours.
What problems can digestion cause?
Digestion can be disturbed due to various causes. For example, a stomach flu (gastroenteritis) triggers diarrhea and vomiting.
Irritable bowel syndrome (colon irritabile) is associated with abdominal cramps, flatulence, diarrhea or constipation.
In celiac disease (gluten intolerance), the digestion of cereals is disturbed: the body cannot tolerate the protein gluten it contains. The mucous membrane of the small intestine is damaged, which also impairs the absorption of other nutrients.
