Vipaka: Taste Conversion During And After Digestion

Dravya Guna helps in the knowledge and identification of a number of medicinal plants from which the active principle are extracted. The actions of these drugs depend upon their Rasa panchakas.

Rasa Panchaka

Rasa Panchakas include –
Rasa – taste: Example, Guduchi is bitter in taste
Guna – qualities. Example: Sesame oil is unctuous
Vipaka – taste conversion after digestion – Jaggery undergoes sweet taste conversion (Madhura Vipaka)
Veerya – Potency of the herb  – Black pepper is hot in potency (Ushna Veerya)
Prabhava – special effect of the herb. Brahmi is Medhya – brain tonic
Vipaka is the taste at the end of digestion. It determines the specific action of an herb on Doshas.

Definition of Vipaka

Vipaka is the transformed state of ingested substance after digestion. It is also called as ‘ Nisthapaka’.
It is said to take place at the time of division of ‘rasa’ and ‘mala’ after digestion is completed, but in it is finalised after the next paka by ‘Bhutagnis’ in liver where most of the drugs are metabolized.

Vipaka cannot be demonstrated directly. It is an in inferential entity and such entities are always debatable.
The change in the tastes (of substances) that occurs at the end of digestion by the association of (coming in contact with and being acted upon) the ‘Jataragni’ (fire in the stomach vis-à-vis digestive juice of the alimentary tract) is called as vipaka as told by Ashtanga Hridaya sutrasthana.

Three types of Vipaka (Taste conversion after digestion):

As per Ashtanga Hrudayam Sutrasthana 1st chapter,

त्रिधा विपाको द्रव्यस्य स्वाद्वम्लकटुकात्मकः॥१७॥
tridhā vipāko dravyasya svādvamlakaṭukātmakaḥ||17||

Swadu (Madhura) Vipaka – Sweet taste conversion after digestion
Amla Vipaka– Sour taste conversion after digestion
Katu Vipaka – Pungent taste conversion after digestion.

Characteristics

Charaka and Susruta did not make any serious attempt to define vipaka precisely. It is Vagbhata who exclusively defined it as the factor, which is the final outcome of the transformation of ‘rasa’ through the action of ‘Jataragni’.

Etymological interpretation

A specific ‘Rasa’ emerging at the end of the digestive state is ‘Vipaka’. The digestive end product, which is not going to change, is also called ‘Nishtapaka’.

The human body exhibits continuous transformation. The etymological meaning of ‘Sharira’ is one, which is being degraded. According to the theory of Moment Fractional Existence, the human body continuously experiences state of genesis, stasis and degradation. It is true for humans as well as everything in this universe; even a minute molecule.

This change is brought about by a specific power called ‘Paka’. Transformation is the action of ‘Paka’ which inturn is brought about by the effect of heat. The effect produces certain drastic changes in each and every object in the universe. ‘Paka’ forms the body, constituted with different forms of ‘Panchamahabhootas’.

This ‘Paka’ is responsible for all the actions. ‘Paka’ can be defined as the transformation with the help of ‘Ushna Shakthi’. ‘Agni’ brings about this ‘Paka’. ‘Agni’ sustains life, brings about digestion and it is self-digestive of all the molecules of the body. In the body also, the diet has to be imbibed in the bodily constituents.

Prapaka

‘Paka’ is the overall digestive juice, and the first part of digestion is called as Prapaka stage. As this is subjected to different stages according to the time and site, it is also called ‘Awasthapak’. This concept of prapaka is very important regarding the dietary dravyas. Poisons, alcohols and dravyas having a specific potency do not undergo a predigestive stage. Their actions start spontaneously.

Dietary and mechanical dravyas are both subjected to change during digestion. Both undergo mutually contrasting changes and hence this division into dietary dravyas and medicinal dravyas. Generally, dietary dravyas are rasa predominant and medicinal dravyas are virya predominant.

Diet forms the main component of any treatment; even though dietary drayas are only the start of the treatment. It is clear that we cannot compare diet and medicine by the same norms. On the Contrary, there is a marked difference between the two. Also, actions of medicinal dravyas are considered on the basis of time and site.

