The purpose of this article is to provide a basic acquaintance with the digestive system and its functions, which are the keys to proper and personalized nutrition.
Our digestive system is a long and very complex pipeline. It is through this pipe that the food we put into our bodies passes, breaks down, is partially absorbed, and finally, the remains exit as feces. We have a tendency to dismiss the importance of understanding how this system works; most of us think that whatever we put in will simply be digested, regardless of what it is.
But this is not the case. Our digestive system is sensitive, and its activity affects our development, our health, and the ongoing operation of all our body systems. If we do not digest food properly, the body cannot build itself or nourish its organs. If we eat unsuitable and harmful food, the situation will only worsen.
Let us compare the digestive system to our largest organ: the skin. Like the skin, the digestive system is exposed to and affected by what we put into it, with the main difference being that we cannot see it. If we put a substance on our skin that causes sensitivity, we immediately see it, feel it, and avoid it. In the digestive system, we might feel gas, stomach pain, or heartburn, but because we don’t see the damage, our willingness to stop consuming the irritant is lower.
Compared to the skin, the digestive system is much larger. If we were to take it with all its folds and curves and spread it out entirely, it would cover a whole tennis court. Cells located along the entire system secrete about seven liters of water, hormones, and various enzymes during a single day to assist in the digestive processes. [1] Understanding how it works and being attentive to its needs are essential skills for maintaining health.
The evolution of the digestive system is examined in two ways: by looking at the structure of the organs and by examining the movement of food (gut kinetics). The evolutionary story told by both measures is that of a herbivorous digestive system, the kind found in mammals whose primary food comes from plants. [2]
On the Movement of Food in the Digestive System
Regarding kinetics, what determines the nature of the digestive system is not necessarily the food that enters it (herbivorous animals sometimes eat small prey or insects), but the nature of the movement. Movement in the system can be very slow (in herbivorous animals) or very fast (in carnivorous animals). [3]
In a herbivorous digestive system, animal-based food will stay for 26 hours or more, while in a carnivore’s system, food stays for about 2.6 hours. [4] In the human digestive system, food stays for 26 hours or more, and meat passes through the human system even slower than plant food. [5] Thus, in terms of movement, our digestive system is similar to that of herbivorous animals, those that tend to eat more plant-based food.
However, there are some differences in the structure of the digestive system between different herbivores, differences likely created during evolution due to different nutritional strategies. For example, in humans, the small intestine is longer than the large intestine, whereas in most monkeys, the large intestine (which processes fibers) is the larger and more significant part. This is likely because humans consumed fewer fibers during their evolution. [6]
What creates the movement of food in the intestines is peristalsis. Peristaltic movement is a wave-like motion created by involuntary smooth muscles found along the entire digestive tract and activated by the autonomic nervous system, similar to the heart muscle. Often, our stomach pains come in waves. This is clearly seen when children suffer from such pain. These waves are the peristaltic waves. Something bad we ate or an inflammation in the intestines hurts intensely every time the system creates a wave trying to move the food forward through the pipeline.
Those Responsible for Regulating Food Movement: Dietary Fiber
Dietary fibers are polysaccharides (chains of sugars) that we are unable to break down in the intestines. Fiber can be found in any whole or natural plant food: in whole grains and legumes, and especially in leaves, vegetables, and fruits.
Even though we seemingly do not need them to nourish the body, since they don’t break down and thus aren’t absorbed from the intestines, fibers have significant roles in the digestive process and are essential to it. The first and primary role of fiber is regulating the movement of food (peristalsis). They are responsible for the regulation of food movement within the pipes, and as a result, they also regulate the rate of nutrient absorption and affect the digestion of fats, sugars, and more. Additionally, they serve as a substrate for the growth and activity of the microbiome (the bacterial system) in the large intestine. [7]
The Regulation of Food Movement in the Intestines
The role of fiber in regulating food movement is well-known. One of the earliest and most interesting studies on the subject was by Dr. Denis Burkitt, an Irish physician and researcher who worked extensively in Africa in the second half of the 20th century. During his work, Burkitt noticed that he saw almost no serious bowel or digestive diseases on the continent, while he witnessed a worrying rise in such diseases in the Western world, particularly in Britain.
