E-ISSN:2456-3110

Review Article

Anti-oxidant Activity

Journal of Ayurveda and Integrated Medical Sciences

2024 Volume 9 Number 10 OCTOBER
Publisherwww.maharshicharaka.in

A pharmacodynamic study of Charaka Vishaghna Mahakashaya with its correlation with anti-oxidant activity

Gusai T1*, Tomar BS2, Chawla SK3
DOI:10.21760/jaims.9.10.26

1* Tanvi Gusai, Post Graduate Scholar, Dept of Agada Tantra, Shree Krishna Government Ayurvedic College, Kurukshetra, Haryana, India.

2 Brijendra Singh Tomar, Professor, Dept of Agada Tantra, Shree Krishna Government Ayurvedic College, Kurukshetra, Haryana, India.

3 Satbir Kumar Chawla, Assistant Professor, Dept of Agada Tantra, Shree Krishna Government Ayurvedic College, Kurukshetra, Haryana, India.

Background: Free radicals are by products of normal cellular metabolism. A balance of free radicals and antioxidants is required for proper physiological functioning. The balance between the production of free radicals and antioxidant defences in the body has important health implications.

Aim and objective: to study the pharmacodynamics of Vishaghna Mahakashaya & the probable mode of action through which it can act as antioxidant.

Discussion: Vishaghna Mahakashaya includes 10 individual drugs which might act as Vishaghna. Vishaghna property can be understood as the property to act against Visha. Rasapanchaka of the Vishaghna Mahakashaya are predominantly Madhura, Tikta & Kashaya Rasa, Laghu & Ruksha Guna, Ushna Virya & Katu Vipaka and proved the concept of Vipritarthkari Chikitsa of Visha.

Conclusion: Vishaghna Mahakashaya can have potential antioxidant activity. And can play an important role in counteracting the excess free radical. Further studies can be conducted to study its role as antioxidant.

Keywords: Antioxidant, Free Radcial, Vishaghna, Vishaghna Mahakashaya

Corresponding Author How to Cite this Article To Browse
Tanvi Gusai, Post Graduate Scholar, Dept of Agada Tantra, Shree Krishna Government Ayurvedic College, Kurukshetra, Haryana, India.
Email:
Gusai T, Tomar BS, Chawla SK, A pharmacodynamic study of Charaka Vishaghna Mahakashaya with its correlation with anti-oxidant activity. J Ayu Int Med Sci. 2024;9(10):157-164.
Available From
https://jaims.in/jaims/article/view/3691

Manuscript Received Review Round 1 Review Round 2 Review Round 3 Accepted
2024-09-13 2024-09-24 2024-10-04 2024-10-14 2024-10-24
Conflict of Interest Funding Ethical Approval Plagiarism X-checker Note
None Nil Yes 13.36

© 2024by Gusai T, Tomar BS, Chawla SKand Published by Maharshi Charaka Ayurveda Organization. This is an Open Access article licensed under a Creative Commons Attribution 4.0 International License https://creativecommons.org/licenses/by/4.0/ unported [CC BY 4.0].

Introduction

Agada Tantra has always been a part of our life & society. In the past it was used as means to manage & diagnose the bites of Sarpa, Keeta, Loota, Mushakadi, etc., to protect the king from getting harmed by the enemies, Visha Kanya, knowledge about Dushit Aahar, Jala & Bhoomi along with their treatment.

With the modernisation of world, there has been modernisation of the world Visha. Visha can be defined as the substance that after entering the body causes the vitiation of the Dhatus (bodily tissues) and also deteriorates the health of human being i.e., it causes ‘Vishada’(sadness).[1]

The world Visha can have both physiological as well as mental aspect with respect to Sharirika & Mansika Vyadhi.

In present time the word Visha can be understood correlated and understood through following modern terms:

  • Processed food
  • Food adulterants
  • Environment toxicology (air, water, & soil pollution)
  • Use of plastics
  • Cosmetic toxicology (use of chemical-built soap, creams, lotions, toothpaste etc.)
  • Occupational toxicology (use of agricultural pesticides, exposure to radiation)
  • Alcohol intake
  • Unhealthy dietary pattern
  • Drug abuse of substances like heroin, opium, cannabis & other re-creational drugs
  • Cigarette smoking

In this article we are going to understand the free radical, antioxidants & how Vishaghna Mahakashaya mentioned in Charaka Samhita can represent potential antioxidant property.

