Digestion process and causes of indigestion based on Avicenna's view and modern medicine

Ali Reza Derakhshan,Mahdi Yousefi,Sohrab Dehghan,Arman Zargaran,Mahmood Khodadoost

1Department of Persian Medicine,School of Persian and Complementary Medicine,Mashhad University of Medical Sciences,Mashhad,Iran.

2Department of Traditional Medicine,School of Traditional Medicine,Shahid Beheshti University of Medical Sciences,Tehran,Iran.

3Department of History of Medicine,School of Persian Medicine,Tehran University of Medical Sciences,Tehran,Iran.

4School of Traditional Medicine,Shahid Beheshti University of Medical Sciences,Tehran,Iran.

Abstract

Keywords:Persian medicine,Digestion,Indigestion,Malabsorption,Avicenna

Background

Food digestion is considered one of the most important physiologic processes in our body.Several factors play a key role in proper digestion and absorption such as mixing action of stomach and intestinal motility,enzyme secretion and activity,appropriate mucosal function,optimum blood supply and normal microbiome [1].Consequently,any defect in these factors can affect the absorption of nutrients which may result in development of a wide range of disorders such as neurologic dysfunctions [2],skin diseases [3],respiratory diseases [4] and rheumatic disorders [5].Since conventional medical therapies may have side effects or do not provide satisfactory results,enthusiasm for complementary and alternative medicine (CAM) is growing.In the field of gastrointestinal (GI) illness [6,7],the most prominent use of CAM is dedicated to functional GI disorders.

Persian medicine (PM) has a history of more than 4000 years [8].It has been commonly practiced in ancient Persia and prospered there as well as Muslim countries.PM is a system containing various methods for prevention,diagnosis and treatment of many ailments since the ancient times [9].PM gives a lot of attention to the GI system and the physiology of digestion and absorption in particular.In PM with its holistic approach,any disorder of GI system can affect the function of other organs such as nervous,respiratory,circulatory and rheumatologic systems[10-12].

Abu Ali Al-Hussain Ibn Abdullah ibn Sina better known as Avicenna (born 980,near Bukhara,Persia[now in Uzbekistan]—died 1037,Hamadan,Iran) is regarded as the prince of physicians whose innovative ideas had a major contribution to development of medicine in medieval times.More than 400 books in the fields of medicine,philosophy,astronomy and other branches of science are attributed to him [13].His masterwork isAl-qanoon fi al-teb(Canon of Medicine) which comprises five books.In fact,Canon of Medicineis the summary of medical knowledge of its time.It is known as an outstanding achievement in the history of medicine,which was celebrated until 17th century [14-16].Avicenna’s view was basically based on the humoral theory which was mainly founded by ancient Greeks and evolved by Persian physicians and was the mainstream medical system in the west until 17th century [17].This theory holds that the human body is brought into being as a result of mixing of four basic substances known as humors which are in balance in a healthy state.Any imbalance in these four humors (i.e.excess or deficit of one of them) can result in diseases and disabilities [18].Avicenna paid a lot of attention to GI disorders and discussed several GI topics in his encyclopedia in detail[19].

Considering that explaining the Avicenna’s teachings as an indicator view in history of medical science could provide a framework for comparing medical knowledge of the past and modern medical sciences.In this article we aimed to study the views of Avicenna on the subject of digestion,indigestion,its causes and outcomes and tried to bridge between his teachings and modern evidences.

Mainbody

Digestion process

Avicenna described the process of digestion in the first volume of theCanon of Medicineunder the heading“creation of humors”.According to his views,the food goes through four digestive stages from the beginning to the end of the digestive process.Food particles undergo transformation in each stage until they are ready to be used by the target organ.Here we briefly introduce these stages of digestion from the perspective of PM based on Avicenna’sCanon of Medicine[20].

The first stage starts from the oral cavity and is completed in the stomach.Avicenna believed that the process of chewing and excretion of saliva were the main factors involved in oral digestion,therefore they have put a lot of emphasis on proper chewing of the food.Ingested food then enters the stomach and the first stage of transformation is completed there.It is important to know that liver,spleen,diaphragm and abdominal muscles are also involved in this stage of digestion.By this time,food is transformed into a semifluid substance calledChylus.Chylusmoves from the stomach and intestine to theMasariqa(mesenteric veins) and then Baab (portal vein) and ultimately reaches the liver.The waste product of the first stage of digestion will form the feces.

