Ayurvedic principles of digestion and metabolism: A glance with modern perspective.
- Preface: Samhita granthas(ancient texts) have emphasised the importance attached by Ayurveda to nutrition and its proper utilisation. Sage acharyas praise that the body is the outcome of food. Evenso, disease is the outcome of food. Food is the factor which sustains and supports the Deha-dhatus(tissue elements), Ojas(the factor of resistance to disease and decay), Bala(strength or capacity to perform physical work) and complexion among others. This food depends upon agni to contribute to the nourishment of the body. It is obvious that the body-elements or sharir dhatus can’t be nourished and developed when food is not properly digested by agni. (Charak chi. 15:5)
- Avastha Paka
- Defination: Gastri-intestinal digestion of food has been described as Avastha Paka or change in the state or form of food substances in the Amashay(stomach and small intestine) and Pakwashaya(large intestine), in the course of the digestive process.
- Two phases of this Paka-the Prapaka and Vipaka have been envisaged. The term Prapaka has been as Pratham Paka or the first(primary)change. The term vipaka, on the other hand, has been defined as changes to which the food that has undergone Prataham Paka is again subjected to further changes under the influence of jatharagni(Pachak Pitta).
- These latter changes have been described in terms of the Rasa or taste of the end products of gastro-intestinal digestion viz., Madhur or sweet, Amla or Sour and Katu or pungent(acrid). Both prapakas and vipaka involve, so to say, Pithara paka or physical changes. These do not involve Pilu paka or chemical change of the food stuffs ingested.
- The Madhurbhava of Avasthapaka(Prapaka)
- Prapaka or the preliminary(primary) phase of digestion of food-stuffs ingested, commences from the time the food is introduced into the mouth. This aspect of digestion and the digestion in the upper portion of the Urdhwamashay(the fundus of the stomach) are comprehended by madhura bhava. The insoluble starches are by now rendered completely soluble and the taste of the end products of this reaction is sweet or madhur, for which reason, this aspect of prapaka of avasthapaka is called as madhurabhava.
- The Amlabhava of Avasthapaka(Prapaka)
- The Madhura bhav of the avasthapaka is seen to be brought to an end by the hydrochloric acid secreted by the cells of the mucous membrane of the stomach. This marks the commencement of the amlabhava or the acid(sour) phase of prapaka. This involves the conversion of the insoluble proteins into the soluble, under the influence of the enzyme pepsin, in the presence of hydrochloric acid. This aspect of prapaka does not seem to have anything to do with the digestion of the end products of madhur paka i.e. substances, the taste of which is madhur. The process of conversion of the insoluble proteins into soluble eptones, in the course of the amlabhava of prapaka can be written as follows-
Proteins prosteoses peptones
- The outcome of this phase of digestion is the production of acidified chime, which has been characterised as Vidagdha(partly or not fully digested). The food in this state is not yet fit for absorption and utilisation in metabolic process.
- The partly digested food acquiring the quality of sourness when moved down from the Amaashay comes into contact with Pitta sthana. Here, the Accha pitta is secreted. Accha pitta is meant Aghana or light, also interpreted as Swatcha or clear. The concept of Accha pitta comprehends the gall bladder bile and pancreatic juice which together have been shown to be responsible for the subsequent digestion of fats, proteins and carbohydrates in the small intestine.
- Jatharagnipak refers to the intestinal digestion and the process involved in it. According to charak, vagbhat substances that possess six different tastes viz., madhura(sweet), Amla(sour, acid), lavan(saltish. Saline), katu(acrid, pungent), Kashay(astringent), amd tikta(bitter) yield, towards the end of Jatahragnipaka, three tastes(substances that exhibit three tastes), viz., madhura(sweet), amala(sour, acid) and Katu(acrid, pungent).
- It was pointed out that the concept of Jatharagni Paka represents the generalization, mainly, of the process and substances involved in intestinal digestion. Jatharagni is itself is, in so far as the gastro-intestinal digestion is concerned, a complex of powerful digestive substances, and in main gastro-intestinal cathepsins.
- According to ayurvedic physiology, bhutagni-paka follows Jatharagnipaka and it completes the process if intestinal digestion. It is only after the completion of Bhutagnipaka that the formation of Ahar rasa is completed and rasa shoshan or absorption of rasa is possible. The outcome of jatharagnipaka are two viz.,
a) the bhinna sanghata or the splitting of the complex food substances into their ultimate elemental units or molecules. These are five distinct bhautic or physico-chemical groups viz., Parthiv, Apya, Agneya(tejas), Vayavya and Akashya.
b) the activation of the nascent agni-bhuta which forms part of the penta-bhautic structure of each one of the five groups of molecules.