Drugs act by Rasa, Paka, Guna, Veerya or Prabhava. Vipaka is time related. Consideration of vipaka is incorrect without specific digestion being completed. The study of vipaka has resulted just from the exceptional behaviour of Rasas.

Avastha Paka

When the food comes into contact with (digestive enzymes), then the process of Avasthapaka (digestion) begins with Madhura paka or Prapaka (the sweet phase) in the Amasaya (oral cavity to first part of duodenum). Later food is ejected into Pachyamanashaya (second part of duodenum to ileum), where it undergoes Amala paka (the acid phase). Finally the food reaches the Pakwashaya (caecum to rectum) where the process of digestion ends with katu vipaka (alkaline phase).

The comparative study of these three stages from modern physiology will provide the exact picture:
The food first under goes digestion in saliva chiefly under the influence of salivary amylase (ptyalin). Usually ptyalin acts on boiled starch. Hence cooked food is considered for Avastha paka. Ptyalin converts starch up to the maltose.

The main action ptyalin takes place in the stomach before the HCI concentration becomes high. On an average it continues for 30-40 min. Digestion of protein and fats in not possible in saliva or before gastric juice involvement. Up to this stage the process of digestion was denoted as Prapaka or Madhura paka in Ayurveda.

Gastric juice does not possess any carbohydrate-spitting enzyme, but gastric HCI can carry on some hydrolysis of sucrose. Protein and fat digestion begins in gastric juice and completes with the action of succus entericus.

Gastric juice contains proteolytic enzyme pepsin, which digests them up to peptones. Gastric lipase breaks neutral fat into 1ml of glycerol and 3 ml of fatty acids. This occurs in Pachyamanashaya according to Ayurveda.

Next, the action of Achcha pitta (pancreato-biliary secretions) begins:
Pancreatic amylase acts on starch and dextrin. Maltase acts on maltose.
Pancreatic trypsin converts protein into peptones and amino acids.50-60% of protein digestion is over in pancreatic juice. Here tryptic digestion ends erepsin action begins.

Fat digestion mainly occurs in duodenum by pancrctic lipase (steapsin) Emulsification of fat takes place by bile salts, fatty acids, cholesterol, monoglycerides, lysolecithins and proteins. Bile salts make the fats water-soluble. Fat digestion is nearly completed in pancreatic juice. Fat is converted in to glycerol and fatty acids.

Till this state, the process of digestion is considered as Amla avastha paka in Ayurveda.
Succus entericus contain amylase and maltose, which helps in the conversation of carbohydrates in to glycose. The other disaccharides taken in food are hydrolyzed by lactase and sucrase (investase) present in this juice.

Digestion of protein in intestinal juice depends upon tryptic activity. Erepsin consists of a mixture of enzymes viz. aminopeptidases and dipeptidases which act in alkaline medium. They convert lower peptides completely into amino acids.

Lipase in Succus entericus is insignificant since fat digestion will be almost over in pancreatic juice.
Apart from these digestive changes the digested food loses most of the water content once stool formation is over in large intestines. This final form is slightly alkaline in reaction. This third phase is mentioned as Katu Avastha paka in Ayurveda.

Nishta Paka

After the food is subjected to Jataragni paka and Bhutagni paka, the nutrients absorbed along with the chyle from the small intestine (Adho-amashaya) will have a further transformation know as ‘ Nishta Paka’.

Karma nishta is the conclusive action of a dravya. Chakrapani explains Nishtapaka or Vipaka as Karma Parisamapti i.e., conclusion of action in its entirety. Even though various Rasas undergo transformative changes, the contribution made by their special functions due to further transformative changes is know as vipaka or Nishta paka. The final actions will increase the symptoms of Kapha, Sukra etc.

The concept of Nishta paka vis-à-vis post digestive changes, which occurs in Adho amasaya after absorption under the influence of Bhutagni paka and Dhatvagni paka etc., this process is denoted as metabolism. Modern physiology believes that- ‘All metabolic reactions are connected in a single integrated whole in which individual links can replace one another’

During Nishta paka the Pancha Bhuta amsas of a dravya will be converted into its basic components like Prithvi, Ap, Tejas, Vayu & Akasha. The Sthayi dhatu poshana will be done by this Vipaka.

