Digestion: this is how it works
Martina Feichter studied biology with an elective subject pharmacy in Innsbruck and also immersed herself in the world of medicinal plants. From there it was not far to other medical topics that still captivate her to this day. She trained as a journalist at the Axel Springer Academy in Hamburg and has been working for since 2007 - first as an editor and since 2012 as a freelance writer.
More about the experts All content is checked by medical journalists.
Carbohydrates, proteins and fats are the main nutrients in humans. But in order for the body to be able to use them, it first has to mechanically chop them up and break them down with the help of enzymes. That is exactly what digestion does!
Soon nothing works without eating and drinking: The human body depends on receiving sufficient fuel in the form of food as regularly as possible. On the way from the oral cavity to the intestine, schnitzels, bananas, rolls & Co. are broken down into usable and absorbable components by mechanical and chemical digestion. The indigestible remains of the chyme are excreted as stool.
Starting shot in the mouth
Digestion starts in the mouth with the mechanical crushing of the food ingested: the teeth chew each bite into smaller pieces. Hardly anything can oppose them: The teeth (more precisely the tooth enamel) are the hardest substance in the body.
The strong tongue muscle mixes the chyme and mixes it with the saliva that is released into the oral cavity by the various salivary glands. These deliver around 1.5 liters of the aqueous secretion every day.
Attack of enzymes
By coarsely chopping food up, the teeth not only make it easier to swallow. This also increases the surface area of the food and thus the attack surface for the digestive enzymes contained in the saliva. They set the chemical (enzymatic) digestion in motion: the so-called alpha-amylase breaks down carbohydrates, such as those found in bread, first into larger pieces and then - if you chew long enough - into double sugar. That's why a long chewed bite of bread tastes sweet at some point.
Saliva also contains what is known as the base lipase of the tongue, which can break down fats. This enzyme plays an important role mainly in newborns. The reason: In newborns, the function of the pancreas - which in adults supplies most of the fat-splitting enzymes - is not yet fully developed.
Transport down
The mucous substances in the saliva make the food pulp more slippery so that it can be swallowed more easily. The musculature in the wall of the esophagus transports the pulp with peristaltic movements down to the stomach.
Acid attack in the stomach
There it suddenly becomes really acidic thanks to the stomach acid: The hydrochloric acid in the gastric juice can lower the pH value to 1 to 1.5. It's even more acidic than vinegar (pH 3). As soon as it lands in the stomach, the chyme reduces the acidity that prevails here, but only slightly to pH values of 3 to 4.
Gastric enzymes like it sour
This is a good thing, because the acidic environment is absolutely necessary for the chemical digestion of food: the hydrochloric acid activates the enzyme precursor pepsinogen into the active enzyme pepsin. This immediately begins to break down proteins in the food. In principle, the pepsin would not stop at the stomach wall, which also consists of plenty of protein. A protective mucous coating on the inner wall prevents the stomach from digesting itself in this way.
Along with the food pulp, enzymes have also slipped from the mouth into the stomach. The base lipase of the tongue is not affected by the high degree of acidity that prevails here. It continues to work on the digestion of fats - together with the fat-splitting enzyme of the stomach (gastric lipase). The amylase, on the other hand, doesn't like it so sour. The digestion of the carbohydrates therefore pauses in the stomach and only continues in the intestine.
Acid kills germs
In addition to activating enzymes, the low pH value in the stomach has another important function: it kills microorganisms that have been ingested with food. In this way, the porridge is sterilized, so to speak.
Farewell in portions
Through contractions, the muscular stomach wall ensures that the gastric juice, digestive enzymes and chyme are well mixed. The whole mixture is called chyme. At the exit of the stomach it is finally released in portions into the next section of the digestive tract by a sphincter muscle (called gastric gatekeeper or pylorus): the three to five meter long small intestine.
Next station: small intestine
The chyme from the stomach is received in the first section of the small intestine, the duodenum, by a new load of digestive juices - from the secretions of the liver and the pancreas.
The pancreatic juice
The secretion of the pancreas contains bicarbonate - a substance that is also found in baking powder as a propellant: It neutralizes the chyme acidified in the stomach, because otherwise the enzymes in the small intestine would not be able to work.
These enzymes are also supplied by the pancreas. There are mainly amylases (for carbohydrate digestion), proteases (for protein digestion) and lipases (for fat digestion). Some of these enzymes are released from the pancreas as inactive precursors and only activated in the small intestine.
The bile
The liver produces the bile, which, after being temporarily stored in the gall bladder, drives the digestion of fat in the small intestine: the bile acids contained in it ensure that the dietary fats are emulsified - otherwise they cannot mix with the aqueous digestive secretions. The emulsification creates numerous small fat droplets, which can then be attacked by the fat-splitting enzymes from the pancreatic juice (pancreatic lipases).
Balance of fat digestion
Most of the fat digestion takes place in the small intestine and only to a lesser extent in the mouth and stomach. The resulting breakdown products (such as free fatty acids) and fat-soluble vitamins are then absorbed through the intestinal wall with the help of the bile acids.
Balance of carbohydrate digestion
The carbohydrates from pasta, bread, potatoes and biscuits mainly consist of multiple sugars (polysaccharides such as starch), sometimes also of double sugars such as sucrose (table sugar) or lactose (milk sugar). They are broken down by the amylases in the mouth and especially in the small intestine into the simple sugars (monosaccharides) glucose, fructose and galactose. Only in this form can the carbohydrates reach the blood through the intestinal wall.
Balance of protein digestion
The proteins in food are broken down in the stomach and especially the small intestine by the protein-splitting enzymes into individual amino acids or short chains of two or three amino acids (di- and tripeptides). These can then be absorbed through the intestinal wall.
And what happens in the colon?
All nutrients that can be used by the body are absorbed in the small intestine. The rest is transported into the large intestine by peristaltic movements of the intestinal wall. A large part of the water contained is removed from the food residues here.
In addition, the intestinal bacteria living here attack the food residues: some of the indigestible components (fiber) can be used by the microbes for energy production. This often produces gases (methane, hydrogen and carbon dioxide) - between 400 and 1,500 milliliters per day. As intestinal winds (flatulence), they escape through the anus to the outside.
The rest of the food components, with which the intestinal bacteria cannot do anything, is finally excreted in the stool. In addition, the stool consists of rejected cells from the intestinal mucosa and bacteria from the intestinal flora.
Bowel movement: how often is normal?
How often someone does their “big business” depends on many different things, such as the amount and composition of the food. A diet rich in fiber (such as whole grains, vegetables, fruit) stimulates digestion, while a diet low in fiber (white flour products, sweets, etc.) makes the intestine sluggish.
The range of the "normal" stool frequency is wide: some people have bowel movements three times a day, while others have a much slower digestion and only empty their bowels three times a week. For the doctor, both are considered normal.
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