According to Ayurveda, Ahar sambhavam vastu rogashch ahar sambhavah, which means, body is the outcome of food and so is the disease. Under normal conditions, food provides energy for body‟s functions and building material for growth and repair. Most of the food substances require changes before utilization. These changes begin with the culinary and end in the intestine with digestion. Cooking and digestion both require heat energy given in any form. Heat energy required for digestion is known as pachakagni in Ayurveda. The entire process (digestion-metabolism) of conversion of food is performed successively by Pachakagni, Bhootagni and Dhatwagni. The present article focuses on how does the normalcy of these Agnis causes ahar sambhavam vastu and abnormality causes rogastu ahar sambhavah on the basis of Ayurvedic and modern science.