The ‘Madhura Nishta paka’ results in the final formation of glucose and its conversion into glycogen for the utility of tissues. Metabolic end-products chatacterized as ‘ Amla Nishta paka’ and ‘ Katu Nishta paka’ may be the outcome of cellular respiration.

The term Amla Vipaka may refer to intermediate metabolites (keto acids) as the lactic acid and pyruvic acids, which are the 3-carbon compounds. These are laghu in nature when compared to amino acids, glucose, glycerol and fatty acids and guru, as compared to the end-products of cellular respiration which are characterized as Katu Vipaka.
Thus, the terms Amla & katu vipakas, in the context of Nishta paka may refer to the basic molecular structure of the given substance.

Avasthapaka, Nishtapaka difference

Chakrapani further substantiates his view that during Dhatu paka process Rasa dhatu produces Kapha as its mala and Rakta dhatu produces Pitta as mala. On the other hand during the Ahara pachana process through Jataragni Prakrita Kapha, Pitta and Vata are produced in the Avastha paka.
The Avastha paka and Nishta paka of Ayurvedic herbal drugs must be studied carefully with this concept.

Vipaka types

Different Types of Vipaka:
There are different views and theories over the number of vipakas also.
Shadvidha Vipaka Vada – 6
Niyata or Yatha Rasa Vipaka Vada-definite Vipaka concept.
Anityata Vipaka vada-indefinite vipaka concept.
Pancha vidha vipaka vada – 5
Trividha vipaka vada – 3
Dvividha Vipaka vada – 2

Shadvidha Vipaka vada: 
Yatha Rasa Vipaka Vada- this concept proposes that each Rasa will under go its individual vipaka and result in six vipakas respectively e.g. madhura rasa gives rise to Madhura vipika; Amla rasa leads to Amla vipaka etc., However, many Acharyas did not agree with this concept owing to several exceptions to this general belief that each Rasa will have its own vipaka e.g. vrihi (Rice) is madhura in taste and undergoes Amla vipaka, similarly Amalaki is Amla in taste and under goes madhura vipaka.

Badanta Nagarjuna strongly opposed this concept by stating that Rasa and Vipaka are expressed at different times and they also act upon the body at different times. Even their perception is different and effects are also different. Therefore Vipaka need not depend upon Rasa alone.But, both Susruta and Nagarjuna have quoted Shadvidha vipaka vada by mentioning it as the view of other Acharyas.

According the following are the vipakas based upon six tastes:
Madhura Rasa                                   Madhura Vipaka
Amla rasa                                            Amla Vipaka
Lavana Rasa                                        Lavana Vipaka
Katu Rasa                                             Katu Vipaka
Tikta Rasa                                             Tikta Vipaka
Kashaya Rasa                                        Kashaya Vipaka

Susruta provided an example to substantiate this view-“the plants like Shali (rice)Yava, Mudga etc., will produce the same plants through their seeds respectively but not other plants. Similarly one rasa can under go its respective vipaka but not any other vipaka”.

Aniyata Vipaka Vada:
The believers of this view are of the opinion that the Vipaka is finalized by the dominant Rasa only. The Rasa, which is predominant in a particular dravya, will nullify the effect of other rasas and emerge itself as the vipaka in the end.

Therefore vipaka cannot be assumed on the basis of the original rasa of a dravya e.g. Aswagandha is Tikta rasa but Katu vipaka; Pippali is katu rasa but Madhura vipaka. Many scholars do not accept this theory because indefinite approach has no value in scientific medicine.

Pancha Vidha vipaka Vada: 
Sushruta mentioned about the five Vipakas on the basis of Panchabhutas e.g.; Akasha dravya under goes Akasha bhutagni paka, vayu dravya under vayu bhutagni paka etc.

