Food is ingested and digested using juices and enzymes to allow the assimilation of the nutrients that compose it, before the rest of the food bolus is evacuated. Let's take an overview of the steps that make it up.
#1 – Ingestion
Digestion begins as soon as food enters the mouth, but even a little before, when we see or smell food! These visual and olfactory stimuli trigger the process of salivation, a valuable phenomenon for digestion. Indeed, saliva contains enzymes, amylases, which will begin the digestion of carbohydrates upon chewing. The mechanical action of the teeth is also essential: well-ground foods are more easily digested foods. This is one of the reasons why it is important to take your time and chew your food well, because this automatically facilitates digestion. By focusing on chewing, we also promote the feeling of satiety, because we generate the production of histamine in the brain, it is a neurotransmitter that modulates the sensations of satiety.
#2 – Journey to the stomach
The food bolus passes to the stomach through the esophagus, the opening and closing of which is controlled by sphincters. It is driven by peristalsis: the set of muscular contractions which allow the progression of the food bolus to the stomach, even while lying down! However, we do not recommend it, to avoid any risk of gastric reflux.
#3 – Acid cocktail in the stomach
Arriving in this organ, the food will undergo a more robust treatment combining mechanical and chemical processes. On the one hand, the muscles of the stomach carry out mixing, while the secreted gastric juices attack the food mass to transform it into porridge called “chyme”. The quantities of gastric juices are also around 2 liters per day! Being composed mainly of hydrochloric acid, the juices can cause heartburn when produced in excess. Fortunately, the walls of the stomach are most of the time well protected from this corrosive substance thanks to the mucus it secretes. Once the food bolus is reduced to chyme, it travels to the small intestine.
#4 – The small intestine: the marathon
It is the longest part of the digestive tract, measuring between 5 and 8 meters, but above all it is the main site of digestion: it is there that food is assimilated and made usable by the cells. of the organism. The wall of this organ has one more particularity compared to the other organs of the digestive tract: it is made up of numerous folds, called villi, which considerably increase the surface area for absorption of nutrients. When it enters the small intestine, the chyme will still be watered with digestive juices, in particular those intended for the breakdown of lipids. But throughout the organ, it will be deprived of the nutrients that make it up to pass into the blood or lymphatic circulation.
#5 – Drying in the colon
The few constituents that have resisted the small intestine, that is to say the fibers, certain proteins, and water, will be able to continue their journey in the large intestine, also called the colon. This one is much smaller, between 80 and 150 cm, and frames the small intestine. It is in this organ that the remains of chyme, still liquid, are transformed into a solid mass: the stools. To get an idea: out of 1.5 L of ingested residue, only 150 g of stools are formed, by dehydration, then evacuated. Bacteria play an essential role in the colon, digesting the last nutrients, fermenting plant fibers, and putrefying protein residues. They also produce vitamins and fatty acids which provide energy and benefits.
#6 – Rectum and anus: the finish line
The rectum is the last segment of the digestive tract, it resembles a light bulb. It is once the feces have reached this stage that we feel the need to have a bowel movement. The anus represents the last obstacle for stools, it is controlled by two sphincters.
In reality, we only evacuate 5% of our food intake. Through different mechanisms, the body uses our food bolus to transform food into nutrients which serve as fuel for all organs. Digestion is therefore a crucial process for our health, but nowadays more and more people are affected by digestive problems which impact their overall health. These are often synonymous with an unbalanced microbiota. Indeed, the probiotic bacteria that live in our intestines are responsible for many phenomena related to digestion. For example, certain bacteria are responsible for increasing the assimilation of nutrients, others facilitate the breakdown of nutrients, and others participate in maintaining healthy permeability of the intestines. One of the most studied therapeutic avenues at the moment therefore concerns probiotics, these small bacteria which already populate our intestines, but which we have difficulty maintaining naturally because of our current lifestyles. It is now increasingly accepted that probiotic supplementation helps improve many intestinal disorders.
Sources:
[1] Boudet, S. (2013). From initial conceptions to scientific knowledge: the case of digestion. Doctoral thesis: University Institute of Teacher Training. 95 p.
[2] The secret life of our intestines. Femina version. October 2019.
[3] Wilkins, T., & Sequoia, J. (2017). Probiotics for Gastrointestinal Conditions: A Summary of the Evidence. American family physician , 96 (3), 170–178.
[4] Helander, H.F., & Fändriks, L. (2014). Surface area of the digestive tract - revisited. Scandinavian journal of gastroenterology , 49 (6), 681–689. https://doi.org/10.3109/00365521.2014.898326