Materials and Methods

Free radical

Free radicals are the products of normal cellular metabolism.

A free radical can be defined as an atom or molecule containing one or more unpaired electrons in valency shell or outer orbit and is capable of independent existence. The odd number of electron(s) of a free radical makes it unstable, short lived and highly reactive.[2] Because of their high reactivity, they can abstract electrons from other compounds to attain stability. Thus, the attacked molecule loses its electron and becomes a free radical itself, beginning a chain reaction cascade which finally damages the living cell.[3]

Free radicals are generated either by normal cell metabolism or by external sources (pollution, cigarette smoke, radiation, medication).

Free radicals are involved in many pathological conditions such as many types of diabetes, neurodegenerative diseases, cardiovascular diseases (CVDs), cancer, cataracts, asthma, rheumatoid arthritis, inflammation, burns, intestinal tract diseases, progerias and ischemic and post-ischemic pathologies.[4]

Antioxidant[5]

Antioxidants are endogenous or exogenous substances which inactivate the free radicals. These substances include the following:

  • Vitamins E, A and C (ascorbic acid) e.g., citrus fruits, guava, amla, pumpkin, almonds etc.
  • Sulfhydryl-containing compounds e.g., cysteine and glutathione.
  • Serum proteins e.g., ceruloplasmin and transferrin

In the most general sense, a natural or synthetic antioxidant directly or indirectly functions to minimize damage to biomolecules (mostly proteins, lipids, and DNA) caused by reactive oxygen species (ROS) and/or reactive nitrogen oxide species (RNOS).

Vishaghna Mahakashaya / Gana has been mentioned at several places in the Samhita’s.

Charaka Samhita[6]

  • Mentioned under 50
  • हरिद्रामञ्जिष्ठासुवहासूक्ष्मैलापालिन्दिचन्दनकतकशिरीषसिन्धुवारश्लेषमातक इति दशेमानि विषघ्नानि भवन्ति। च.सू.4/16
  • Haridra, Manjishta, Suvaha, Sukshma Ela, Palindi, Chandan, Katak, Shirisha, Sindhuvara, Shleshmatka

Table 1: Raspanchak of Vishaghna Mahakashya

NameRasaGunaViryaVipakaKarma
Haridra[11]Tikta, KatuRuksha, LaghuUshnaKatuKapha-Pittahara, Varnya, Prameha, Shotha,
Manjistha[12]Madhura, Tikta,KashayaGuru, RukshaUshnaKatuShotha, Raktaatisara, Kushta, Visarpa
Suvaha[13]
(Rasna)
TiktaGuruUshnaKatuKapha-Vatahara, Shotha, Vatika-Amya
Sukshma Ela[14]Katu,LaghuSheetaKatuKaphanashak, Shvasa, Kasa, Mutrakrucha
Palinidi[15]
(Trivit)
MadhuraLaghu Ruksha TikshnaUshnaKatuVatahara, Shotha, Udarroga, Pitta Jvara
Chandan[16]
(Rakta)
Tikta, MadhuraGuruSheetaKatuRaktapittahara, Chakshushya, Vrishya
Kataka[17]Madhura, Kashaya,Laghu,SheetaMadhuraKapha- Vatahara, Chakshushya
Shirisha[18]Madhura, TiktaLaghu,Ushna (Anushna)KatuVishaghna, Shoth, Visarpa, Vranaropan
Sindhuvara[19]Katu, Tikta,
Kashaya
LaghuUshnaKatuVata-Kaphahara, Chakshusya, Keshya, Krimighna, Shotha
Shleshmatak[20]MadhuraSnigdha, Guru, PichilaSheetaPhala-Madhura
Tvak- Katu
Pitta- Kaphahara, Visphota, Visarpa

Sushruta Samhita[7]

  • Mentioned under 37 Ganas.
  • Gana Aragvadhadi Gana, Lodhradi Gana, Eladi Gana, Shyamadi Gana, Patoladi Gana, Utpaladi Gana.