The five vipakas are:
Parthiva vipaka
Apya vipaka – Guru vipaka
Taijasa vipaka
Vayavya vipaka – Laghu vipaka
Nabhasa vipaka

Actually these five Vipakas will come under two vipaka only viz. Guru and Laghu vipakas.

Trividha Vipaka Vada: 
This is the widely accepted concept of Vipaka. It is mainly emphasized and proposed by Charaka, Vagbhata, Parasara etc.
The six rasas will under go three vipakas as denoted here:
Madhura vipaka – Madhura & Lavana Rasas
Amla vipaka – Amla Rasa
Katu vipaka – Katu, Tikta & Kashaya Rasas

In Ashtanga Samgraha we find a question of Parashara where in he questioned that “ if Tikta and kashaya rasas will under go Katu vipaka, how they will act as Pitta samaka (pitta subsiding factors)”.

Therefore he accepted three vipakas with a little modification:
Madhura Vipaka – Madhura, Lavana, Tikta Kashya Rasa
Amla vipaka – Amla Rasa
Katu vipaka – Katu Rasa
This Trividha vipaka concept is other wise know as ‘Rasa vipaka vada’ as it is purely based upon the Rasas.

Yogendranath Sen proposed it as “Tridoshavipaka vada” since the drugs which are under going these three vipakas will either increase or decrease Tridoshas .
They are:
Madhura Vipaka – Kapha Vardhaka
Amla Vipaka Pitta Vardhaka
Katu Vipaka Vata Vardhaka

Shivadas Sen described that Kapha and Kapha-Vata leads to Madhura vipaka; Kapha-Pitta leads to Amla vipaka and Vata, pitta & Vata-pitta leads to Katu vipaka. However do not agree with this view because rasa can only be transformed into Vipaka and therefore their effects on dosha will be seen. But dosha alone cannot determine vipaka.
Dwividha Vipaka Vada:

Susruta mentioned Vipaka as mainly two types viz., (i) Madhura vipaka and ii) Katu vipaka. These two are respectively known as Guru vipaka and laghu on the basis of their predominant guna.

Badhanta Nagarjuna proposed the concept that qualities like Seeta, Snigdha, Guru, Pichchila etc., are responsible factors for Vipaka. He also concluded that the transformation of food or drug material would end with genesis of either Guru guna or Laghu guna. Hence vipakas are two only viz., Guru vipaka and Laghu vipaka. He condemned the concept of three vipakas and stated that Vipaka of a dravya is two types- “ Chira kalika” and “Achira kalika”

According to this view the six Rasas will under go only two vipakas in the following manner-
Madhu vipaka – Madhura, Amla, Lavana Rasa
Katu vipaka – Katu Tikta & Kashaya Rasas

Nagarjuna’s view is expressed in the following way:
Guru Vipaka – Seeta, Snigdha, Guru, Pichchila gunas.
Laghu vipaka – Laghu Ruksha, Visada, Tikshna gunas.
Trividha Vipaka Vada vis-à-vis Dwividha Vipaka Vada

Susruta mentioned that the Trividha Vipaka vada is incorrect because Amla vipaka can neither exist by means of qualities of bhutas nor mentioned in the classical texts. On the other hand Pitta will have Katu rasa and Amla rasa in normal state and vitiated state.

Here the Amla state is only temporary and vitiated phase. If vitiated state is also considered, then one must accept the Lavana vipaka. It is because vitiated Kapha attains Lavana Rasa. Hence vipakas are considered as two.

The Madhura vipaka will act as Vata and Pitta samaka while Katu vipaka subsides Kapha. In this way all the doshas are covered by two Vipakas.

Yogendranath Sen elaborated his views on this concept and proposed that-Charaka accepted Amla,Katu rasa for normal Pitta while Susruta accepted only katu rasa. According to Susruta, Amlatwa is attributed to Pitta in vitiated state only.

Trividha Vipaka vada is mainly based upon Tridoshas while Dwividha Vipaka vada is mainly based upon Pancha bhutas.

