Abstract The process of food digestion is one of the most important physiologic processes in human body. In this review, we are seeking the views of Avicenna, the most distinguished Persian medical scholar about digestion and indigestion. Avicenna’s view was based on the humoral theory. Avicenna has focused scrutiny on the process of digestion. He divided this process into four phases including gastric, hepatic, intravascular and intra-organ digestion. A defect in any of these phases can lead to disturbance in other stages. Avicenna approached the problem of indigestion through factors of diet, lifestyle and inherent structural characteristics of digestive organs. Modern medicine confirms Avicenna's opinion about the start of digestion from the mouth, the role of the stomach in digestion and the role of the liver in the metabolism of foods. Overeating or eating certain foods, snacking between meals, eating variety of different foods together, intense physical activity, sexual activity after a meal, stress and sleep insufficiency are among factors that may be linked to indigestion in modern medicine viewpoints and also have been mentioned in Avicenna's teachings. It seems rational to consider the medical approaches recommended by Avicenna for future studies in the field of digestive disorders.

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Persian Medicine
Digestion process and causes of indigestion based on Avicenna's view
and modern medicine
Ali Reza Derakhshan1
, Mahdi Yousefi1
, Sohrab Dehghan2
, Arman Zargaran3
, Mahmood Khodadoost4
1
Department of Persian Medicine, School of Persian and Complementary Medicine, Mashhad University of
Medical Sciences, Mashhad, Iran. 2
Department of Traditional Medicine, School of Traditional Medicine, Shahid
Beheshti University of Medical Sciences, Tehran, Iran. 3
Department of History of Medicine, School of Persian
Medicine, Tehran University of Medical Sciences, Tehran, Iran. 4
School of Traditional Medicine, Shahid Beheshti
University of Medical Sciences, Tehran, Iran.
*Corresponding to: Mahmood Khodadoost, School of Traditional Medicine, No.8, Shams Alley, Vali-e-Asr Street,
Tehran, Iran. Email: mkhodadoost@sbmu.ac.ir.
Highlights
This manuscript reviews digestion process and the problem of indigestion from the perspective of Avicenna
and aims to establish a link between the perspective of Avicenna and the evidence of modern medicine.
Traditionality
Avicenna (born 980, died 1037, Hamadan, Iran) is regarded as the most distinguished Persian medical
scholar. His masterwork Canon of Medicine became the mainstream medical system in the west until 17th
century. Avicenna paid a lot of attention to gastrointestinal disorders and his view was based on the humoral
theory.

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Abstract
The process of food digestion is one of the most important physiologic processes in human body. In this review, we
are seeking the views of Avicenna, the most distinguished Persian medical scholar about digestion and indigestion.
Avicenna’s view was based on the humoral theory. Avicenna has focused scrutiny on the process of digestion. He
divided this process into four phases including gastric, hepatic, intravascular and intra-organ digestion. A defect in
any of these phases can lead to disturbance in other stages. Avicenna approached the problem of indigestion
through factors of diet, lifestyle and inherent structural characteristics of digestive organs. Modern medicine
confirms Avicenna's opinion about the start of digestion from the mouth, the role of the stomach in digestion and
the role of the liver in the metabolism of foods. Overeating or eating certain foods, snacking between meals, eating
variety of different foods together, intense physical activity, sexual activity after a meal, stress and sleep
insufficiency are among factors that may be linked to indigestion in modern medicine viewpoints and also have
been mentioned in Avicenna's teachings. It seems rational to consider the medical approaches recommended by
Avicenna for future studies in the field of digestive disorders.
Keywords: Persian medicine, Digestion, Indigestion, Malabsorption, Avicenna
Abbreviations:
CAM, Complementary and alternative medicine; GI, Gastrointestinal; PM, Persian medicine.
Competing interests:
The authors declare that there is no conflict of interests regarding the publication of this paper.
Citation:
Ali Reza Derakhshan, Mahdi Yousefi, Sohrab Dehghan, et al. Digestion process and causes of indigestion
based on Avicenna's view and modern medicine. Traditional Medicine Research, 2019, 4(3): 140-147.
Executive Editor: Cui-Hong Zhu.
Submitted: 22 September 2018, Accepted: 20 January 2019, Online: 15 February 2019.