Table 2: Visha Guna

SNVisha GunaCharaka [21]Sushruta [22]Vagbhata [23]Sharanghdhar [24]
1.Laghu+++-
2.Ruksha+++-
3.Aashu+++-
4.Vishad+++-
5.Vyavayi++++
6.Tikshna+++-
7.Vikasi++++
8.Sukshma++++
9.Ushna+++-
10.Anirdeshiya+---
11.Apaki-++-
12.Avyakt Rasa--+-
13.Chedi---+
14.Madavaha---+
15.Jeevitahar---+
16.Yogavahi---+
17.Agneya---+


Table 3: Ingredients Phytochemical & their Pharmacological actions

SNVishaghna DravyaPart usedPhytochemical constituentsPharmacological action
1.Haridra[25]
Curcuma longa
Zingiberaceae
RhizomeCurcuminoids, curcumin, desmethoxycurcumin, dihydrocurcuminAntibacterial, cholagogue, insecticidal, antifungal, anti-inflammatory, antiprotozoal, cns depressant, antifertility, antarthritic, hypocholesteremic, antihepatotoxic, antihistaminic
2.Manjistha[26]
Rubia cordifolia
Rubiaceae
StemAlizarin, purpurin, xanthopurpurin, munjistin, glucose, sucroseAntioxidant, antibacterial, anticancer, anti-inflammatory, anti-tumor, antiviral, hemostatic, anti-lipid peroxidative activity, hypogylcemic.
3.Suvaha[27]
Pluchea lanceolata
Asteraceae
Root, leaf, whole plant,Quercetin, quercitrin, isorhanetin, pleuchioside, pleuchiolAnti-inflammatory, anti-oedema, spasmolytic, anti-implantation, analgesic.
4.Sukshma ela[28]
Elettaria cardamomum
Zingiberaceae
Seedα- pinene, sabinene, myrcene, limonene, cineol, cymeneHepatoprotective, anti -inflammatory, analgesic, antispasmodic, anti-microbial, anti- fungal
5.Palindi[29]
Operculina turpethum
convolvulaceae
Rootα- & β-Turpetheins, coumarin, 4’-0-methylpigenin, luteolinAntibacterial, anti-inflammatory, cathartic, anti-helminthic, cardiac depressant and spasmodic to smooth and skeletal muscles.
6.Rakta Chandan[30]
Pterocarpus santalinus Linn.
Fabaceae
Heart woodSantalin A, santalin B, isopterocarpolone, pterocarpol & pterocarptriolHypogylcaemic, antispasmodic, coagulant, nematicidal, anti-inflammatory, anti-arthritic, cns depressant, anticonvulsant, depressant, anti- androgenic, anti-bacterial, antipyretic, anti- allergic.

7.Katak[31-34]
Strychnos potatotroum
loganiaceae
Seeds, rootdiaboline (major alkaloid) and its acetate ,brucine, loganin, mannose, sucrose, arachidonic, lignoceric, linoleic, oleic, palmitic, and stearic acids.Anti diabetic, anti- inflammatory, anti-ulcer, hepatoprotective, antioxidant, anti-arthritic, antinociceptive, antipyretic effect, anti-diarrheal, diuretic, contraceptive efficacy, anti microbial.
8.Shirisha[35]
Albizzia lebbek
Mimosoideae
Bark, flower, seed,Melanoxetin, okanin, phytosterol, flavonoids, alizziageninAnti protozoal, hypogylcemic, anticancer, spermicidal, abortifacient, anti-asthmatic, anti- allergic, analgesic, antifertility, anti-fungal, anti- ovulatory, anti-anaphylactic, antibacterial, hypotensive, cns depressant, and bronchodilator.
9.Sindhuvara[36]
Vitex negundo
Verbenaceae
LeafHentriacontane, β-sitosterol, β- sitosterol acetate, stigmasterol, vanillic acidAnti-inflammatory, antibacterial, moderate cns depressant, anti-fertility, anti-spasmodic, analgesic, hepatoprotective, estrogenic, anti-convulsant, anti-arthritic, diuretic, antimicrobial, anti-parkinsonian, anti-psychotic, anti- depressant, antihistamine releasing activity, mosquito repellent activity, antifeedant, anti-filarial, juvenomimetic, antiandrogenic.
10.Shleshmatak [37-40]
Cordia dichotoma
Boraginaceae
Stem bark, fruitα-amyrins, betulin, octacosanol, lupeol-3rhamnoside, β-sitosterol, β-sitosterol-3glucosideNormoglycemic, wound healing activity, anti-microbial, antifungal, analgesic, anti-bacterial, cytotoxic activity, anti-oxidant activity, anti-diabetic activity, anti-helminthic activity, anti- ulcer, anti-inflammatory, anti-implantation activity.