Badhanta Nagarjuna also condemned the concept of three Vipakas. He delineated three reasons in support of his view:
Kalatah: Some substance will be digested very lately and some will be digested immediately. Therefore Vipaka is two types viz., Chira kalika and Achira kalika
Gunatah: On the basis of qualities Vipaka can be only two varieties viz., Guru & Laghu Vipakas.
Rasatah: The transformation of Rasas will occur in two forms viz., Katu-Tikta-Kashaya Rasas – Laghu Vipaka and Madhura-Amla-Lavana Rasas – Guru Vipaka.

Yogendranath elaborately discussed on Amla vipaka. Usually prithvi bhuta is responsible for Guru Vipaka and agni bhuta is responsible for Laghu Vipaka. Hence it is difficult to decide the Amla Vipaka since Amla is the combination of Prithvi and Agni bhutas.

He quotes that Charaka accepted Pitta as Amla as well as Katu in normal state. But Susruta accepts only katu for the pitta in the normal state. On this issue both the Acharyas differed and therefore Trividha & Dwividha Vipaka concepts are proposed respectively.

Some scholars will consider Amla Vipaka as a part and parcel of Madhura Vipaka and some include it under Katu Vipaka. Even Charak described Madhura as Guru Vipaka and Amla and Katu as Laghu Vipakas in one context.
Before more description in made on Vipaka, it is essential to finalize the concept of Vipaka.

It appears that the concept of Vipaka as mentioned by Charaka is mainly aimed at Ahara. At the same time Susruta’s concept of Dwividha Vipaka vada reflects the drug metabolism. On review of literature the author could not trace out even a single oushahdha dravya undergoing Amla vipaka. So it will be quite appropriate to conclude that Trividha and Dwividha Vipaka concept will be reflecting the food and drug metabolism respectively in Ayurveda.

The reason for not mentioning these two aspects separately and directly is that Ayurveda mainly uses natural products/herbs in the therapeutics. Thus drugs metabolism as described in modern pharmacology is not directly applicable in every aspect of Vipaka concept. However herb are used as Ahara as well as Oushadha. For example, Pushkara mula, which contains inulin as primary metabolite is, used in diet while the secondary metabolites flavonoids are, used in medicine.

From these discussions, a common conclusion may be drawn that the concept of Dwividha Vipaka is more suitable in the context of Dravya guna Vijana with special reference to herbs.

Vipaka properties, functions

Properties And Functions of Vipaka:
Vipaka of a drug will affect the doshas, dhatus and malas also. Charaka very lucidly described the qualities and actions of three Vipakas. They are tabulated below:

Types of Vipaka
Madhura Vipaka – Snigdha Guru Guna, Enhances Kapha, Decreases vata-Pitta, improves dhatus and Sukra. Increases the quantity of stools & urine

Amla Vipaka – Snigdha Laghu Guna, Enhances Pitta         Subsides Vata Sukrahara            -do-
Katu                       Ruksha Laghu       Enhances Vata,        Subsides Kapha Sukra hara    Decrease the quantity of stools                                                                                                                                                                                           & urine

Similarly Susruta and Badhanta Nagarjuna have explained the qualities and effects of guru and laghu vipkas. But, they did not mention about the effect on sukra dhatu by the two vipakas. Their properties are tabulated.
 Type of Vipaka        Bhuta Predominance      Effect on Doshas                   Effect on Malas
Guru(Madhura)            Prithvi & Ap                            Kapha Vardhaka                         Srishta vit- mutra (increase in                                                                                                         Vata- pitta hara                                   quantity of stools & urine)
Laghu                             Vayu, Agni                                   Kapha hara &                            Baddha Vit-Mutra (decrease in                                                  Akasha                                       Vata-pitta vardhaka                     quantity of stools & urine)

In this context it was clarified that the guru and Laghu Vipakas need not be dosha vardhaka and samaka respectively in all cases.
Vagbhata considers that Vipaka exhibits its effects similar to those of Rasas e.g. Madhura Rasa properties and action can be seen with Madhura Vipaka etc.