He argued that the transition from a traditional diet to a modern Western diet brought about the increase in bowel diseases, and the key change was the sharp decrease in fiber consumption. He discovered that a low-fiber diet significantly slows down the speed of food movement and reduces stool weight. In his study, he had people swallow 25 tiny plastic pellets and checked how long it took for them to exit. Among 100 white South African students eating a Western diet, the average transit time was 48 hours. Among 500 black South Africans eating a traditional diet, it was 33 hours, and their stool weight was nearly double. Burkitt links the recurrence of various Western diseases to the beginning of white flour production and the decline of fiber. [8] Burkitt was also a researcher who strongly advocated for the natural squatting position during bowel movements. He wrote a global bestseller titled: Don’t Forget Fibre in Your Diet to Help Avoid Many of Our Commonest Diseases.
There is currently a wide research debate about the role and benefit of fiber in human nutrition. [9] However, the prevailing view among researchers is that fiber is essential for intestinal and digestive health and contributes to food movement, the development of a diverse bacterial system, and overall gut health. [10]
Substrate for the Bacterial System
Beyond regulating movement, researchers are increasingly studying the role of fiber as a substrate for bacteria in the large intestine. Bacteria cannot thrive without food, and the “soil” they grow on is the dietary fiber we eat. Even if we introduce many bacteria into the system, they will not survive without the proper substrate.
It is now known that the modern Western diet creates a microbiome completely different in composition from that created by a traditional diet. Researcher Jeff Leach argues that prebiotics were an integral part of the ancient diet, with evidence showing significant consumption of roots rich in inulin and other fibers across the globe. These fibers are significant for the thriving of beneficial Lactobacillus bacteria in our intestines. [11] More and more studies point to the role of fiber in creating a healthy fermentation environment for gut bacteria and its impact on our health. [12]
To clarify terms: the fibers we consume from whole plant foods, which serve as the food for bacteria, are called Prebiotics. This is different from Probiotics, which are the bacteria themselves. Many people take probiotic capsules after antibiotics, but there are many debates about their effectiveness. [13] Evidence increasingly points not only to their ineffectiveness in some cases but to the potential damage they can cause.
The food industry often fortifies products with fiber or probiotics to give them a health halo. This has been done recently with fiber in industrial chocolate with artificial sweeteners. The addition of fiber is indeed important, but it does not erase the harm of the base product if it is damaging. This is an attempt to whitewash harmful food instead of producing food from nutritious ingredients. A balanced, non-industrial diet provides all the fiber a human needs.
The Structure of the Digestive System and its Basic Functions
Where does digestion begin? Unlike what many think, it starts in the mouth. The mouth has two roles: chewing, which prepares the food for the stomach, and the initial breakdown of substances by enzymes in the saliva. It also serves as a gatekeeper. If something is unsuitable or tastes bad, we immediately spit it out. This is a healthy and important instinct.
From the mouth, food travels through the esophagus to the stomach. The stomach is a muscular sac that breaks food down into a paste before it reaches the intestines. Much like a concrete mixer, it uses peristaltic movement to break down food chemically and mechanically. The stomach has different areas with unique roles and secretes substances to assist in the process. [14]
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Capacity: When inactive, the stomach shrinks to about 55ml. At full capacity, it reaches 1.5 liters. But like a mixer, when it is too full, it cannot do its job. If we eat more than a liter in volume, we damage the digestive process and become more thirsty, as the stomach will signal it needs water to dilute digestive juices.
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Maimonides (The Rambam) wrote on this: “The main thing is to avoid satiety that leads to stretching the stomach… if the stomach is stretched beyond its nature, its activity weakens and it will not be able to push the food… it will then demand water, not because the person is thirsty, but to ease the weight of the food.” [15]
To optimize the stomach, we should eat simple foods, not those composed of many different types. In the 19th century, Dr. William Beaumont discovered that every food is digested differently and for a different length of time. [16] For example, meat stays in the stomach for six hours or more, while bread stays less, and fruit even less. The conclusion is that we should eat simple food and separate our foods as much as possible. While the stomach is sophisticated, simple foods ease its function.