Ashtanga Samgrah[8,9]

  • He has mentioned both Mahakashaya and
  • Vishaghna Mahakashaya Manjistha, Shleshmatka, Rajni, Suvaha, Shirisha, Palindi, Chandana, Katak, Nirgundi.
  • Ganas Anjanadi Gana, Patoladi Gana, Aragvadhadi Gana, Rodhradi Gana, Arkadi Gana, Eladi Gana, Shyamadi Gana.

Ashtanga Hrudaya[10]

  • Mentioned only Ganas
  • Anjanadi Gana, Patoladi Gana, Aragvadhadi Gana, Rodhradi Gana, Arkadi Gana, Eladi Gana, Shyamadi Gana.

Discussion

There are 50 Mahakashaya as per Acharya Charaka. One of the Mahakashaya taken for study was Vishaghna Mahakashya. There are ten drugs in each Mahakashaya i.e., Haridra, Manjistha, Suvaha (Rasna), Sukshma Ela, Trivrut, Rakta Chandan, Katak, Shirisha, Shleshmatak & Nirgundi. These Mahakashaya can be used both internally and externally. Moreover, out of the Panchvidha Kashaya Kalpana, it can be further used in many other dosage forms such as Swarasa, Kalka, Kwath, Phanta or Hima Kalpana. The dominant rasa are Madhura & Tikta (33%) followed by Katu & Kashaya (33 %) consecutively. Among Guna’s of Vishaghna Dravya, Laghu Guna (40%) seems to be dominating followed by Ruksha Guna (20%), Pichila , Snigdha ( 7%) & Tikshna Guna ( 6%) respectively. Ushna Veerya (50%), followed by 40% Sheeta Veerya with 10 % of Anushna Veerya.

In Vipaka also, Katu (90%) is the dominating Vipaka followed by the Madhur Vipaka (10%).

jaims_3691_01.JPG

jaims_3691_02.JPG


jaims_3691_03.JPG

jaims_3691_04.JPG

Rasapanchaka of Vishaghna Mahakashya

It can be seen that these Vishaghna Mahakashya Dravya’s will act against the Visha by the virtue of their Dravyaguna properties. It is observed that these Vishaghna Mahakashya possess Dravyguna proprties which seems to be similar to the properties of visha. These Dravya’s possess the same potential similar to Visha to fight against the Visha, so that it can reach the places where Visha has been placed in the body. Although among the rasa where Madhur & Tikta seems to be prevailing, will help in the process of fighting the Visha along with the healing. As per Acharya Charaka, madhura Rasa & Tikta Rasa is responsible for Vishaghna property which justifies its role as Vishaghna.[87]

These 10 drugs individually possess antioxidant properties proved through modern methods. Various research has been conducted on the constituents of Vishaghna Mahakashaya accounts for its antioxidant properties.

Properties of Vishaghna Mahakashaya when compared to Visha Guna, Laghu, Ruksha, Ushna & Tikshna Guna seems to be common. Similarly, when compared with Oja Guna, Snigdha, Sheeta, Pichila & Madhura Guna seems to be common. When the formulation possesses properties like Visha, it will counteract against Visha with almost same strength, as Visha acts in the body. With the similarities in properties of Oja, it will help in preventing the after effects of Visha by its Snigdha, Sheeta, Pichila & Madhura Guna.

Conclusion

It is observed that Vishaghna Mahakashya can act antioxidant and act against free radcials through its Dravyaguna property. Similarly, the free radicals act at cellular and tissue level, and seems to act in similar way to Visha such as, Marmaghna i.e., it effects the vital organs, by its Sukshama Guna it is acting at cellular level and destroying the healthy cells. Due to its Vyavayi & Vikasi Guna, it travels freely in Rakta and causes the further vitiation of other Doshas. Free radicals are not limited to any specific system or organ and may get situated anywhere i.e., Vishada Guna. Due to its Laghu Guna, it is difficult to treat. So, the Vishaghna Mahakashaya which is defined as the Mahakashaya will act against or counteract the effect of Visha. The concept of Vipritarthkari Chikitsa is also justified. Therefore, analytical study can be done to establish its role as antioxidant.