Before the effects of Vipaka are concluded it is important to know about the degree of variation of Vipaka. According to Charaka there are three degrees of variation for Vipaka.
Uttama (superior)
Madhyama (average)
Avara/Alpa (inferior)

Gangadhara also mentioned that gunas like Snigdha, Ruksha etc., would decide the degree of variation of Vipaka.
Vipaka          Superior     Average             Inferior
Madhura        Madhura        Amla                      Lavana
Katu                 Kashaya         Katu                      Tikta

Assessment of vipaka:
Vipaka is assessed finally after the complete metabolism of the drug and though the final effect of the drug. That means the end phase of bio-transformation resulting in ultimate therapeutic effect is the source to assess Vipaka.

Vipaka cannot be perceived directly. It can be assessed with the help of inference. Chakrapani is of the opinion that Vipaka shall be assessed even in the absence of Pratyakshatwa.

Yogendranath Sen also accepted the same view and stated that Vipaka can be determined by means of final therapeutic effect. For example, the effect of food or drug can be finalized by its effect on dosha-dhatu-malas in the form of increase or in their activity.

Sivadas Sen further quoted some exceptions and described that Vipaka cannot be assessed basing upon Rasa or Virya alone. Some times it must be assessed without their help also.e.g. Vrishyatwa of Pippali indicate that it under goes Madhura Vipaka; Srishta Vinmutra of Saindhava lavana indicate its Madhura Vipaka etc.

Experimental Model to Assess Vipaka: 
The author proposes an animal model for the assessment or determination of Vipaka of unknown drugs. It will also help to reassess the Vipaka of classical drugs.
The main parameters shall be measurement of faeces and urine quantities and ii) mating behavior and testicular biopsy of the animals selected for study.

The proposed models consist of two groups of animals Preferably albino rats or rabbits may be picked up. One group will be given the drug and the other group is kept as control group. The study should be the conducted in the metabolic cages by segregating each animal in individual cages.

The quantity of faeces and urine shall be measured every day for one week or 10 days. The mating behavior of the animal may also be documented simultaneously or separately. Afterwards the animal may be sacrificed to study the testicular histology. All these aspects may be compared with the control group.

The drugs, which increased the quantity of urine, faeces and spermatogensis, may be labelled as Madhura Vipaka and vice versa may be labelled as katu Vipaka.

Difference Between Rasa and Vipaka:
It is already discussed that Vipaka is the final out come of Rasa transformation inside the body. Thus a question may arise that when Vipaka is dependent of rasa what are the necessity to explain both then entities separately? We find the following points which differentiate both of them:
Dissimilar characters or properties
Differences in the time taken for their actions
Difference in their therapeutic effects
Dissimilarity in the their perception.

                             Rasa                                                                                 Vipaka: 
It is known by means of Rasanendriya (tongue)                 It is known by means of its final effect on the body.
Quick and local effects are seen                                              Delayed and systemic effects are seen.
It can be directly detectable                                                     It will be detected by inference.

P.V. Sharma is of the opinion that Rasa mainly affects the mind and Vipaka mainly affects the body. This view may not stand food for logic and scientific discussion since Medhya drugs, which are used in psychiatric conditions, are acting both on mind and body. E.g. Guduchi with Madhura paka and Manduka parni with Tikta Rasa are both acting as Medhya Rasayanas. Both the drugs are acting upon psyche and soma.

Dr.K. Nisteswar in his text on Dravya guna differed with P.V. Sharma’s view. He also disagreed with the concept of local action and systemic action of Rasa and Vipaka respectively. But the author is of the opinion that Rasa usually reflects the local effects of a drug than the systemic effects. Field of action of Rasa is limited to Rasanendriya only.

However, if the drug is once administered to a man it crosses the field of tongue and under goes the conversion into Virya and Vipaka. Hence some of the system effects are attributed to Rasa.

Which is stronger factor to cause medicine action? Taste or Vipaka?

Superiority Of Vipaka:
Among the constituents of a dravya that is responsible for various pharmacological actions. This again depends upon proper or improper transformation. If a drug is properly metabolized it produces good effects and improperly metabolized, drug is responsible for ill effects. Sushruta denotes these two properties as ‘Samyak Vipaka’ and ‘Mithya Vipaka’.