- The bhutagni thus activated, digests the substance of that group.Thus the agni constituent of the predominantly parthiv molecule, spoken of as parthivagni, digests the substance of the molecule. The basic structural factors of the body, such as the rasadi dhatus, unceasingly undergo destruction by their own agnis and these are always being reformed by dhatvaharas(nutrients of the dhatus or tissues) derived from the four kinds of food ingested. This view, in the parlance of modern physiology, will amount to this, that the tissue of the body are being reformed as rapidly as they are destroyed, in the course of metabolism.
- The material stuff with which the tisssues of the body are composed can be described in two ways, viz.,
i. in terms of basic organic and inorganic substances, such as proteins, carbohydrates, fats, vitamins and minerals or
ii. in terms of the chemical structure of the ultimate elemental units of the proximate constituents of the food ingested. It may be noted that a parthiva substances can alone contribute to an increase of the parthiva constituents of the body.
- Katu Avasthapaka:
- The thirds aspect of avasthapaka is spoken of as the katubhava. This aspect relates to the acrid and pungent nature of reactions that occur in the Pakwashaya or large intestine. Charak says that, “the material passed down from the amashay, having reached the pakwashaya, is dehydrated and converted into lumps by heat-an acrid and pungent gas being produced in the process”. This description is supported by modern researchers. These have shown that during the passage of the intestinal contents through the small intestine, the process of absorption, with the exception of water, is normally completed. In the large intestine more ofwater and salts are absorbed. The material left over is converted into faeces which leaves the body. The large intestine is inhabited by a large population of avaikarika or sahajkrimis or bacterial flora of which escheria coli is ordinarily the more predominant variety. These putrefactive flora are concerned with the alteration of the products derived from the digestion of proteins. They bring about the putrifaction of these protein residues, leading to the liberation of various kinds of pungent gases, possessing disagreeable and often foul odur e.g. indol, skatol, phenol, hydrogen sulphide and ammonia.
- The neural influence:
- The neural influence over the several functions of the amashay-urdhwa and adho-and pakwashaya is attributed to the samana and apana vayus. Prana vayu which is said to have its seat in murdh(head) and hridaya(heart) is stated to be responsible for the act of the swallowing of the food-annapraveshkrut. Samana vayu, while being located near agni, is stated to move throughout the kostha. It is stated to enable the reception, digestion, the separation fo the sara(nutrients)from kitta(food residue) and the propulsion of food. Apana vayu, on the other hand is stated to be located in the pakwashay and move the faeces downwards, evacuates urine, ejaculates semen in the male, discharges artava and brings down foetus.
- It is seen that the functions ascribed to samana vayu, by Ayurveda are performed, for the most part, by the intrinsic nerves of the stomach and intestine. The enteric plexuses function in maintain the peristaltic movement along the digestive tract, the central nerves, viz., the para-sympathetic and sympathetic, exert a regulating effect upon gastro-intestinal peristalsis. The peristaltic wave mechanically break-up the intestinal contents which are well macerated and get thoroughly mixed up with the pancreatic juice, bile and intestinal juices. As parts of the semi-solid mass are brought in contact with the absorbing surfaces of the intestinal wall, the absorption of the digested portion of the nutrition takes place.
- The large intestine has two intrinsic nerve plexuses. It is in addition innervated by both the sympathetic and para-sympathetic nerves. The proximal part of the colon is innervated by fibres derived from the superior mesenteric plexus. This is partly sympatheitic(from lumbar root) and partly parasympathetic(from vagus). The distal part of the colon is enervated by sympathetic fibres which reach it from the upper lumbar roots i.e. the pelvic splanchnic branches, inferior mesenteric plexsus and nerves. The parasympathetic supply derived from the 2nd to 4th sacral roots by the way of the hypo- gastric plexus in which are scattered ganglia to the wall of the colon. The sympathetic is inhibitory to the musculature of the colon, with the exception of the ileo-colic sphincter to which it is motor, while the parasympathetic is motor to all except the ileo-colic and anal sphincters. Hence, in the view of the forgoing observations, it may be said that the stulantra or pakwashay is also under the control of saman vayu while, the predominantly para-sympathetic i.e., cranio sacral inervation will explain the role of apana vayu.