Agni means digestive or metabolic factors. In panchamahabhuta (five fundamental elements), it means fire. It is responsible for transformation or change in form of an element. In human biology, it is responsible factor for digestion and metabolism. It is also a synonym of Pitta dosha.
Agni includes all factors responsible for digestion and metabolism / transformation having predominance of agni mahabhuta.(Code:SAT-B.488)
- Jatharagni: The factors responsible for digestion and metabolism at gastro-intestinal tract.
- Bhutagni : The factors responsible for digestion and metabolism at organic level. These are five types of agni, those act after jatharagni, but before dhatvagni on the food and its metabolites. They are: parthiva (pruthvi dominant), apya (aap dominant) , taijasa (teja or agni dominant), vayavya (vayu dominant) and nabhasa (akasha dominant). They act on the corresponding substrate based on mahabhuta composition to make them homologous to that of the bodily constituents.
The intensity of agni is influenced by dosha and is of four types:
- Sama (normal, due to the balanced state of all three dosha)
- Vishama (irregular due to the dominance of vata)
- Tīkshna (intense due to the dominance of pitta)
- Manda (low due to the dominance of kapha)
The first type of normal digestion (samagni) needs 12 hours (4 yama) for complete digestion of food, and 6 hours (2 yama) for proper digestion of medicine. [A.S. Sutra Sthana 11/53] The time taken for complete digestion suggests digestive pattern of that individual.
Dosha are the main functional unit of human physiology. Dosha also govern the physiology of digestion. Samana vayu functions nearby the abode of digestive factors (jatharagni), kindles agni. Prana vayu and apana vayu supports the functions of agni. Pachaka pitta when devoid of its liquid property, is responsible for digestion.[A. Hr. Sutra Sthana 12/10-11] Kledaka kapha is responsible to moisten the ingested food. Thus the process of digestion is driven by dosha. Factors responsible for complete transformation and absorption of food (ahara parinamakara bhava) explain this phenomenon in-depth. [Cha.Sa.Sharira Sthana6/14],[Cha.Sa.Chikitsa Sthana 15/6-8]. The vitiated dosha leads to the formation of ama.
Importance and assessment of agni
The proper quantity of food depends on the strength of agni (factors responsible for digestion and metabolism). The strength of agni varies according to season, age and other factors. Thus the amount of food will also vary as per the power of agni. Suitable variation in quantity of food helps maintaining proper digestion and metabolism. The quantity further subjects to heavy to digest (gurutva) and light to digest (laghutva) nature of food. Nourishing benefits of diet is subjected to the status of agni. Thus agni is an important facilitator between health and food. Assessment of the status of agni is an essential component in maintenance of health as well as while treating diseased conditions.
Singh A, Patwardhan K. et. al. developed and validated a self-assessment tool to estimate strength of agni. The study also evaluates the practical utility of developed tool by recording serum lipid parameters. Lipid parameters vary significantly according to the status of [agni]]. 
Agni performs various functions of digestion, metabolism and assimilation. Gastric secretions is a digestive fluid, formed in the stomach and contain numerous compounds including hydrochloric acid (HCL), pepsin, lipase, mucin. Kulatunga et al assessed the status of agni in the patients of pandu (anemia and blood deficiency disorders) and find out its relationship with the acidity of gastric secretions by use of fractional test meal examination. Their study concluded that HCL reduction in patients of anemia seriously affects the protein and iron absorption; thus Hypochlorhydria (found in 72.8% of the patients) indicates hypofunction of agni. 
- National AYUSH Morbidity and Standardized Terminologies Electronic Portal by Ministry of AYUSH Available on http://namstp.ayush.gov.in/#/sat
- agbhata. Ashtanga Hridayam. Edited by Harishastri Paradkar Vaidya. 1st ed. Varanasi: Krishnadas Academy;2000.
- Singh A, Singh G, Patwardhan K, Gehlot S. Development, Validation and Verification of a Self-Assessment Tool to Estimate Agnibala (Digestive Strength). J Evid Based Complementary Altern Med. 2017 Jan;22(1):134-140. doi: 10.1177/2156587216656117. Epub 2016 Jul 4.
- Eswaran HT, Kavita MB, Tripaty TB, and Shivakumar. Formation and validation of questionnaire to assess Jāṭharāgni. Anc Sci Life.2015 Apr-Jun; 34(4): 203–209.
- Patil VC, Baghel MS, Thakar AB. Assessment of agni (digestive process) and koshtha (bowel movement with special reference to abhyantara snehana (internal oleation). Ancient Sci. Life. 2008; 28:26-28
- Kulatunga R D H, Rai N P, Ali Z. Status of Agni in Pandu Roga (anemia) and its association with the acidity of gastric secretions-A Clinical Study. IAMJ: Volume 7, Issue 1, January – 2019.