Some scholars may consider the former as Samana pratyarabdhatwa and the later as Vichitra pratyayarabdhatwa. For example, Chitraka, which undergoes Katu Vipaka according to its Rasa, Guna, & Vipaka will be responsible for Baddha vit-mutra. While, Pippali which under goes Madhura Vipaka irrespective of its Rasa, Guna & Virya will act as Sukra Vardhana.

Badhanta Nagarjuna explains the superiority of Vipaka on the basis of four points. They are:
Nimittatwa (responsible factor) — Stimulation or suppression of the doshas is under the control of Vipaka. Hence it is important.
Dhatupadehat (tissue construction): Tissue construction or building up various tissues of the body is possible through digestion /metabolism. Therefore Vipaka is important.
Vipakakshepatwa (dependency for therapeutic effects): All the food and materials will depend upon proper and improper Vipaka to exhibit either good effects or ill effects. Hence Vipaka is important.
Shastra Pramanya (emphasis by classics): Classical texts/treatises quote Vipaka as an important entity of dravya. Hence it is superior.

Modern Interpretation Of Vipaka: 
Avastha paka & Nishta paka are mainly described in relation to food metabolism in Ayurveda. It is essential to review the definition of digestion and metabolism in relation to Avastha and Nishta paka.

Digestion – it is the process of bio-chemical transformation of complex and larger food particles in the gut enzymatically into a simple form suitable for absorption and assimilation, the complex and higher molecules being unsuitable for absorption.

Metabolism – Metabolism of a food substance is a series of specific biochemical reactions occurring within the living organism from the time of its incorporation into the cell or tissue till its excretion, of which some are concerned with tissue synthesis and others with tissue break-down what are termed as anabolism and catabolism respectively.

Though natural products are used as medicine in Ayurveda, drug metabolism of these herbs etc. shall be considered differently since some of the digestive and metabolic states differ from that of food substances. For example, Swarasa, Kalka, Churna etc, preparation may have not any starch reaction with saliva, as they are not boiled. But Kashaya, Lehya etc., will undergo the salivary digestion also.

On the other hand the secondary metabolites of the plants i.e., active principles etc., will under go certain chemical changes inside the body under the influence of liver and tissue micro-enzymes resulting in respective pharmacological actions. The drug effects produced through the process of metaboilzation shall only be considered as Vipaka.

The unmetabolized active principle of drug responsible for pharmacological action may be considered as Virya. E.g. the third phase of effects of morphine is seen after conjugation (i.e., after vipaka) while first two phases of its effects are due to Virya (unmetabolized active ingredient).

The concept of Vipaka in Ayurveda will cover the drug metabolism or pharamacokinetics dealt in modern pharmacology. Depending upon this view the fate of an herbal drug administered orally may be explained in this way:

Herbal drug

The above diagram represents the macro molecules of the herbal drugs stored in various tissues of the plants. These chemical constituents will be arranged in four categories viz., hydrophilic polar, lipophylic and highly lipophylic.

Hydrophilic – They enter blood stream immediately after absorption and reach the site of action or target organ. The end products are eliminated through bile and urine.

Polar – they will readily enter into the body fluids and under go second phase of biotransformation and finally attain bio-inactivation. This process is also completed quickly compared to lipophylic compounds.

Lipophylic- they undergo first phase of biotransformation initially and become activated or inactivated. If activated the polar components are formed which will further undergo second phase of biotransformation to becomes inactive.

Highly lipohylic – They are sequestered and stored in the adipose tissue.

From these findings it may be tentatively concluded that the drug action of hydrophilic and polar components may be similar to that of Virya, while lipophylic components acting on end organ is due to vipaka. The former one does not require complete biotransformation while the latter one does require.

Polar compounds bye-passing phase-I metabolism demonstrate the concept of Vyavayi, Vikasi, Asukari etc., gunas. The main difference between Vipaka and Virya will be that- Vipaka acts through “ distribution” while; Virya acts through “absorption”.

The two phase of metabolism may be represented as:
Phase I – Functionalization reaction.
Phase II – Synthetic or conjugating reaction.