References

1. U.R.Sekhar, A Textbook Of Agada Tantra, Introduction To Visa, Chapter-2 , Chaukhambha Sanskrit Sansthan; first ed. 2006 pg 6.
2. Phaniendra A, Jestadi DB, Periyasamy L. Free radicals: properties, sources, targets, and their implication in various diseases. Indian J Clin Biochem. 2015;30(1):11-26. doi:10.1007/s12291-014-0446-0
3. Mukherji SM, Singh SP. Reaction mechanism in organic chemistry. Madras: Macmillan India Press; 1986.
4. Mukherji SM, Singh SP. Reaction mechanism in organic chemistry. Madras: Macmillan India Press; 1986.
5. Mohan Harsh, Textbook of Pathology; 6th edition chapter 3, Jaypee Brothers Medical publishers (P) Ltd; 2010 pg 33.


6. Pandeya Kashinath & Chaturvedi Gorakhnath, hindi commentary on Charaka Samhita, Sutrasthana, Shadavirechanashitiya Adhyaya, Chapter 4 verse 16 Chaukhambha Sanskrita Sansthana 2016 pg 83.
7. Shastri Ambikadutta, Sushruta Samhita, Sutrasthana; Dravyasamgrahaniya Adhyaya, Chapter 38; Verse 7,15,25,30,34,53, Chaukhambha Sanskrit Sansthana. 2018, pg 182-189.
8. Tripathi Ravidutt, Asthtang Samgraha, Sutrasthana; Mahakashaya Sangraha, Chapter 15 verse 21 Chaukhambha Sanskrit Sansthana 1999, pg 300.
9. Tripathi Ravidutt, Asthtang Samgraha Sutrasthana; Vividhgana adhyaya Chapter 16 verse 8,9,12,21,23,38,39, Chaukhambha Sanskrita Sansthana 1999 pg 312-326.
10. Tripathi Ravidutt, Asthtang Hridaya Sutrasthana; Shodhanadigana Sangraha Chapter 15 verse 14,15,17,27,29,44,45. Chaukhambha Sanskrita Sansthana 2017, pg 223- 239.
11. Chunekar K.C., commentary on Bhavaprakasa Nighantu; Haritkyadi varga, verse 196,197 Chaukhambha Bharti academy 2015 pg 111.
12. Chunekar K.C., commentary on Bhavaprakasa Nighantu; Haritkyadi varga verse 190,191 Chaukhambha Bharti academy 2015 pg 107.
13. Chunekar K.C., commentary on Bhavaprakasa Nighantu; Haritkyadi varga verse 163,164 Chaukhambha Bharti academy 2015 pg 76.
14. Chunekar K.C., commentary on Bhavaprakasa Nighantu; Kapooradi varga verse 63 Chaukhamba Bharti academy 2015 pg 212.
15. Chunekar K.C., commentary on Bhavaprakasa Nighantu; Guduchyadi varga verse 193,194 Chaukhambha Bharti academy 2015 pg 383.
16. Chunekar K.C., commentary on Bhavaprakasa Nighantu; Kapooradi varga verse 17, Chaukhambha Bharti academy 2015 pg 182.
17. Chunekar K.C., commentary on Bhavaprakasa Nighantu; Aamradiphala varga verse 108 Chaukhambha Bharti academy 2015 pg 572.
18. Chunekar K.C., commentary on Bhavaprakasa Nighantu; Vatadi varga verse 14 Chaukhambha Bharti academy 2015 pg 506.
19. Chunekar K.C., commentary on Bhavaprakasa Nighantu; Guduchyadi varga verse 113, 114 Chaukhambha Bharti academy 2015 pg 329.
20. Chunekar K.C., commentary on Bhavaprakasa Nighantu; Aamradiphala varga verse 105, 106, 107 Chaukhambha Bharti academy 2015 pg 471.