The 14th-century philosopher Ibn Khaldun wrote that diseases are common among city dwellers because of the abundance and variety of their meals, creating a strange compound unsuitable for the body. [17] Theories of food combining, such as those by Herbert Shelton, were developed around this understanding. [18]
The Intestines
After the stomach, food moves to the small intestine (6 to 8 meters long), where most chemical digestion and nutrient absorption occurs. Enzymes and bile assist in this process. [19] Finally, it moves to the large intestine (about 1.5 meters), where stool is formed, water is absorbed, and bacterial fermentation occurs. This process can take a day or more. [20]
Stool is usually a taboo subject, but this is problematic. If we don’t talk about it, we can’t diagnose problems. Many people live for years with diarrhea or constipation without treating it because they are ashamed. I highly recommend discussing stool with family or those close to you.
The way we sit on the toilet is also crucial. Physiologically, sitting is the worst way to pass stool. If you watch small children, they naturally squat. We should do the same, either by squatting on the toilet or using a small stool to raise the legs.
Finally, pay attention to the nature of your stool. Irregularities are a sign that something, either what we eat or our emotional state, needs addressing. Treating symptoms with drugs only worsens the situation in the long run. You can use the Bristol Stool Chart as a general guide to see if your digestion is healthy. Lastly, gas is also a result of fermentation and is affected by whether our food was suitable for our system.
References and Notes
[1] Tortora, G. J., and Derrickson, B. H. (2008). Principles of Anatomy and Physiology. John Wiley & Sons.
[2] Milton, K. (1999). “Nutritional characteristics of wild primate foods: do the diets of our closest living relatives have lessons for us?” Nutrition, 15(6), 488 to 498.
[3] There are also omnivores, but these originate from mammals with either a herbivorous or carnivorous digestive system.
[4] Milton, K. (2003). “The critical role played by animal source foods in human evolution.” The Journal of Nutrition, 133(11), 3886S to 3892S.
[5] People have a tendency to think that as soon as they eat something and go to the bathroom, what comes out the other side is what they just ate, but this is not the case.
[6] Milton, K. (2003). “The critical role played by animal source foods in human (Homo) evolution.” The Journal of Nutrition, 133(11), 3886S to 3892S.
[7] Eastwood, M. A. (1992). “The physiological effect of dietary fiber: an update.” Annual Review of Nutrition, 12(1), 19 to 35; Tungland, B. C., and Meyer, D. (2002). “Nondigestible Oligo and Polysaccharides (Dietary Fiber): Their Physiology and Role in Human Health and Food.” Comprehensive Reviews in Food Science and Food Safety, 1(3), 90 to 109; Salminen, S., et al. (1998). “Functional food science and gastrointestinal physiology and function.” British Journal of Nutrition, 80(S1).
[8] Burkitt, D. P., Walker, A. R. P., and Painter, N. S. (1972). “Effect of dietary fibre on stools and transit times, and its role in the causation of disease.” The Lancet, 300(7792).
[9] For example: Park, Y., et al. (2005). “Dietary fiber intake and risk of colorectal cancer: a pooled analysis of prospective cohort studies.” JAMA, 294(22).
[10] Stevens, C. E., and Hume, I. D. (2004). Comparative Physiology of the Vertebrate Digestive System. Cambridge University Press.
[11] Leach, J. D. (2007). “Prebiotics in ancient diets.” Food Science and Technology Bulletin: Functional Foods, 4, 1 to 8.
[12] Stevens, C. E., and Hume, I. D. (2004). Comparative Physiology of the Vertebrate Digestive System. Cambridge University Press.
[13] Lerner, A., et al. (2017). “Horizontal gene transfer in the human gut.” Frontiers in Immunology, 8, article 1630.
[14] Tortora, G. J., and Derrickson, B. H. (2008). Principles of Anatomy and Physiology. John Wiley & Sons.
[15] Maimonides (The Rambam), Aphorisms on Asthma (The Medical Aphorisms of Moses Maimonides), Part Five (Translation by the author).
[16] Beaumont, W. (1833). Experiments and Observations on the Gastric Juice, and the Physiology of Digestion.
[17] Ibn Khaldun, The Muqaddimah: An Introduction to History. Translated by Franz Rosenthal.
[18] Shelton, H. M. (2012). Food Combining Made Easy. Book Publishing Company.
[19] Tortora, G. J., and Derrickson, B. H. (2008). Principles of Anatomy and Physiology. John Wiley & Sons.
[20] Tortora, G. J., and Derrickson, B. H. (2008). Principles of Anatomy and Physiology. John Wiley & Sons.