On the other hand certain compounds are insoluble in the body fluids and resistant to the chemical/enzymatic reactions in the GIT. Therefore they are eliminated through stools and do not exhibit any biological action e.g. mineral oil, barium sulphate.

Some compounds, which are freely soluble in body fluids and resistant to chemical changes, are relatively non-toxic and rapidly excreted e.g. aromatic and aliphatic compounds.

The following aspects of drug metabolism are essential for every drug:
Rates and site of absorption of both the drug and its metabolite.
Plasma and tissue levels.
Plasma protein binging.
Rate of metabolism and half-life of the drug in blood and tissues.
Rates and routes of excretion.
Sites of drug metabolism

Liver is the primary site of drug metabolism and metabolism of all substances which enter blood circulation via the GIT (through portal circulations). The oxygenated blood per fuses through liver cells where nutrients are removed or stored.

Drugs are metabolized into water-soluble derivatives and returned to circulation for excretion by the kidneys. Bile also acts as one of the means of excretion of metabolic products through faeces.

Metabolism of drugs and foreign substances in the liver is carried out by a number of specific and non-specific enzymes. Generally, drug metabolism may involve the following types of biotransformations.

A single step conversion of biologically active compound to an inactive compound, which is excreted;
A two-step conversion, in which there is first inactivation followed by conjugation with glucuronic acid;
A two-step process in which an inactive compound is converted to a biologically active compound followed by inactivation and excretion;
A two-step process in which there is first a change in activity of an active compound followed by inactivation.

Types of Metabolic Conversions: 
Oxidation – it is normally the first step involved in the drug metabolism unless the drug compound possess functional groups like –OH, -SH, -NH2, -CO2H which are capable of conjugation. A complex of non-specific microsomal enzymes present in the liver catalyze metabolic oxidation of a large variety of compounds both endogenous substances like steroid hormones and exogenous substances like drugs and pollutants. The most important enzyme involved in this type of oxidation is cytochrome p- 450

Reduction – Metabolic reductions are carried out by enzyme systems, which make use of NADPH as a hydrogen donar.
Hydrolysis – Esterases such as pseudocholine esterase present in plasma will catalyze the hydrolysis reactions of drug substances.
Conjugation Reaction – in case of drugs and other molecules containing group like -OH2, -SH, -NH2 OR –CO2H conjugation with glucuronic acid, sulphate, amino acids and peptides is the major pathway for metabolic elimination. However, in the absence of such reactive groups, oxidation, reduction, hydrolysis etc., precedes the conjugation reaction.

These aspects confirm that the phenomenon through which a drug molecule gets converted into different form is termed as ‘ biotransformation’ or metabolism. Most of the metabolized end products are usually more polar in charter than the parent drug molecule. This increased polarity renders the metabolite less absorbable through the renal tubules and also makes it transient in the body. There will be a disparity due to the genetic differences among different races.
References:
1. Charaka Samhitha
By: R.K. Sharma  Pub: Chowkambha Sanskrit Series Office, Varanasi Edn:2000
2. Susruta Samhitha – Vol 1 (Sutra Sthana)
3. Asthanga Samgraha – Vol 1 (Sutra sthana)
4. Ashtanga Hridaya – Vol 1 (Sutra sthana) Translated By: Prof .K.R SRIKANTHA MURTHY
Pub: Krishna Das  Academy,Varanasi  Edn: IV, 1999
5. Charaka Samhitha – VOL-III (Chikitsa sthana) By: R.K.Sharma, Bhagwan Das
Pub: Chowkambha Sanskrit Series Office, Varanasi Edn: 1999
6. Rasa Vaisheshika – VOL-I (Sutra sthana)
7. Ayurveda Pharmacology And Therapeutic Uses of Medicinal Plants
By: Vaidya V.M Gogte
8. Introduction to Dravya Guna, By: Prof. P.V.Sharma
Pub: Chowkambha Orientalia, Edn: 1995
9. Dravya Guna Vijnana By: DR. J.L.N. Shastry, Pub: Chowkambha Orientalia, Edn: 2004