In modern physiology,digestion is defined as the process by which the ingested food is dissolved and broken down to simple chemical compounds with the help of enzymes excreted from the GI tract.This process starts in oral cavity as Avicenna mentioned.The amylase and lipase present in saliva digest the starch and lipids and continue their action in the stomach.Chewing the food for a longer time helps the amylase and lipase exert their action on starch in a proper way[1,21].

Besides,by breaking down the food to small pieces,the act of chewing increases the area of contact for the enzymatic action in oral cavity.As mentioned earlier Avicenna has pointed out these two factors in digestion with his remarkable precision [22].Avicenna cited the organs adjacent to GI tract to have a role in digestion process and specifically noted the liver,spleen and diaphragm.Avicenna believed that liver help gastric digestion by itshararat(heat).hararatin PM has wide meaning and does not refer solely to the temperature of the liver.In this concept the enzymatic activity of an organ or release of ATP within cells can be explained below the definition ofhararat[23].Role of liver in digestion can be explained by the release of bile from gallbladder.It has been proved that liver malfunction can reduce the production of bile salts.In addition,anatomical defects in the biliary system may reduce the delivery of bile salts to the digestive canal [1].Diaphragm is mainly involved in the respiratory process.However,it has an assisting role in vomiting,defecation and urination by increasing intra-abdominal pressure as well as inhibiting gastric reflux by increasing the pressure on the esophagus[24].

Although spleen plays no direct role (like secreting digestive enzymes)in the digestive process,it has been proven that dysfunction of spleen can lead to dysfunction of other digestive organs by dysregulation of immune system as well as gut associated lymphatic tissue dysfunction [25].It should be noted that Avicenna regards the role of the spleen in the digestion process as related to the spleen vessels,not the splenic tissue itself.Today we know that pancreatic enzymes play a major role in digestive process while Avicenna has not mentioned this fact.This can be due to two reasons: limitation of the knowledge regarding the action of pancreas or the possibility that he has considered the pancreas and spleen as a single organ.

Table1 Four stages of digestion from the perspective of Avicenna and the evidence of modern medicine

The second stage of digestion takes place in the liver.Avicenna explained that the blood of the portal vein is distributed in the liver and the liver transforms theChylusinto a product calledChymus,which is consisted of the humors.Humors are basic substances in human body that are divided into four general categories: Blood (Dam),Phlegm (Balgham),Yellow bile (Safra) and Black bile (Sauda).Each of these humors have their own characteristics and functions.In other words,normal humors are created in the liver as the final products of the hepatic digestion.This description is likely to match the fact that the main function of the liver is to process the blood,break down,and metabolize the nutrients.As described by Avicenna these humors enter a vessel that emerges from the convex part of the liver and join the body circulation,which we know today that this description is related to inferior vena cava.The waste products of this stage are entering the bloodstream and excrete from blood into urine by the kidneys as stated in modern physiology.

The third stage of digestion takes place in the blood vessels,which further prepares the nutrients for delivery to the tissues.This phase of digestion is in fact an intermediate stage that prepares the blood and its ingredients for the fourth stage (intra-organ digestion).The fourth stage of digestion takes place in the various tissues of the body separately by which the food is transformed into a specific tissue.In other words,after passing the three stages of the digestion and after the specific reactions at each stage,the food is converted into substances transmitted by the blood stream to each target tissue and eventually in the fourth stage,it is used for growth and regeneration in each tissue.The waste products of the third and fourth stage of digestion form the sweat,which is excreted through the skin.Enzymatic reactions inside vessels such as endothelial lipase and protease activities can be considered as evidence of intravascular metabolism and digestion [26,27].Although molecular medicine was not known at the time of Avicenna,he considered each tissue as a part of digestion process.Degradation and catabolism of food molecules,which occurs within cells,can match this opinion.Table 1 summarizes Avicenna's description of four digestive stages and related evidence of modern medicine.