21. Pandeya Kashinath & Chaturvedi Gorakhnath, hindi commentary on Charaka Samhita, Chikitsasthana, Vishachikitsa, Chapter 23 verse 24, Chaukhambha Sanskrita Sansthana;2016 pg 629.
22. Shastri Ambikadutta,Sushruta Samhita, kalpasthana; jangamvisha adhyaya, Chapter 2; Verse 19, Chaukhambha Sanskrita Sansthana ;2018, pg 31
23. Kaviraj Atrideva Gupta, Hindi Commentary On Astanga Samgraha, Visha Pratishedha Adhyaya, Chapter 40 Verse 12, Choukhambha Krishnadas Academy, Varanasi; 2019 pg. 341
24. Shrivastav Shailja, ‘Jivanprada’ Hindi Commentary on Sharangdhar Samhita, Chaturtha Adhyaya, Poorvakhanda, Chapter 4 Verse 23, Chaukhambha Orientaila, Varanasi 2017, pg. 34
25. Sharma P.C., Yelne M.B, Dennis T.J., Database on Medicinal Plants Used in Ayurveda, Haridra, volume 1, 2005, Documentation and Publication Division Central Council for Research in Ayurveda & Siddha; pg.152-55
26. Sharma P.C., Yelne M.B, Dennis T.J., Database on Medicinal Plants Used in Ayurveda, Manjistha, volume 5, 2005, Documentation and Publication Division Central Council for Research in Ayurveda & Siddha: pg. 171
27. Sharma P.C., Yelne M.B, Dennis T.J., Database on Medicinal Plants Used in Ayurveda, 2005, rasna, Documentation and Publication Division Central Council for Research in Ayurveda & Siddha: pg. 375-78
28. Sharma P.C., Yelne M.B, Dennis T.J., Database on Medicinal Plants Used in Ayurveda, 2005, sukshma ela, volume 5, Documentation and Publication Division Central Council for Research in Ayurveda & Siddha: pg.391-94
29. Sharma P.C., Yelne M.B, Dennis T.J., Database on Medicinal Plants Used in Ayurveda, trivrita, volume 1, 2005, Documentation and Publication Division Central Council for Research in Ayurveda & Siddha: pg. 462-64
30. Sharma P.C., Yelne M.B, Dennis T.J., Database on Medicinal Plants Used in Ayurveda, Rakta Chandan, volume 7, 2005, Documentation and Publication Division Central Council for Research in Ayurveda & Siddha: p361-64
31. The Ayurvedic Pharmacopoeia of India, Government of India Ministry of Health and Family Welfare Department Of Ayush, Part-1, vol.4, p. 41
32. Vol. 10. New Delhi: CSIR; 1976. Wealth of India. Raw Materials. Sp. W. Publications and Information Directorate; pp. 66–7


33. 12th ed. London: Balliere-Tindall; 1983. Trease and Evans, Pharmacognosy; p. 36.
34. Yadav KN, Kadam PV, Patel JA, Patil MJ. Strychnos potatorum: Phytochemical and pharmacological review. Pharmacogn Rev. 2014;8(15):61-66. doi:10.4103/0973-7847.125533
35. Sharma P.C., Yelne M.B, Dennis T.J., Database on Medicinal Plants Used in Ayurveda, Shirisha, volume 1 2005, Documentation and Publication Division Central Council for Research in Ayurveda & Siddha: p.445-47.
36. Sharma P.C., Yelne M.B, Dennis T.J., Database on Medicinal Plants Used in Ayurveda, nirgundi, volume 3 2005, Documentation and Publication Division Central Council for Research in Ayurveda & Siddha: p. 450-55
37. The Ayurvedic Pharmacopoeia of India, Government of India Ministry of Health and Family Welfare Department of Ayush, Part-1, vol.6, p. 152
38. Hussain N, Kakoti BB. Review on ethno botany and psychopharmacology of Cordia dichotoma. J Drug Deliv Ther. 2013;3(1):110–113
39. Nariya PB, Bhalodia NR, Shukla VJ, Acharya RN. Antimicrobial and antifungal activities of Cordia dichotoma (Forester F.) bark extracts. Ayu. 2011; 32:585–589. doi: 10.4103/0974-8520.96138
40. The Ayurvedic Pharmacopoeia Of India Part -Ii (Formulations) Volume -I First Edition Government Of India Ministry Of Health And Family Welfare Department Of Ayurveda, Yoga & Naturopathy, Unani, Siddha And Homoeopathy, New Delhi 2007 Contents [Internet]. [cited 2024 Oct 18]. Available from: https://dravyagunatvpm.wordpress.com/wp-content/uploads/2009/02/api-2-general1.pdf