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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 is Al-qanoon fi al-teb (Canon of
Medicine) which comprises five books. In fact, Canon
of Medicine is 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 the Canon of Medicine under 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’s Canon 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 called Chylus. Chylus moves from
the stomach and intestine to the Masariqa (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

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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 its hararat (heat). hararat in 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 of hararat [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.
Table 1 Four stages of digestion from the perspective of Avicenna and the evidence of modern medicine
Avicenna's view Modern medicine's view
Gastric digestion Digestion begins in the oral cavity
with crucial role of chewing and
saliva.
Food is transformed into a semifluid
substance called Chylus.
In addition to stomach, the liver, the
spleen and the diaphragm have a role
at this stage.
The role of the pancreas is not
expressed explicitly by Avicenna,
although he may have considered the
pancreas as part of the spleen.
Mechanical and chemical digestion begins in the oral
cavity with the effect of chewing and salivary amylase
and lipase.
Digestion (specially proteins) continues at this stage
with a key role of gastric juice and pancreatic enzymes.
Role of liver in this stage can be explained by the
release of bile from gallbladder.
Dysfuction of spleen can lead to dysfunction of other
digestive organs by dysregulation of immune system as
well as gut associated lymphatic tissue dysfunction.
The diaphragm can play a role in digestion process in
some cases by increasing intra-abdominal pressure.
Hepatic digestion The blood passes through Masariqa
(mesenteric) and Baab (portal) veins
and distributes in the liver.
The hepatic digestion leads to
metabolization of food and formation
of four humors.
These humors enter a vessel that
emerges from the convex part of the
liver and join the body circulation.
The superior mesenteric vein collects blood from small
intestine and forms the portal vein in combination with
splenic vein. Portal vein carries blood from
gastrointestinal tract to the liver.
Liver process the blood, break down, and metabolize
the nutrients. The blood that enters the liver is drawn
into the sinusoids and after metabolization enters into
the inferior vena cava via hepatic vein.
Intravascular
digestion
Occurs in the blood vessels.
Prepares humors for consumption in
tissues.
Enzymatic reactions inside vessels such as endothelial
lipase and protease activities can be considered as
evidence of intravascular metabolism and digestion.
Intra-organ
digestion
Digested food reaches the tissue and
digestion completed in each tissue
specifically. The result is tissue
growth and regeneration
Intracellular enzymatic reactions and degradation and
catabolism of food molecules can match this opinion.

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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 the
Chylus into a product called Chymus, 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 of Sou-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

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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.
Table 2 Possible causes of indigestion: Avicenna's view and modern medicine
Causes Avicenna’s views Modern studies
Intra-abd
ominal
causes
Structural abnormality or
imbalance of humors in
abdominal organs specially
stomach, intestine or liver.
Peptic ulcer, gastroesophageal reflux disease, gastric or esophageal
cancer, pancreatic or biliary disorders, intolerance to food or drugs are
considered as organic causes of dyspepsia [31].
Dietary
factors
Overeating
Inappropriate fasting
Eating foods which have high
potential of rotting (i.e. fresh
milk, fish and watermelon)
Eating a variety of foods in one
meal.
Eat before digestion of the
previous meal
Not following the right order of
eating
Eating large amounts of food at a meal is effective in inducing
indigestion [33].
Fatty foods, spicy foods, carbohydrate-containing foods and some
other foods such as pickles vinegar, soft drinks, grain, tea, salt, pizza,
watermelon, red pepper, and macaroni are the foods that cause the
highest exaggeration of symptoms in functional dyspepsia. In other
hand, apples, rice, bread, caraway seed, honey, quince, and walnut can
alleviate symptoms. Milk and dairy products were controversial in
dyspepsia [34, 35] but they have been implicated as trigger for
Irritable bowel syndrome [36].
Higher prevalence of snacking between meals have reported in
patients with dyspepsia [37].
Differences in digestibility and absorption of commonly ingested
short-chain carbohydrates can lead to exacerbation of symptoms in
functional gastrointestinal disorders [38].
Any alteration in eating habits and diet can affect the overall function
of the gut [13, 15].
Lifestyle
factors
Intensive physical activity after
meals
Sexual intercourse after meals
Mental and emotional stress
after meals
Bathing after meals (There is no
study about the relationship
between bathing and
gastrointestinal disorders)
Drinking more or less than
needed
Sleeping more or less than
needed
Exposure to very cold or very
hot or polluted weather
Regular physical activity is associated with lower symptoms in
irritable bowel syndrome but endurance sports is associated with more
gastrointestinal symptoms [34].
Excessive eating and drinking may result in greater hemodynamic
changes associated with sexual activity [16]. Furthermore, it has been
reported that large meals are a risk factor for death after sexual
intercourse [39].
Stress can influences activities in the gut and lead to dysfunction of
gastrointestinal system through disruption of homeostasis in the gut
[13, 40].
Difficulty falling asleep or sleep insufficiency has been identified as a
risk factor for dyspepsia [34].
Air pollution can change gut microbiota and immune function [41]. It
also can be a potential trigger of IBS [42]. Specific alteration of
rhythm in temperature stress produces somatic and visceral pains in
IBS [43].

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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.
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