Digestion Explained Simply
What is actually digestion?
On the surface, digestion seems like a simple process: You are hungry, so you eat and your energy gets replensished. The process then begins from the beginning again. But the processes taking place within our body are actually very complex with each organ having a very specific task. Our body’s work can be made easier or harder depending on the food we give it. In this article you will learn more about the efficiency of your digestive system, the organs involved and their respective tasks.
It all starts in the mouth
Digestion begins in the mouth with the mechanical break down of food by chewing. During this step, the food is coarsely crushed by the teeth with the help of enzymes in the mouth. First carbohydrate chains are split by the enzymes. The long-chain carbohydrate “starch” is split into smaller, sweet-tasting carbohydrates (mono- and disaccharides), which is why wholemeal bread, for example, tastes slightly sweet after a while.
By eating unconsciously with a smart phone in your hand you chew less and end up swallowing half chewed food.
The action of swallowing takes the food, which has been softened by saliva, through the throat and oesophagus to the next level of digestion.
The stomach is responsible for several tasks. Food is temporarily stored here while it is pre-digested. The stomach wall contains glands, which together with their secretions form the stomach lining and prevent the stomach from digesting itself. The stomach has an acidic environment due to the secretion of enzymes and acids and would otherwise damage itself. The so-called gastric acid has a PH value of 1. To put this into context: water has a value of 7 on a PH scale of 1 – 14 (1 = acidic, 14 = alkaline). The acidic gastric juice coarsely dissolves fat droplets and denatures protein chains, making them easier to break down.
The stomach also contracts and relaxes in order to mix the food and gastric juice. Carbohydrates are not digested any further in the stomach because the carbohydrate-splitting enzyme amylase is inhibited. The lining cells of the stomach mucosa form a protein that is necessary for the absorption of vitamin B12. You can find everything you need to know about vitamin B12 in our vitamin guide. The stomach coarsely breaks down food into smaller components and passes it on to the duodenum. The third task of the stomach is to neutralize the food, i.e. to kill bacteria and disinfect it so that no germs can enter the body.
Depending on the type of food and the proportion of nutrients, the food is in the stomach for different lengths of time. A high fat content, for example, extends the time in the stomach.
The intestine is where most of the digestion takes place. Almost all the nutrients and water are absorbed in the duodenum, the narrow small intestine and the wider large intestine.
The food continues from the stomach into the duodenum in a controlled manner via a locking ring. Here, the enzyme digestion that begun in the mouth is continued. In order to activate the enzymes (e.g. amylase) and thus the digestion, the food bolus, that was acidified by the gastric juice, must be re-neutralised. This task is performed by glandular secretions. The food bolus is then enriched with enzymes produced in the pancreas, which further split up the components of the food. The bile ensures further digestion of the fats.
The small intestine is responsible for the absorption of nutrients during digestion. It is five to six metres long and has a very large surface area due to its bulges, depressions and folds. A fine fluff of microvilli increases the surface area to about 200m². This maximises absorption of nutrients and water through the intestinal mucosa. The intestinal juice is secreted via glands, which dissolves the nutrients into their smallest water-soluble components. These components are finally released into the blood via the intestinal wall. The blood then distributes the nutrients to the required organs and tissues or to storage locations.
In the small intestine, the carbohydrates are broken down into their smallest components. Disaccharides are split into glucose and fructose. Lactose is split into glucose and galactose. People with a lactose intolerance lack the enzyme needed to break down lactose and therefore have problems digesting milk sugar. Our articles give you all the necessary nutritional advice regarding intolerances.
The bile helps mix the fats with the digestive enzymes, which facilitates digestion. Protein is split by enzymes secreted in the duodenum (such as trypsin) into amino acids which are absorbed into the blood through the intestinal wall. Fat-soluble vitamins are absorbed and stored in the liver and fatty tissue. Excess water-soluble vitamins that are not required are excreted via the kidneys.
The small intestine absorbs eight to nine litres of water daily.
The large intestine (also called the colon) is responsible for further recovery of water and salts. It has a larger diameter than the small intestine and is about one meter long. However, the wall is smooth and not enlarged by protrusions or depressions. Almost 90 percent of the water in food can be absorbed in the large intestine along with the electrolytes.
All that remains at this point is fibre that cannot be processed by the body and the enzymes present. Dietary fibres are divided into soluble and insoluble ones. Soluble ones are easily soluble in water and result in a gel-like consistency which is conducive to transport. In the large intestine, they are partially broken down by bacteria and the fatty acids produced are absorbed. The bacteria in the large intestine also produce vitamins such as B12, but these can only be absorbed to a limited extent.
You can find more information on dietary fibres in our bloated belly article.
Some microorganisms such as bacteria form the intestinal flora. As described, they are responsible for the breakdown of cellulose, i.e. dietary fibre. The sensitivity of these organisms becomes apparent when taking antibiotics. Antibiotics destroy the bacterial strains and this can lead to digestive problems, since the bacteria takes six months to regenerate. In addition, the bacteria change the food pulp into faeces through gases and dyes.
The bile for the digestion of fats is produced here and then stored in the gall bladder. From there it reaches the intestines via various glands. In addition, all the absorbed nutrients from the intestines reach the liver via the blood. There, currently unneeded nutrients (vitamins, fats, carbohydrates, etc.) are removed from the blood, stored and released back into the blood as needed.
The nervous system and musculature
The body influences digestion via the nervous system. The parasympathetic nervous system is responsible for relaxation and provides most of the required blood to the digestive tract. This is inhibited by sport. Our Crash Diet article explains this antagonistic principle.
The digestive tract consists mainly of smooth muscles. These function unconsciously and cannot be controlled. The nervous system described above provides for the necessary stretch attraction in the final intestine and thus for a brisk emptying of the intestine. These increase or decrease the transport and thus the digestion speed.
With a surface area of 300 – 500m², our digestive tract forms the largest point of contact to the outside world. A great variety of particles from the outside world enter our body via food. This does not only include nutrients, but also toxins such as bacteria and pathogens.
For this reason, a large part of our immune system is located inside our intestine in order to protect it sufficiently. A healthy diet is important, because a healthy intestine is often the reflection of a healthy body and vice versa. Dietary fibres make transport much easier for our body, “fatty” food makes for slower transport and can bring digestion to a standstill.
Our body is dependent on a healthy, balanced diet. Since food is absorbed into our blood and inevitably transported into our body, it is important to eat smart. The body remembers everything and will not forgive you if you have eaten badly for the past 20 years. Treat your body like a temple, give it the best food.
- Campbell, Neil A.; Reece, Jane B.; Urry, Lisa A. (2018): Biology. 10th, updated edition. Edited by Jürgen J. Heinisch and Achim Paululat.