Although the topic of four stage of digestion described by Avicenna and other PM practitioners is based on humoral theory,it largely meets modern findings.Avicenna presented a coherent picture of digestion process and related disorders however in some minor discussions,his teachings differ from current medical science due to lack of access to modern diagnostic and imaging equipment.Although the oral,gastric and intestinal digestion is considered the first stage of digestion from Avicenna’s point of view,it is of great importance because it can affect the next stages of digestion as well.

Indigestion

Definition

Avicenna defined indigestion under the term ofSou-e hazm,as incomplete process of digestion and absorption of food.It is important to note that indigestion at any stage can affect the quality of product in next stages as well.The abnormal products are considered harmful to the body [28].In modern medicine,indigestion is a term that means lack of adequate digestion and includes a wide variety of symptoms.The word that can be used interchangeably with indigestion,is dyspepsia[29].

Symptoms

Avicenna considers unusual foul odor of stool as the cardinal symptom of indigestion.Other symptoms that can be noticed according to the cause of indigestion include bloating,pain or burning sensation of stomach,flatulence and gas passing,nausea,emaciation and abdominal gurgling sounds [14].Today the term of indigestion refers to group of symptoms ranging from pain or discomfort to postprandial fullness,early satiation,abdominal bloating,burping,gurgling sounds or even nausea and vomiting [30,31].Symptoms that mentioned by Avicenna for indigestion have many overlaps with symptoms of dyspepsia especially functional dyspepsia.

Causes

In Avicenna’s view,some organic causes as well as various dietary and lifestyle factors can be involved in indigestion and its possible consequences.Avicenna divided causes of indigestion into intrinsic and extrinsic.Intrinsic factors are mainly related to organic causes whereas extrinsic factors dealing with diet,eating habits and lifestyle parameters such as physical and sexual activity,mental and emotional states and sleeping[14].Today we know that several mechanisms including physiologic,biologic,psychologic and environmental mechanisms are contributing to indigestion [32].Table 2 shows the causes of indigestion from the perspective of Avicenna and related evidence of modern medicine.

From comparative perspective,the subject of indigestion in PM can be related to: (1) Lack of breakdown of macromolecules to monomers resulting in production of dimers or larger molecules and their absorption [44] especially autoimmune diseases (i.e.celiac disease) and allergic disorders (i.e.allergic rhinitis) [45];(2) Interaction among various food particles resulting in the production of intermediate molecules with antigenic properties that can damage the mucosa of GI tract [46];(3) Production of large amount of gas made from insufficient digestion which is absorbed from the GI epithelium leading to change in epithelial permeability [47];(4) Improper quality of the absorbed material can alter the metabolism of GI mucosal cells or affect the next stages of digestion including the liver metabolism[48].

Today the researchers highlight the role of indigestion and malabsorption in the pathophysiology of a wide range of diseases such as a number of GI,developmental,rheumatic,neurologic,allergic and autoimmune and even psychiatric disorders.Moreover,the link between digestive and neurologic system under the term brain-gut axis has been studied in several papers.Currently more and more documents are supporting the concept of a balanced and diverse gut microbiota in maintenance of proper cognition and emotional functioning[49,50].

As it mentioned,most of Avicenna’s teachings on the topic of indigestion causes had direct or indirect evidence.However,some were controversial and some had no relevant evidence.Considering that Avicenna has thought in the framework of humoral medicine,it’s difficult to propose a mechanism for some of his teachings that have no evidence in modern medicine.

Table2 Possible causes of indigestion:Avicenna's view and modern medicine

Prospect

Understanding CAM perceptions on preventive health issues can help illuminate ways in integrative medicine and framing use of CAM in health care and public health systems.Given the importance of digestion from the view of PM and support of considerable parts of its teachings by modern medical findings,it seems rational to consider the medical approaches of this system for future studies in the field of digestive disorders.PM practitioners have provided many recommendations for maintaining health especially proper GI function.Since these recommendations are the result of many years of experience of prominent medical scientists of the past,further integrative research is needed in order to promote public health in this field with safe,affordable and efficacious recommendations in harmony with culture and traditions.